01 – USE OF A DDVH DECOMPOSITION TECHNIQUE TO EVALUATE NORMAL TISSUE COMPLICATION PROBABILITY OF THE LIVER IN THE TREATMENT OF MESOTHELIOMA PATIENTS.

Frank Van den Heuvel, Yolande Lievens

UZ Gasthuisberg - KU Leuven

Dose volume histograms are a common tool to assess the value of a treatment plan for various forms of radiation therapy treatment. The purpose of this work is to applya set of tools to analyze differential dose volume histograms by decomposing them into physically and clinically meaningful normal distributions.

One of the advantages of such a decomposition technique is that it allows to estimate the normal tissue complication probability (NTCP) in inhomogeneously irradiated organs in a more refined manner. Indeed, radiobiological data can be taken into account for every decomposed entity, and the NTCP can be calculated automatically starting from a differential dose volume histogram.

In this study the NTCP for the liver was calculated for 11 patients undergoing IMRT treatment for Mesothelioma. The treatment dose was set at 50.4 Gy in fractions of 1.8 Gy.

The dose volume histograms were obtained from an inverse planning system using a dynamic IMRT implementation (Eclipse, Varian, Milpitas, CA).The calculations were performed as described by Mc Ginn et al. using an error function based expression for NTCP. In the latter article the treatment dose was tailored so that the NTCP to the liver did not increase 10%.

The NTCP ranged from 0.4% to 20%. It was clear and not surprising that the NTCP for patients with right lung involvement was significantly higher. In our conclusion it are these patients specifically who will receive benefit from an IMRT based treatment while the other patients can be treated conventionally with the same complication probability.

Keywords : treatment planning, dose volume histogram, radiobiology

02 – A VARIABLE TG 43 APPROACH TO BRACHYTHERAPY USING 192IR WIRE IMPLANTS.

Frank Van den Heuvel, Marisol De Brabandere

UZ Gasthuisberg - KU Leuven

Currently, Ir-wire applications are still being calculated with a methodology developed in the early 80s. While clinical data support the adequacy of this approach. There are still questions pertaining the way dose is prescribed as well as the way source strength is provided by various standardized laboratories.

The current standard of practice requires source strength to be defined as the Air Kerma strength. The still prevalent Paris technique still utilizes apparant activity.

We propose to alter the characterization of the Ir-wire source to closely follow the AAPM Task Group 43 (TG43) description of brachytherapy sources. This allows the Ir-wire to be easily incorporated in modern brachytherapy systems. In contrast to other sources typefied by TG43 the geometrical length of the Ir-wire is not a fixed constant, which influences the geometrical parameters of the dose distribution. We therefore initiate TG43 calculations for different lengths (e.g. 1 to 14 cm long wires).

A general purpose monte carlo simulation program (MCNPX) is used to generate a cylindrically symmetric dose distribution in: 1) wet Air, 2) Water, and 3) Polystyreen.An additional dose point is added to serve as a reference at the center of the wire in the long direction, 1cm away from the source center in the away direction.

The following characteristics are then scored: The dose rate constant:the radial function and the geometric function . All

functions are parametrized (L) with respect to length. Note that the source strength needs to be determined from the MC-calculations.

This paper aims to:

Determine a generalized form of the TG43 formalism with a parameter L, fitted to the values obtained above.

Estimate the differences of using partial lengths (i.e. a 6 cm wire described by the sum of two 3 cm wires)

Keywords : brachytherapy, TG43, Ir

03 – RADIOTHERAPY SOFTWARE FOR DEPARTMENT MANAGEMENT AND ORGANISATION

Maxime Coevoet, Laurette Renard, Françoise Vanneste, Jean-Marc Denis, Samira Chellouki, Pascale Henderickx, Ayhan Findik.

Clinique Universitaire St-Luc (UCL)

The “Radiotherapy Intranet” is a software developed to improve organisation in a radiotherapy department and to have a transfer of information between the different teams. It gives a global overview of the work to do.

We have made a real study of what is the job of everyone in a radiotherapy department. The result is a checklist of all the things to do. From the consultation to the first treatment day of the patient, trough the first simulation, planning, realisation of cerrobend blocks, … Each checklist is customised for each different patient planning.

Practically, the main page of the Intranet is the list of the different patients names in different background colours. Each colour belongs to one trade (red for physicists, blue for nurse, … ). When the stage is finished the background colour of the patient name will change to the new one. And someone else can deal with the continuation of the treatment preparation.

Radiotherapy is a teamwork. The Intranet can't work if every involved uses it correctly. But now, after one year of use, we can say that it became difficult to work without this huge information database.

Moreover, we are able to get some statistics on the time needed for a particular activity, on improvement of organisation, time lost in the procedures, number of patients, ...

Technically, the Intranet works like a website. The database is running on a local web server, each computer can access to it via a web browser like Internet Explorer, Mozilla Firefox, Epiphany, ... as well under Windows, Linux or Macintosh, and, the most advantage for a radiotherapy department, without any installation on each client.

Is it is an internal website, confidentiality is 100% guaranteed.

Keywords : radiotherapy, software, management

04 – TENTATIVE OF COMPARISON BETWEEN STEP AND SHOOT AND DYNAMIC (SLIDING WINDOW AND HELICAL TOMOGRAPHY) IMRT FOR HEAD AND NECK CANCER USING A SIB APPROACH

Milàn Tomsej (1), V. Tondeur (1), Pascal Fenoglietto (2), Nathalie de Patoul (1), Norbert Aillères (2), Vincent Grégoire (1)

(1) Université Catholique de Louvain (U.C.L.), Cliniques universitaires Saint-Luc Service de radiothérapie oncologique, 10, avenue Hippocrate, B-1200-Bruxelles, Belgique

(2) Centre Val d’Aurelles- Service de radiothérapie, Montpellier- France

Intensity modulated radiation therapy (IMRT) plays an important role in the treatment of patients with head and neck cancer. As the highly conformal dose distribution that is achievable with IMRT makes it possible to envisage an increase in physical dose while still maintaining the dose to the OAR at a reasonable level, several options could be considered to attain this objective.

This has been possible thanks to the rising of quality of inverse treatment planning systems employing very powerful algorithms, focused either on optimisation either on dose calculations. Optimal fluences are found for each beam by the input of dose volume constraints and will generate different shapes of beams using wether a dynamic or a static collimation. Linacs today are capable to treat in IMRT patients using the “step and shoot” or the dynamic delivery or both.

This study tries to compare both delivery techniques (step and shoot and dynamic), with ELEKTA (step and shoot only possible) and VARIAN (step and shoot and dynamic) linacs using the same inverse planning system, ECLIPSE (VARIAN), in which both kinds of beam delivery exist, and is the result of a collaboration of 2 sites owning original equipment, i.e. Centre Val d’Aurelles in Montpellier, France (ECLIPSE and VARIAN linacs) and St-Luc University Hospital, U.C.L. in Brussels, Belgium (ECLIPSE and ELEKTA linacs).

Of course, due to the integration of a new Hi-ART (Tomotherapy Inc.) in Brussels, its dynamic helical delivery mode has also been tested and studied.

Same patients have been exchanged in terms of images and volumes and were chosen to belong to same category,say moderately advanced (T2N0, T2N1, T3N0) squamous cell carcinomas (SCC) of the oropharynx, larynx and hypopharynx, treated using a simultaneous integrated boost approach. Obviously, same dose volume objectives (and/or same optimal fluence files) have been fixed as well as the maximum iterations and of course the geometrical parameters and beam energy.

Treatment plans are performed according to such protocol, and analysis has been realized reporting parameters derived from dose volume histograms and will be discussed here.

Keywords : IMRT, SIB, delivery mode, tomotherapy

05 – VALIDATION OF MRI TO CT SIMILARITY MEASURE VOXEL-BASED FRAMELESS REGISTRATION FOR FRAME-BASED RADIOSURGERY TREATMENT PLAN

F.De Smedt(1,2), D.Wikler(1), M. Op de beeck(1), N.Massager(1), D.Deviendt(1,2), S. Simon(1,2), M. Levivier(1)

(1) Hôpital Erasme, Service de Neurochirurgie, Centre Gamma Knife

(2) Institut Bordet, Service de Radiophysique

INTRODUCTION – Gamma Knife radiosurgery uses a frame-based approach for multimodality medical image registration and treatment application to provide superior accuracy and reproducibility. The recent advent of frameless techniques for the registration of medical images and their accuracy assesment foster the use of multimodal information in radiotherapy and radiosurgery treatment planning systems. Frame-based procedures could benefit from frameless registration in using diagnostic MRI for the treatment planning along with frame-based CT hence reducing organization and timing constraints. Our study aims at assessing the clinical validity of such an approach for Gamma Knife interventions.

MATERIAL AND METHODS – 50 radiosurgery treatment planning (RTP) data from Gamma Knife patients were retrospectively included in our study. For each RTP, the frame-based MRI pulse sequences used in the treatment plan were registered using the similarity measure voxel-based registration provided by the new release of the LGP4C Gamma Knife RTP system (Elekta, Sweden). Registration parameters were extracted from LGP4C for frameless and frame-based algorithms as applied to MRI pulse sequences (Tmri-ct, Tmri-frame) and CT (Tct-frame) series. From these matrices, the error geometric transform generated by the frameless technique as opposed to the frame-based registration was derived: Terror = Tct-frame * Tmri-ct * inv(Tmri-frame). This error transform was then applied to the treatment prescribed isodose volume (PIV) leaving the target volume (TV) unchanged to simulate errors due to the frameless registration on the placement of isocenters. A modified conformance ratio (TVpiv*PIV/TV where TVpiv is the target volume fraction covered by PIV) was computed and compared to the original conformance ratio to assess the validity of the frameless registration.

CONCLUSION – Results and their classification according to treatment indication and volume suggest that registration of frameless MRI to frame-based CT can be achieved. Accuracy and reproducibility is dependent on specific guidelines for the MRI registration strategy. MRI high resolution 3D gradient echo T1 can be registered to CT with accurate and reproducible results when initial conditions are not too far from the final solution while MRI high resolution selective partial inversion recovery T2 SPIR cannot be registered accurately to CT.

Keywords : registration, radiosurgery, Gamma Knife

06 – ACCEPTANCE AND COMMISSIONING OF A SYSTEM FOR HELICAL TOMOTHERAPY : WORKING IN NON-REFERENCE CONDITIONS.

Koen Tournel, Dirk Verellen, Nadine Linthout

Radiotherapy Department ,Oncoligisch Centrum, Academisch Ziekenhuis VUB

Earlier this year a commercial system for delivering intensity modulated radiation therapy using a helical, rotational delivery (TomoTherapy, TomoTherapy inc., Madison US) was installed at the AZVUB. The system consists of a 6MV linac mounted on a slipring, and the center of rotation is located at a source-axis distance (SAD) of 85cm. The beam intensity is modulated using a binary MLC consisting of 2 banks of 32 leaves. Possible field sizes are resp. 1, 2.5 and 5 cm by 40cm. Treatments are performed by continuous movement of the couch while the linac is generating radiation.

The acceptance and commisioning procedure of the system that was provided by the company only consisted of a black-box technique that verified the entire treatment chain as a whole and did not involve intensive testing of different parts as described in NCS report 9 , especially regarding the absolute dosimetry of the system. Because of the particular design (SAD 85cm, no 10x10 field, no flattening filter, no monitor unit specification) it is not possible to apply the standard dosimetry protocols to this system. To be able to describe and check standard and action levels for the different dosimetric parameters of the system a check using an adapted version of the TG51 protocol was applied and-cross linked to the HPA-based calibration of the other accelerators in our institution. Since the ionisation chambers to which the system is customized (Exradin A1SL, Standard Imaging, US) is not widely spread in Europe the chambers were recalibrated in the national standard laboratory and cross-linked to the standard dosimetric protocol for photon beams (HPA) in our department. Since only phantom checks using a homogeneous phantom and company supplied-ionisation chambers were possible on the system a procedure was developed to be able to check dose in a RANDO head-and-neack phantom using TLD and film.

The results of the recalibration of the company-supplied chambers show a discrepancy of 2-3% relative to the value supplied by a US-standard laboratory. After this correction the absolute dosimetry was within 0.5% of the value generated by the other machines in the department and using another protocol. The procedure of calibration of film and TLD-dosimeters proved valid within 2-3%, allowing the use in relative dosimetry.

Keywords : helical tomotherapy, dosimetry, commissioning

07 – DYNAMIC ARC THERAPY: FEASIBILITY FOR PROSTATE CANCER IRRADIATION.

Harzee L., Dessy F., Merlo P., Hoornaert M.T.

Hospital of Jolimont, Haine-Saint-Paul, Belgium

PURPOSE – To study the feasibility of using dynamic arc therapy to treat patients having a prostate cancer for the boost or the total treatment (good prognostic)

MATERIALS AND METHODS – Dynamic arc therapy can be delivered by our Varian linear accelerator Clinac 2100C/D (120 leafs MLC). In this study, two Treatment Planning Systems are used : Eclipse, version 6.5.7.2.24 from Varian and Brainscan version 5.3. from Brainlab. Both TPS use a pencil beam algorithm. Treatment plans using 6 MV photon beams are generated on both TPS with identical field setup. The treatment parameters are then exported to the treatment machine through the Varis network and delivered to the body phantom Omnipro IMRT from Scanditronix. Kodak EDR2 films are placed between each slice. They are digitized with a VIDAR VXR12 Plus associated with the software Omnipro, version 1.4.1.0 from Scanditronix. With this method, 3 comparisons are possible: Eclipse-Brainscan to compare calculated treatment plans, Eclipse-Measurements & Brainscan-Measurements to evaluate the agreement between each calculated dose distribution and the measurement, using the gamma factor (= 3% , = 3 mm). In addition, the dose at the isocenter is determined with a 0.6cc ionisation chamber and relative punctual measurements are made at the isocenter and in the middle of the rectum with a PinPoint chamber 0.015cc from PTW. In the second part of this study, using Eclipse, we try to find the best field configuration (1 or 2 dynamics arcs) to treat this organ and we compare our methods and results with the literature.

RESULTS – The results from the comparison between calculated and measured dose ditribution are discussed. The limitations of both TPS for this type of treatment are presented, as well as the best configuration obtained with Eclipse.

CONCLUSION – These preliminary results, together with an easy and quick treatment delivery, encourage us to go on with comparison with our actual techniques before implementing this technique in clinical routine.

Keywords : dynamic arc, prostate, gamma index

08 – THE EVALUATION OF TWO DIFFERENT IMRT TREATMENT DELIVERY SYSTEMS, HELICAL TOMOTHERAPY AND STEP ‘N SHOOT IMRT : AN ALANINE DOSIMETRIC VERIFICATION.

Michael Duchateau, Dirk Verellen, Bob Schaeken

Xios Hogeschool Diepenbeek, AZ-VUB Jette, ZNA-Middelheim

BACKGROUND AND PURPOSE – Verification of IMRT delivered dose is challenging (small fields, high dose gradients). This study evaluates absolute dose administered with two different techniques; helical tomotherapy (AZ-VUB, Jette, Brussels) and step and shoot IMRT treatment delivery (ZNA-Middelheim, Antwerp), using alanine dosimetry.

METHODS AND MATERIALS – Tomotherapy(TM) IMRT is delivered using the HiART system and the step and shoot IMRT is delivered with a SLi linac (Elekta(TM)). The Elekta SLi uses the Pinnacle(TM) treatment planning system and the Tomotherapy planning station uses a specific developed treatment planning system. Both systems are based on the superposition/convolution principle to perform the dose calculations. An anthropomorphic phantom is used for the measurements. Several different tumour contours, located in the head and neck region, will be copied or drawn on to the phantom. Dosimetry with alanine, using electron spin resonance (ESR), will be used to evaluate the effectively delivered dose of these IMRT delivery techniques. The alanine dosimeters are positioned in the PTV region and organs at risk.

RESULTS AND CONCLUSION – The “copy-to-phantom” methodology is not suitable for a correct comparison because of the software differences in the quality assurance module of both planning systems. Therefore a “phantom-is-patient” methodology will be used for comparison and dosimetric evaluation. Dosimetric verification using film, TLD and alanine will be shown. Preliminary results obtained with alanine on the elekta Sli for a brain lesion show that 2 out of 10 measured points were overestimated with more than 5%. Measurements for the head and neck lesion show more than 5% overestimation in 3 out of 18 measured points and 3 out of 18 points have an underestimation of more than 5%. Performed TLD measurements on the tomotherapy HiArt system did not lead to a firm conclusion because of the TLD positioning in high gradient zones. Film measurements on the tomotherapy HiArt system were evaluated using a gamma technique with following parameters ; search distance : 3 mm, DTA tolerance : 3 mm and dose tolerance : 0,075 Gy (3% of 2,5 Gy).

Keywords : tomotherapy, step and shoot IMRT, alanine dosimetry

09 – CHANGE MANAGEMENT IN RT

Tom Depuydt, Yolande Lievens

University Hospital Leuven Gasthuisberg

The turn of the century brought the large-scale break through of 3D conformal and intensity modulated radiation therapy techniques in the clinical setting. During the 90’s, new technologies were introduced for high-resolution dose delivery. These delivery techniques have nearly reached maturity. However to fully use the power of high-conformal dose delivery up to its potential, the main focus of innovation today is mainly on target volume definition and localization. Definition and localization of the target volume requires advanced image acquisition technology for visualization of anatomy. Some of the required technology and know-how is available in the radiology industry. An integration process of this technology into radiation therapy was commenced and input from technological innovation in radiology became one of the driving forces for innovation in radiation therapy. As a consequence, these new devices found there way to application in radiation therapy much faster then during the previous decades. The last few years, the speed of technological evolution in radiation therapy increased and this will probably continue in the future. Radiation therapy innovating at higher speed influences the process of replacing equipment. Beside the installation of new equipment there seems to be an almost continuous process of upgrading hardware and software in our departments. To handle this continuous process of change one has to incorporate change into the daily routine. Radiation therapy being a complex working environment with its different disciplines working together is a particularly difficult environment to implement new approaches to change.

The radiation therapy department of the University Hospital Gasthuisberg Leuven is at the moment going through the motions of changing about 75% of its RT equipment. This talk is about the impact of such a project on a radiation therapy department with a heavy workload of treating 2400 patients every year on 5 treatment units.

Keywords : change management, innovation, IGRT

10 – MEASURING THE SKIN DOSE OF PATIENTS IN INTERVENTIONAL RADIOLOGY: STANDARDIZATION OF THE CALIBRATION OF THE THERMOLUMINESCENT DOSIMETERS

N. Jans (1,2), L. Struelens (2), M. Breugelmans(1)

(1) XIOS Hogeschool Limburg, Master in Industrial Siences, Nuclear Technology, Diepenbeek

(2) SCK-CEN, Radiation Protection division, Mol

BACKGROUND AND PURPOSE – Interventional radiology deals with radiological procedures where problems of the vascular structure are treated. These examinations can be very complex. The doses for the patients can be very high and next to stochastic effects, also deterministic effects have to be taken into account. The most important tissue of concern, relating to these latter effects is the skin. Therefore it is important that the skin dose of the patient could be measured as accurately as possible. The most direct measurement of this skin dose is done with thermoluminescent dosimeters (TLDs). The calibration of TLDs is crucial in the measurement process, but no standardization exists in the way these calibrations should be performed.

METHOD AND MATERIALS – It is important that TLDs are calibrated in a situation which corresponds to the practice where they are used. By this, we mean the energy used, the way they are packed and treated. If different circomstances are used for the calibration, this might have an influence on the response of the TLD and doses may be calculated less accurate. Several experiments are carried out to investigate the influence of different exposure conditions on the response of TLDs. 2 types of TLDs were used: LiF:Mg,Ti (TLD100, USA) and LiF:Mg,Cu,P (MCP-N, Poland). TLDs were packed into different materials: in plastic bags (whether or not sewed on different kind of tissues), in a plexi standard, behind 2 mm of PMMA, etc. Some of the TLDs were also used without packing. The experiments can be categorised in 4 exposure classes: on a waterphantom, on the Rando-Alderson phantom, on wood and free-in-air. Different energies were taken into account: narrow and broad x-ray reference ISO spectra, a Co- and a Cs-source.

RESULTS AND DISCUSSION – For every exposure class, response curves in function of energy were plotted for the different packing methods. They are calculated as well in reference to air Kerma as to Hp(0.07). As expected, we clearly found differences in TLD response for the different exposure conditions. All these curves are thoroughly evaluated, in the clinical energy field of interest, to conclude, among others, which experiment combination is the least depending on energy, which combination of exposure condition (energy and phantom) and packing method has similar responses as those for the combinations used most frequently in practice, how large are the differences, etc.

Keywords : skin dose, TLD, calibration

11 – THE INFLUENCE OF THE PATIENT SIZE ON THE CONVERSION COEFFICIENTS FOR THE CALCULATION OF THE EFFECTIVE DOSE

Katrien Leysen (1,2), Lara Struelens (2) and Herwig Janssens (1)

(1) Xios Hogeschool Limburg, master in industrial sciences, nuclear technology, Diepenbeek

(2) SCK-CEN, radiation protection division, Mol

BACKGROUND AND PURPOSE – According to the legislation, the registration of patient doses in conventional radiology is necessary. In view of optimisation of patient doses, also the effective dose might be a useful tool. Conversion coefficients are used to calculate this effective dose, with the aid of easily measurable quantities like dose-area product (DAP) or entrance surface dose (ESD). Mostly, those conversion coefficients are used, which are calculated for standard phantoms (length: 179cm and weight: 73,5kg). Because every patient is different, however, inaccuracies are introduced using these standard conversion coefficients. This study evaluates the difference in effective dose and organ dose conversion coefficients between a standard model and two thicker phantoms (one with length: 179cm and weight: 83,5kg and the other with length: 179cm and weight: 93,6kg).

METHODS AND MATERIALS –The simulation of an androgynous phantom is made by the software BODYBUILDER. The advantage of this software is the possibility to add an extra layer of fat. The calculation of the effective dose and a theoretical DAP/ESD-value is delivered using the computer code MCNP4B. This is done for different examinations (abdomen AP and thorax PA) and different clinical spectra. For the mentioned procedures above, DAP and ESD are measured on different patients in some radiology departments. The technical parameters used (tube voltage and tube current), aswell as the patient dimensions, were also registered.

RESULTS AND CONCLUSION – The MCNP calculations lead to 3 tables of conversion coefficients, one standard table and two tables for thicker phantoms. The patient data in the hospitals will tell us how the procedure is performed in function of patient thickness. These results will show us the degree of inaccuracy that is made by using the standard conversion coefficients for effective dose calculations, according to patient thickness. Also the influence of possible changes of tube voltage for thicker patients in practice is investigated.

Keywords : conversion coefficient, effective dose, patient thickness

12 – ACCURACY OF PATIENT DOSE CALCULATION FOR LUNG-IMRT: A COMPARISON OF MONTE CARLO, CONVOLUTION/SUPERPOSITION AND PENCIL BEAM COMPUTATIONS

Barbara Vanderstraeten (1,2), Nick Reynaert (1), Leen Paelinck (2), Indira Madani (2), Werner De Gersem (2), Carlos De Wagter (2), Wilfried De Neve (2) and Hubert Thierens (1)

(1) Ghent University, Department of Medical Physics

(2) Ghent University Hospital, Department of Radiotherapy

We compared full Monte Carlo calculations (performed by our Monte Carlo Dose Engine MCDE) with two different commercial convolution/superposition implementations (Pinnacle and Helax-TMS’s collapsed cone model Helax-CC) and one pencil beam algorithm (Helax-TMS’s pencil beam model Helax-PB) by computing dose distributions within 10 lung-IMRT patients. Treatment plans were created for two photon beam qualities (6 and 18 MV).

For each dose calculation algorithm, patient and beam quality the following dose-volume values were reported: minimal, median and maximal dose (Dmin, D50 and Dmax) for the gross tumor and planning target volumes (GTV and PTV); the volume of the lungs (excluding the GTV) receiving at least 20 and 30Gy (V20 and V30) and the mean lung dose; the 33rd percentile dose (D33) and Dmax delivered to the heart and to the expanded oesophagus; and Dmax for the expanded spinal cord.

Statistical analysis was performed by means of 1-way analysis of variance (ANOVA) for repeated measurements and pairwise comparison of means according to Tukey’s method. Pinnacle showed an excellent correspondence with MCDE within the target structures, whereas the best correspondence for the organs at risk (OARs) was found between Helax-CC and MCDE. Results from Helax-PB were unsatisfying both for targets and OARs.

Additionally, individual patient results were analyzed. Deviations larger than 5% were found in 1 patient for the comparison of MCDE and Helax-CC within the target structures, while for Pinnacle all dose differences were below 5%. Both for Pinnacle and Helax-CC, substantial deviations from MCDE were found within the OARs. Pinnacle showed deviations in dose-volume values within the lungs larger than 5% for two (6 MV) and six patients (18 MV), respectively.

In conclusion, not one pair of the dose calculation algorithms we investigated could provide results that were consistent (within 5%) for all 10 patients for the set of clinically relevant dose-volume indices studied. As the results from both convolution/superposition algorithms differed significantly, care should be taken when evaluating treatment plans, since the choice of dose calculation algorithm may influence clinical results. Full Monte Carlo provides a great benchmarking tool for evaluating the performance of other algorithms for patient dose computations.

Keywords : dose computation, lung IMRT, Monte Carlo

13 – COMMISSIONING OF A NEW VARIAN TPS ALGORITHM (AAA) CONSIDERING STEP AND SHOOT IMRT TREATMENTS : DOSIMETRY COMPARISON WITH MEASUREMENTS AND MONTE CARLO (BEAMNRC) CALCULATIONS USING A ELEKTA SL25 LINAC

Edmond Sterpin, Milàn Tomsej, Nick Reynaert and Stefaan Vynckier

Université Catholique de Louvain - Cliniques universitaires Saint-Luc. Radiotherapy Department

AAA (Anisotropic Analytical Algorithm) is a pencil beam convolution/superposition algorithm involving Monte Carlo derived data for primary photons and for scattered photons and electrons. Moreover, the dose deposition is calculated by using six exponential curves. Inhomogeneities are taken into account anisotropically thanks to photons scatter kernels in multiple lateral directions.

Monte Carlo calculation uses instead physical cross sections to simulate the particles transport depending on the medium. We expect therefore more accurate results for heterogeneities with Monte Carlo calculations than analytical algorithms.

Our Monte Carlo model, using BEAMnrc software and the modeling of the ELEKTA linear accelerator were validated in standard conditions achieving measurements’ and calculations’ comparisons in a water phantom. The model has been tuned by adapting electron energy and the opening of the jaws and the leaves.

AAA has not been validated yet for ELEKTA linear accelerators. We use our model and measurements to study the accuracy of the results given by AAA firstly in a water phantom for standard and IMRT fields. Secondly, further comparisons will be performed considering a phantom containing inhomogeneities. Finally, we compare AAA with Monte Carlo calculations in a CT scan for a complete IMRT treatment considering a head and neck tumor case.

Keywords : Monte Carlo, IMRT, AAA

14 – TANDEM METHOD FOR CALIBRATING DOSE-AREA PRODUCT METERS

Paula Pöyry, Tuomo Komppa and Antti Kosunen

STUK - Radiation and Nuclear Safety Authority

Dose-area product (DAP) meters are used in diagnostic radiology to evaluate radiation exposure to patient. DAP meters are transmission-type plane ionization chambers providing a reading proportional to the surface integral of absorbed dose to air over the effective area of the chamber. To provide accurate results for comparative studies, the DAP meter should be calibrated properly.

A conventional way of calibration of DAP meter of an X-ray equipment, the field meter, is to approximate the surface integral by the product of the field area and the dose measured in the centre of the field, but the accuracy of this method may be inadequate for the most demanding purposes [1]. The accuracy of DAP measurements can be improved by calibrating the DAP meter relative to the surface integral of the absorbed dose. A practical way of doing this at a hospital is to use a tandem method where another DAP meter is used as reference instrument, the reference meter [2]. In the most common variation of tandem method, the field meter and the reference meter are used simultaneously in the X-ray beam. The reference meter has to bee calibrated relative to the incident beam, thus providing for the field meter a calibration relative to the beam incident on the patient.

The response of a typical DAP meter depends significantly on the energy distribution of the X-ray beam. The half-value layer (HVL), generally used to specify the radiation quality, is not adequate alone to determine the response of a DAP meter properly. Even at a constant HVL value, the calibration coefficient of a DAP meter can typically vary about 10 % in X-ray beams with different spectra. For DAP calibration, the radiation quality can be specified by the tube voltage and beam filtration, and an adequate number of clinical radiation qualities must be used to achieve reasonable accuracy.

The tandem method for calibrating field DAP meters is described in this work. The total relative uncertainty of the calibration coefficient (in good practical conditions) was estimated to be ±6 % with a confidence level of 95 %. The work is part of a project to establish a calibration service for DAP meters in Finland and to provide dosimetric guidance for users.

[1] J. P. Larsson, J. Persliden and G. Alm Carlsson, Phys. Med. Biol. 43 (1998) 599.[2] P. Pöyry, T. Komppa, A. Kosunen, Proceedings of the XXXIX annual conference of the Finnish physical society (2005) 247.

Keywords : DAP meter, dose-area product, calibration, tandem method

15 – COMPARISON OF EQUIPMENT PERFORMANCE FOR X-RAY SYSTEMS IN INTERVENTIONAL CARDIOLOGY

Octavian Dragusin, Hilde Bosmans, Kristien Smans, Peggy Vander Henst, Walter Desmet

Catholic University of Leuven, Gasthuisberg Hospital, Department of Radiology

Interventional cardiology procedures are among the most common group of interventional procedures performed in Belgium. The EU Medical Exposure Directive 97/43/Euratom and national legislation consider Catheterization Laboratory as an area of special concern regarding the dose levels.

The paper presents the results of performance tests of 16 interventional cardiac systems (14 systems with image intensifier and 2 systems with flat panel detectors). The main goal of this survey was to assess the requirements for equipment standards and to address the imbalance between the advancing technology and existing standards. Testing was performed in line with current national and international guidelines.

The quality controls included the assessment of the performance of the X-ray generator, the dose levels in fluoroscopy and digital acquisition modes and image quality using a subjective method (the Leeds test object). All doses (the detector and patient entrance dose rates) were measured under AEC (Automatic Exposure Control) and for various scattering material.

Analysing the results showed 2 remarkable results:

for new and old systems, dose levels are manufacturer dependent. Consequently, dose rates were found to be very different between systems, in both the fluoroscopy and digital acquisition modes.

In terms of image quality, all systems were found to have a measured threshold contrast of < 4% and no measurable improvement in image quality was observed for the systems operating at higher dose levels

From these observations, we conclude 2 facts: it is needed to carefully verify working conditions (including the pre programmed settings of the systems) and to check the effectiveness of quality control protocol and test tools.

Keywords : interventional cardiology, quality assurance tests

16 – ACCURACY OF THE TPS CALCULATION OF MATCHED HALF-BEAM DOSE DISTRIBUTIONS

S. Petillion, A. Swinnen, M. De Brabandere, F. Van den Heuvel

University Hospital LeuvenDepartment of Oncology and Experimental Radiotherapy

PURPOSE – For some clinical indications with large planning target volumes (PTVs), the dose distributions are not satisfactory. To improve homogeneity, half-beam blocked fields can be matched. The use of matched half-beams is possible thanks to the advent of independent jaws in most clinical accelerators. This study checks whether a commercial treatment planning system (TPS) is able to accurately calculate the dose in the matching region.

METHODS – Four different isocentric irradiation conditions, each consisting of two half-beams, are considered: matching using the upper and lower jaws, with and without backing MLC-leaves. The total size of the two beams combined is 24 x 20 cm². An EDR2-film is positioned at a depth of 12cm in a polystyrene phantom, and irradiated with 18MV photon beams, delivering 2Gy to the central axis at the film position. Using a densitometer the measured dose distributions are determined, and subsequently compared to the calculated dose distributions.

RESULTS – It is found that the calculated dose distributions are identical for all four setups. The TPS predicts a small dose dip at the connection of the half-beams. In contrast, the measured dose distributions are mutually different. It is shown that if the matching is performed with the upper jaws, the experimental results are influenced by the position of the MLC-leaves. If no MLC-backing is present, there is no dose reduction observed in the matching region. For closed leaves under the jaw (e.g. adjusted to PTV), the film measurement shows a larger dose dip than calculated. If the shaping of the half-beams is performed with the lower jaws, the calculated and the measured dose distributions are comparable, even in the matching region, irrespective of the leaf positions.

CONCLUSIONS – Most likely, the discrepancy of the calculated and the measured dose distributions is due to the inaccuracy in the beam modelling of the TPS, i.e. the geometric penumbra and the tongue-and-groove shape of the MLC are not taken into account.

Keywords : TPS calculation, matched half-beams, dose distribution

17 – FRAMEWORK FOR DIFFUSION TENSOR TRACTOGRAPHY VALIDATION WITH AN ANATOMICALLY REALISTIC SOFTWARE PHANTOM.

Steven Delputte, Rik Van de Walle, Ignace Lemahieu

Medisip (ELIS, UGent)

During the last decade, the development of MRI has led to a diversity of applications. One of the more advanced ones is Diffusion Tensor MRI (DT-MRI). This technique provides us with bulk averaged, microscopic anatomical information of fibrous tissues such as skeletal muscles and brain white matter. Fiber tracking algorithms based on diffusion tensor data are the first techniques capable of reconstructing the three-dimensional architecture of the major white matter axonal bundles within the living human brain. This information can help neuroscientists to better understand brain effective connectivity and white matter diseases (Alzheimer’s disease, multiple sclerosis,...) or can be used for better pre surgical planning.

Although diffusion tensor tractography is very promising, it’s current clinical use is limited due to the lack of a golden standard for validation of this technique. Therefore we propose a method to construct a noise-free synthetic diffusion tensor dataset, based on a priori known tracts that resemble the true white matter anatomy. These ground-truth fibers are found by performing Density Regularized Fiber Tractography (DRFT, Delputte, Proc. ISMRM 2005) on in-vivo data. The diffusion weighted volumes are smoothed with an anisotropic PDE-filter and we use a robust tensor estimation technique to reject outliers from the final tensor fitting. This also removes the need for cardiac gating. DRFT takes into account the architectural milieu of each tract and results in 3D, non-spurious, axonal pathways with a pointwise estimate of probability along each tract.

The construction of the synthetic tensor dataset is based on the framework of A. Leemans (MRM 53, 2005). Instead of using fixed parameters for each fiber and for each point along these fibers however, we use the pointwise architectural information provided by the DRFT results. Thus, we use a varying width and steepness for the saturated Gaussian fiber kernel, a probability estimate, and an actually measured fractional anisotropy (FA) and apparent diffusion coefficient to make the dataset as realistic as possible. Comparing the colour coded synthetic FA volume with the in-vivo one, we find indeed a good correspondence. After adding noise, we can run different tractography algorithms on the datasets and quantitatively compare these reconstructed fibers with the original ground-truth fibers. The framework was also used to optimize internal and operator dependant tractography parameters.

Keywords : diffusion tensor tractography, quantitative validation, synthetic phantom

18 – PHANTOM DESIGN FOR DIFFUSION TENSOR MAGNETIC RESONANCE IMAGING.

E.Fieremans (1), S.Delputte (1), Y.De Deene (2), Y.D'Asseler (1), E.Achten (3), I.Lemahieu (1), R.Van de Walle (1)

(1) Ghent University, ELIS/MEDISIP, Sint-Pietersnieuwstraat 41, B-9000 Ghent, Belgium.

(2) Ghent University Hospital, Department of Radiotherapy, De pintelaan , B-9000 Ghent, Belgium

(3) Ghent University Hospital, Department of Neuroradiology, De pintelaan , B-9000 Ghent, Belgium

Diffusion tensor magnetic resonance imaging (DT-MRI) is a non-invasive imaging method that can measure the motion of water molecules in all directions. In tissues containing a large number of fibres, like skeletal muscle and brain white matter, water tends to diffuse mainly along the directions of those fibres. This way, diffusion tensor imaging (DTI) has access to the organization in space of tissue micro structural components. Moreover, fibre tracking algorithms make is possible to reconstruct the three-dimensional structure of the major white matter tracts within the living human brain.

To determine the accuracy and precision of DTI a validation is necessary which requires a phantom with a well known structure. We’ve proposed a flexible hardware phantom by using parallel fibres made of woven strands tightly held together by a shrinking tube [1].In this abstract the possibility to develop a diffusion phantom for validating DTI in brain white matter is investigated.

To test the possibility of an anthropomorphic head diffusion phantom, fascicles are constructed in different diameters and geometries to imitate some of the major neural fibre tracts: corticospinal tracts, optical tracts, corpus callosum (forceps maior and minor) and fronto-occipital tracts. For this experiment phantom fascicles were placed in gel of 1.25 % (w/w) agarose and 0.275 mM Gd-DTPA to obtain T1 and T2 values similar to those of white matter and to reduce flow artefacts. These fascicles were fixed in a container with a similar shape as the head that contains a hollow pipe presenting the trachea.

To imitate brain white matter in DTI, a phantom material is necessary with similar diffusion characteristics and proton density. Fascicles were made of different fibre materials (silk filaments, silk yarn, Micro Dyneema®, dialysis fibers) - containing a varying number of strands- and were compared for fractional anisotropy (FA) and proton density (PD). Monte Carlo simulations were performed to study the diffusion process within the phantom fascicles. The diameter of the used fibre filament and the packing density pointed out to be the crucial factors to determine the behaviour of the short- and long-time of the diffusion process within the phantom fascicles.

[1] E.Fieremans et Al., ISMRM Proceedings, 1301 (2005).

Keywords : phantom, diffusion, diffusion tensor imaging

19 – EXPLOITING THE CHARACTERISTICS OF A PROTOTYPE ROTATING SLAT COLLIMATED SOLID STATE STRIP DETECTOR: DUAL ISOTOPE IMAGING.

Roel Van Holen, Steven Staelens, Stefaan Vandenberghe, Yves D'Asseler and Ignace Lemahieu

ELIS-MEDISIP, Ghent University (UGent), Ghent, Belgium

OBJECTIVES – Solid state detectors offer exquisite energy resolution compared to scintillation detectors. A dual isotope 123I/99mTc subtraction study investigates if the parathyroid (PT) localization with the prototype Solstice gamma camera is superior to traditional gamma camera scintigraphy with a NaI detector crystal.

METHODS – A thyroid/parathyroid phantom was built from CT data. The thyroid volume is 25ml and the two parathyroids have a volume of 0,2ml and 0,5ml. The phantom was filled with 123I and 99mTc activities based on literature values of thyroid and PT adenoma tracer uptake. The total activity (Technetium and Iodine) in the phantom was about 6 MBq, based on a patient injection of 750 MBq. Using the prototype Philips SOLSTICE camera (rotating slat collimator on a solid state strip detector) we performed a 20 minutes acquisition. Using the same phantom setup, the measurement was repeated on a classical gamma camera (AXIS, Philips).For the Solstice data, photopeak windows of 10% were set around 140 keV for 99mTc and 159 keV for 123I before reconstruction to projection images using Monte Carlo based MLEM. Projection data from the Axis simulation were binned in a 256 by 256 pixels matrix using a 15% energy window for 99mTc (140 keV) and a 10% window for 123I (159 keV). Subtraction of the iodine image (accumulating only in the thyroid) from the simultaneously acquired technetium image (accumulating in both the thyroid and parathyroids) yielded the image of the PTs. After qualitative evaluation, the images were scored on the basis of detectability of PT glands D=(s-μ)/σ where s represents the image counts in a region of interest drawn in the PT and μ and σ are the mean and standard deviation of the surrounding background activity respectively.

RESULTS – Visual inspection already shows the Solstice image to be superior to the Axis image. We can point out where the parathyroid glands are on the Solstice image while on the axis image, the localization can’t be done this accurately. Quantitative analysis of the images showed the detectability D in the Solstice imaging to be better than the Axis imaging by a factor 1,37 for the bigger adenoma and by a factor 1,71 for the smaller gland.

CONCLUSIONS – Due to the superior energy resolution and sensitivity of the Solstice gamma camera, the localization and detection of the PT glands using the 99mTc/123I subtraction technique is showed to be clinically more performant than the classical gamma camera approach.

Keywords : energy resolution, dual-isotope imaging, parathyroid scintigraphy

20 – A REVIEW ON THREE DIMENSIONAL RADIATION GEL DOSIMETRY

Yves De Deene

Radiotherapy (P7), Ghent University Hospital, De Pintelaan 185, 9000 Gent

The first gel dosimeters date back to the 1950's when a radiation-induced colour change in dyes imbedded in a gel were used to investigate doses [1]. In 1984, Fricke gel dosimeters that could be read out using NMR relaxometry were proposed [2]. These gels were based on the conventional Fricke solution that was dispersed throughout a gel matrix. In these Fricke gels the Fe2+ ions are converted to Fe3+ ions upon irradiation. The difference in paramagnetic properties of both ions results in a change in the NMR spin-spin relaxation rate (R1) that is radiation dose related. Due to predominantly diffusion-related limitations, alternative polymer gel dosimeters were subsequently suggested [3]. In polymer gel dosimeters, monomers are dispersed in a hydrogel matrix. The monomers undergo a polymerization reaction of which the degree is a function of absorbed radiation dose.

The radiation-induced formation of polymer influences the NMR proton spin-spin relaxation rate. First attempts to use polymer gel dosimeters to visualize the dose distribution of a conformal radiation treatment showed that necessary care should be given to the NMR imaging techniques [4].

As the polymerization reaction is inhibited by oxygen, all free oxygen has to be removed from the gels. For many years this was achieved by perfusing the gel with nitrogen. Recently, the use of anti-oxidants has been proposed to cancel out the inhibition by oxygen [5]. Emphasis has been layed on the radiation properties of the different gel dosimeters [6].

As well as MRI, other quantitative techniques for measuring dose distributions include optical and x-ray CT, vibrational Raman spectroscopy and ultrasound. Many of the recent developments in the field of radiation gel dosimetry are communicated at the international DOSGEL conferences [6,7].

[1] Day M J and Stein G J 1950 Nature 166: 146-7.[2] Gore J C, Kang Y S and Schulz R J 1984 Phys. Med. Biol. 29: 1189-97. [3] Maryanski M J, Gore J C, Kennan R P and Schulz R J 1993 Magn. Reson. Imaging 11: 253-8.[4] De Deene Y, De Wagter C, Van Duyse B, Derycke S, Mersseman B, De Gersem W, Voet T, Achten E and De Neve W 2000 Magn. Reson. Med. 43: 116-25.[5] Fong P M, Keil D C, Does M D and Gore J C 2001 Phys. Med. Biol. 46: 3105-13.[6] DOSGEL 2004 Proc. Third Int. Conf. on Radiotherapy Gel Dosimetry (Ghent, Belgium) ed. De Deene Y and Baldock C.[7] DOSGEL 2001 Proc. Second Int. Conf. on Radiotherapy Gel Dosimetry (Brisbane, Australia) ed. Baldock C and De Deene Y.

Keywords : polymer gel dosimetry, 3D radiation dosimeters, quantitative NMR

21 – A SINGLE MEASUREMENT-BASED METHOD FOR ABSOLUTE QUANTIFICATION OF METABOLITES BY PROTON MAGNETIC RESONANCE SPECTROSCOPY

M.S. Özdemir(1), Y .De Deene(2), E.Achten(3), Y.De Asseler(1) , I.Lemahieu(1)

(1) Gent University,Department of Electronics and Information Systems,Sintpietersnieuwstraat 41,9000 Gent, Belgium

(2) Department of Radiotherapy,Gent University Hospital, De Pintelaan 185, 9000 Gent, Belgium

(3) MR-Department (-1K12),Gent University Hospital, De Pintelaan 185, 9000 Gent Belgium

Magnetic Resonance Spectroscopy (MRS) is a non-invasive technique capable of directly measuring the chemicals within the body. Results are typically expressed as ratios between metabolite signals which vary between malignant and benign tissue. Therefore, malignancy can be characterized by utilizing the ratio change between certain metabolites of interest. However, the ratio-based results may not be plausible in many cases as the change in ratio may arise from the changes in the numerator as well as the denominator. Thus, presenting the ratio-based results can be misleading and absolute quantification of the metabolite concentration is preferable as it resolves possible ambiguities resulting from ratios. For absolute quantification of metabolites, a reference signal must be acquired which can be either an internal or external standard. Internal references have the advantage that they are insensitive to systematic errors associated with B1, Bo field inhomogeneities, flip angle and other pulse sequence related factors. Creatine (cre), a metabolite in the brain spectrum, is most commonly used as an internal reference. However, recent studies showed substantial regional and pathology-related changes in Cre [1]. An alternative reference is unsuppressed tissue water signal as brain water content is relatively well known and pathology related changes are relatively small. Nevertheless, water referencing may not be convenient in certain MRS studies since it may be prohibitively time consuming to record both water suppressed and unsuppressed spectra. However, we previously showed that a single measurement by which only an unsuppressed water spectrum is acquired can be adequate to perform absolute quantification [2]. By using Singular Value Decomposition (SVD), metabolite signals can be extracted from the spectrum of unsuppressed water signal which is used as an internal reference. In this study, a single measurement-based method to quantify proton MR spectra is presented and tested by phantoms. The limitations of the technique such as the signal loss in the extracted metabolite spectrum due to SVD and gradient-induced frequency modulation side bands are discussed and possible solutions to these shortcomings are given.

References

[1] Stockler et al, Pediatr Res.,1994; 36(3):409-413[2] Ozdemir et al, 22 nd ESMRMB,2005,18,222-223

Keywords :magnetic resonance spectroscopy, absolute quantification, SVD

22 – AN ADAPTIVE OFF-LINE RADIATION SCHEME FOR TREATMENT OF PROSTATE CANCER USING CONE-BEAM CT SCANS

Tonnis Nuver, Jasper Nijkamp, Rianne de Jong, Annemarie Lakeman, Monique Smitsmans, Peter Remeijer, Marcel van Herk, and Joos Lebesque

Department of Radiation Oncology, The Netherlands Cancer Institute – Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands.

BACKGROUND – We have developed an adaptive scheme for treatment of prostate cancer based on kV cone-beam CT images that are obtained in the first six treatment days. With this scheme a better estimate of the average prostate position and the average rectum shape are obtained, and the PTV margin can be reduced from 10 to 7 mm.

METHODS – The cone-beam CT scans obtained in the first 6 treatment days are matched on the planning CT scan using the pelvic bones. The obtained translations are used to correct the patient position. A Shrinking-Action-Level protocol is used for this set-up correction. Automatic 3D grey value matching is then used to match the prostates of the cone-beam CT scans to the prostate of the planning CT scan. Each match yields three translations (T_LR, T_CC, T_AP) and three rotations (R_LR, R_CC, R_AP). Mean values for these translations and rotations are calculated and used to move the prostate of the planning CT scan to the average position. For the six cone-beam CT scans the outer rectal wall is delineated. For each CT slice 50 equidistant points are placed on each of the rectum contours. The first of these 50 points is placed at the dorsal side of the rectal wall. Corresponding points can then be distinguished for the rectums of the various scans. The coordinates of corresponding points are averaged to obtain points on the surface of the average rectal wall. At the end of the first treatment week a new IMRT treatment plan is calculated, which is based on the average prostate and rectum, and a PTV margin of 7 mm. During treatment weekly cone-beam CT scans used in the set-up protocol are also used to verify that the prostate is inside the PTV.

RESULTS – So far, 10 prostate cancer patients were successfully treated with our adaptive treatment scheme. The PTV volume was on average 28% smaller than for conventional treatment. For all verification scans the prostate was positioned inside the PTV volume. The average dose received by the rectum reduced by 0.0-21.4%, and the equivalent uniform dose (a=12) reduced by 0.5-4.1%.

CONCLUSIONS – This is the first routine clinical application of soft tissue image guidance for the prostate using kV cone beam CT. It does not only correct for patient set-up error, but organ motion as well, making the planning a more accurate representation of the real treatment. Contrary to adaptive treatment schemes that use implanted markers, our method is non-invasive and allows for calculation of an average rectum.

Keywords : IGRT, prostate, kV cone-beam CT

23 – THREE DIMENSIONAL RADIATION DOSIMETRY IN LUNG-EQUIVALENT REGIONS BY USE OF A RADIATION SENSITIVE GEL FOAM: PRELIMINARY RESULTS

Yves De Deene

Radiotherapy (P7), Ghent University Hospital, De Pintelaan 185, 9000 Gent

A polymer hydrogel foam is proposed as a potential three-dimensional experimental dosimeter for radiation treatment verification in low-density tissue such as the lung. A gel foam is created by heavy mixing a radiation sensitive polymer gel mixture in an anoxic atmosphere. The gel foam sets after the addition of a strong anti-oxidant (tetrakis-hydroxy-phosphonium salt) and during slowly rotating the gel foam mixture. The gel foam dosimeter is based on the dose dependent polymerization of vinyl monomers that are confined in the gel. The degree of polymerization has a measurable effect on proton NMR properties. Optical microscopy of the gel foam dosimeter reveals good similarity with the microstructure of lung tissue. The mass density of the gel foam dosimeter is in the order of 0.25 - 0.35 g/cc.

Both NMR spin-spin relaxation rate (R2) and magnetization transfer ratio (MTR) have been used to visualize the dose distribution of the gel foam dosimeter. It is found that MTR has significant advantages as compared to R2 for mapping the dose distribution. Spin-spin relaxation dispersion has been observed making R2 dependent on the density of the gel dosimeter and on the echo time intervals in a multiple spin-echo sequence.

A method to extract the electron density of the gel dosimeter by use of NMR proton density images is also presented. The post-irradiation stability of the gel foam with respect to the NMR properties has been investigated. The spin-spin relaxation dispersion and NMR diffusion measurements can be used to extract information on the gel foam microstructure in a non-invasive way. Computer models are proposed that simulate the behavior of the water molecules in the gel foam dosimeter describing restricted diffusion and relaxation dispersion that is attributed to the mesoscopic magnetic field inhomogeneities caused by the nitrogen bubbles.

These preliminary results show that the gel foam dosimeter has significant potential in the experimental verification of radiation treatment planning software that can be applied to low-density structures. Research is underway to investigate how the density and microstructure of the gel foam dosimeter can be tuned.

It also demonstrates the possibility of using quantitative NMR to obtain relevant information (such as proton and electron density) that could be used as input to radiation treatment plans.

Keywords : polymer gel dosimetry, low-density dosimeters, quantitative NMR

24 – A NEW COMPUTER CODE MEDICUS FOR DETERMINATION OF CROSS SECTIONS OF NUCLEAR REACTIONS USED FOR THE MEDICAL RADIOISOTOPE PRODUCTION

A. I. Ibishi, and A. Hermanne

Vrije Universiteit Brussel, VUCY, Laarbeeklaan 103, 1090 Brussels, Belgium

The new computer code MEDICUS has been developed in MATLAB 6.5, Mathematica 5, and Lahey Fortran 95 programming languages. Especially are calculated cross sections of nuclear reactions, used for production of medical radioisotopes, that are important in cancer radiotherapy, such as: flourine 18 (18F), phosphorus 32 (32P), cobalt 60 (60Co), copper 67 (67Cu), strontium 89 (89Sr), yttrium 90 (90Y), strontium 92 (92Sr), indium 111 (111In), iodine 125 (125I), iodine 131 (131I), cesium 137 (137Cs), samarium 152 (152Sm), samarium 153 (153Sm), lutetium 177 (177Lu), rhenium 186 (186Re), rhenium 188 (188Re), and iridium 192 (192Ir). The nuclear cluster structure of these elements has been analyzed [1]. Suitable radioisotopes for radioimmunotherapy are investigated [2]. The many-nucleon equations are solved in the framework of the EMBNCM theory, DWBA, Direct Nuclear Cluster Reactions, Pre-equilibrium Reactions, Optical Model, and Exciton Model with Cluster Emission [1]. Excitation energies up to 200 MeV, and incorporate several other improvements in calculation approaches. Our calculations shows dependence between the nuclear cluster structures of radioisotopes such as: 32P, 60Co, 67Cu, 90Y, 92Sr, 131I, 137Cs, and beta radiation energy, chemical stabilities, accumulation properties, radioisotope half-life, and destroyed tumour cells. Possibilities for injection of radioisotopes such as: 67Cu, 90Y, 177Lu, 186Re, and 188Re, with monoclonal antibodies into a cancer patient [2], in vivo behaviour of labeled compounds to the specific organ, and effects of different nuclear cluster structures of radionuclides, to catalyze binding, and destruction of tumours by antibodies, are investigated. Code MEDICUS based on EMBNCM, gives better fit to the data, than actually used codes. We have taken into account reaction mechanisms [1]. Our results are important for the development of the new anti-cancer vaccine.

[1]. A.I.Ibishi, New Method for Calculation of Nuclear Cluster Structure of Nuclei, AIP Conf. Proc. 768, 395, Melville, New York, USA, 2005. http://link.aip.org/link/?APCPCS/768/395/1[2]. Wilder et al, Radioimmunotherapy: Recent results and future directions,J. Clin. Oncol. Vol 14, 1383 - 1400, 1996.

Keywords : MEDICUS, cross-sections, medical radioisotopes.

25 – MINIMIZING THE TIME SPEND ON PATIENT SPECIFIC QA FOR IMRT

Jan Verstraete, Bianca Vanstraelen, Tom Depuydt, Frank Van den Heuvel

UZ Gasthuisberg Leuven

Although IMRT improves the conformity of the treatment, it requires more time for quality assurance (QA). Essentially QA for IMRT has 3 main levels: QA of the treatment planning system, QA of the treatment equipment and patient specific QA. Some centers focus primarily on the first 2 and tend to rely on the stability of there equipment. Limited effort is put in the patient specific QA and some centers no longer execute these checks. At UZ Leuven we are following another approach and kept doing patient specific QA while minimizing the time spend on this procedure. This is necessary with about 100 IMRT patients or around 530 fields per year.Until September 2005 we focused on the use of absolute film dosimetry to perform routine patient specific QA. In this methodology, a hybrid plan is created for every treatment field in a RW3 phantom and the dose is calculated at a depth of 3 cm. The fields are delivered on the treatment machine to the phantom (one film per field) using the actual treatment plan at a zero gantry angle. To rule out differences in film development conditions, all the films are developed at the same time and digitized using a VIDAR 12-bit scanner. With two supplementary films, a specific sensitometric curve per patient is created to convert all the films to absolute dose with in-house build software called IMRTool. This software allows quantitative comparison of 2D dose distributions using the gamma index calculation. The constraints used in our department for gamma index calculations are 3% of the local dose and 3mm for distance with an agreement score of 90%. In less than 2% of the cases the agreement score is lower than 90%. In one case the speed of one leaf was incorrect. For the other cases the agreement score was within the 90% if the QA procedure was repeated.Over the years this procedure was optimized but still it takes about 12 minutes of machine time (with two dosimetrists) and about 80 minutes of processing data to execute the whole procedure. Since June 2005 we can acquire dosimetric electronic portal images. As the dosimetric information is readily available after irradiation no processing time is needed other than a comparison between the measurements with the EPID and a prediction of the EPIDs signal calculated in the TPS. At this moment this measurement is scheduled in a short treatment machine slot of six minutes (and one dosimetrist) and the evaluation process is performed semi-automatically by the IMRTool software.

Keywords : IMRT, QA, patient

26 – ESTIMATING A CONFIDENCE LIMIT FOR THE INTEGRAL EVALUATION OF IMRT TREATMENTS FOR HEAD & NECK TUMOURS.

B. Schaeken (1,2), B. De Ost(1), D. Van Gestel(1), D. Van den Weyngaert(1).

(1) Department of Radiotherapy, ZNA- Middelheim, Antwerp, Belgium

(2) Nuclear Technological Centre NuTeC, Xios Hogeschool Limburg, Hasselt.

MATERIAL AND METHODS – The alanine dosimeters consist of finely dispersed L-a-alanine crystals (75%w) suspended uniformly in a matrix of cellulose (17%w) and additives to facilitate the process of direct tabletting using an eccentric press. The dosimeters are read out with an X-band desktop EPR spectrometer (Bruker, EMS104) with a cavity operating in the TM110 mode. The dosimeters have a diameter of 4.9 mm to fit within the quartz holder of the resonant cavity and are 1 mm to 5 mm long (mass density of 1302 kg/m3 ± 0.5% (k=1)). Dose is measured as the peak to peak height of the central line in the absorption spectrum. The alanine detectors show favorable dosimetric properties: linear dose response, no energy or dose rate dependence, no fading and tissue equivalence. The combined uncertainty is 2.2% (k=1) @ 6 Gy and rapidly decreases with increasing dose. IMRT treatments are checked with the “copy to phantom” methodology.

RESULTS – 225 alanine measurements were performed for 26 patients which received IMRT treatment for head & neck tumours. The calculated- to measured dose was 1.016 ( s =4.0%). If we restrict data to patients treated with less than 50 beam segments, Dcal/Dmeas = 1.008 ( s =3.6%). To study the influence of daily setup errors to the effectively delivered dose, the results of a 1 time 20 Gy set up was compared to a 10 times 2 Gy set-up for the same treatment. For the latter, our treatment team was installing the phantom each day on the treatment table, as they would do in clinical routine, to administer 2Gy each session. In this way a total dose of 20 Gy was accumulated in the detectors. The ratio of the calculated dose to the measured dose was 0.991 (N=29; sd =5.1%) versus 1.011 ( sd =2.8%) for the 1 x 20 Gy and 10 x 2 Gy respectively. In the 1 x 20 Gy set up, 4 measuring points were found outside the 5% tolerance level, for the 10 x 2 Gy experiment this was only the case in 2 measuring points. Verification of the delivered dose in the treatment of whole breast cancer shows that Dcal/Dmeas = 1.00 (N=121; sd =0.03). For the treatment of brain tumours: Dcal/Dmeas = 1.01 (N=41; sd =0.015).

CONCLUSION – Alanine/ESR dosimetry has proven to be a reliable technique that facilitated the introduction of complex treatment modalities at our department. Numerous dose measurements allowed us to establish a confidence limit for the whole treatment in function of the degree of modulation (complexity).

Keywords : confidence limit, IMRT, alanine dosimetry

27 – MONIQA, MONITOR QUALITY ASSURANCE: VALIDATION AND INITIAL RESULTS OF A DYNAMIC PATTERN FOR DAILY QUALITY CONTROL OF MEDICAL SCREEN DEVICES

Jurgen Jacobs, Tom Deprez, Frank Rogge, Hilde Bosmans

UZ Leuven, GasthuisbergLUCMFR

PURPOSE – We have developed a complete environment (MoniQA) for quality control of all types of radiological screen devices, including specific viewing stations for digital mammography. The European Protocol for quality control of digital mammography systems (EUREF) prescribes tests for viewing stations, including daily tests. Because these proposed tests have limitations (static patterns, learning effect, difficult to use), we decided to develop a new dedicated pattern for daily quality control. We report on the practical workout and on the first validation of this pattern against the patterns of well-known protocols like the ones proposed by the AAPMtg18 and the DIN.

METHODS AND MATERIALS – MoniQA was developed in JAVA. Besides a main application to show patterns and to collect user data, it also consists out of a framework to simplify the creation of new patterns. We implemented all patterns needed by the AAPMtg18 protocol, by the DIN protocol and also a new dedicated pattern which we call the MoniQA pattern. This pattern contains all elements necessary to perform a full daily quality control: luminance, resolution, geometric and general image quality checks. This is a dynamic pattern. Every time another pattern is created according certain rules. This makes sure that there is no learning effect for the observer. We compared the results of the different elements in the protocols and the needed time. Four observers (both expert and novice) performed the three already mentioned protocols on twelve different monitors. We included high end, 5MP monitors for mammography as well as screen devices for general radiology in our test. The observations were made under clinical conditions.

RESULTS – We tested our software tool in a medical environment and it proved to be stable. The comparison of the different results indicated that our pattern is able to show at least the same possible malfunctions. In overall we saw that an evaluation of the MoniQA pattern only took 50% of the time of the AAPMtg18 protocol and resulted in a time gain of 15% compared with the DIN protocol.

CONCLUSION – Our preliminary results demonstrate that with our new pattern we can easily and quickly monitor the quality of medical screens on a daily basis. The results of our tests show that our approach gives at least the same results as the well-known protocols. Combined with our software environment, we implemented a powerful framework to perform the quality control of screen devices.

Keywords : monitor quality assurance

28 – DTI OF NORMAL APPEARING SPINAL CORD IN ELDERLY

Van Hecke W, Leemans A, Sijbers J, Parizel PM, Van Goethem JWM

University Hospital Antwerp

PURPOSE – Like the brain, the spinal cord contains many sensory and motor pathways that can be visualized with diffusion tensor imaging (DTI). The DTI technique measures the self-diffusion, or random thermal motion, of the endogenous water in nerve tissue. Our objective was to evaluate quantitative DTI values of the spinal cord in an older patient population with normal appearing spinal cord and without spinal cord symptoms.

METHODS – Diffusion tensor measurements of the cervical spinal cord were performed on 17 patients referred for degenerative spinal disease. All patients had a normal appearing spinal cord on MRI and none had clinical spinal cord symptoms. The mean patient age was 50.3 ± 10.8 years (min – max years). Acquisition parameters were as follows: TR: 10.4 s; TE: 100 ms; diffusion gradient: 40 mT/m; number of slices: 60; b: 700 s/mm; TA: 12” 18’; diffusion measurements in 60 directions. The cervical spinal cord was selected by manual delineation of an ROI on all axial DTI slices. The data were analyzed using a propriety software tool “ExploreDTI” developed at our institution. Fiber tracking was performed using different values of fractional anisotropy (FA) for seed point selection and also to end fiber tracking. The mean FA and mean diffusivity (MD) were calculated for the entire cervical spinal cord. Statistical analysis was performed using a Mann-Whitney U-Test.

RESULTS – A mean FA of 0.572 ± 0.024 was found with parameter settings of an FA for seed point selection of 0.7 and an FA to stop tracking of 0.3. Under the same conditions a mean MD of (1.08 ± 0.08) 10-3 mm2/s was found.

CONCLUSION – FA values of the cervical spinal cord in our patient population are lower than other values published in the literature. We postulate that this might be (in part) related to patient age. Studies in the brain showed a decrease of the FA in older people [1]. In the future we will enlarge the control group and incorporate younger people to investigate the exact relationship between the age and the quantitative DTI parameters. In addition the influence of multipler sclerosis (MS) on the quantitative DTI parameters of the cervical spinal cord will be studied.

[1] Salat DH, Tuch DS, Greve DN, van der Kouwe AJ, Hevelone ND, Zaleta AK, Rosen BR, Fischl B, Corkin S, Rosas HD, Dale AM: Age-related alterations in white matter microstructure measured by diffusion tensor imaging; Neurobiol Aging. 2005 Aug-Sep;26(8):1215-27. Epub 2004 Dec 19.

Keywords : diffusion tensor imaging, spinal cord, quantitative measures

29 - CHARACTERIZATION OF THE CR SYSTEMS.

Joris Nens

VUB

GOAL – To characterize the response of computed radiography (CR) systems for different beam qualities.

MATERIALS AND METHODS – Two CR-systems of Kodak (CR400 and CR800) are tested on 3 X-ray tables. On all tables, the response curves have been made. To see the dependence of the response to the energy spectrum, the IP has been exposed with the same dose for different kV and filter conditions. For both CR-systems the differences between the cassettes has validated.

RESULTS AND DISCUSSION – For the response curves, differences were measure of 1.6% for the slope and 0.4% for the offset, for the CR800 exposed with different tubes but read on the same CR-reader. The response varies 12% when the kV is changing in a range of 40 to 100 kV and about 5 to 6% when changing for no filter up to 6 mm Al. The differences in response between the different cassettes are 0.84% for the same type and 1.52% for all the cassettes together. These tests are the first stage in the development of an automatic quality control.

Keywords : CR, quality control

30 – FORENSIC MEDICAL RADIATION PHYSICS IN BELGIUM.

P. Vander Henst (1,2,3), H. Bosmans (1,2), G. Marchal (1), T. Harding(3)

(1) Department of Radiology - UZ Gasthuisberg

(2) LUCMFR

(3) Institute for Forensic Medicine – University of Geneva

PURPOSE – Notwithstanding the fact that “forensic science” speaks to everyone’s imagination, it still seems to be far away from the daily activities of the medical radiation physicists. A theoretical analysis should give an answer to the question whether medical radiation physics could be “highlighted” as a needed speciality within the forensic science.

METHODS AND MATERIAL

Theoretical analysis in relation with daily routines

Although the ED97/43 entitle him as an expert in radiation physics and technology applied for medical exposure, the medical radiation physicists do not appear to consider the fact that their well defined responsibilities could make them face liability charges.

in relation with the first mandatory appointments by courtsThe first mandatory appointments in Belgium indeed stated the in theorem. for 5 cases by courts.

Peer Review Method The peer review method has been used to compare medical radiation physics

as a forensic science to other closely related forensic sciences and their methods.

RESULTS – The theoretical outcome of the previous methods has been reassessed against national and international publications, and was put into the legal, socio-economic and ethical reality. It revealed a possible direct involvement at different levels of the medical radiation physicist in forensic science: - in a legal proceeding - in an expertise - the routinely given expert informationThe results from the peer review revealed that indeed medical radiation physics was recognised as an important part in multidisciplinary medico-legal forensic cases.

CONCLUSION – The result of the study got directly confirmed in daily situations

Keywords : forensic radiation science

31 – COMPARING DIFFERENT COLLIMATION TECHNIQUES (M3,120 MLC,CIRCULAR ARC) IN STATIC AND DYNAMIC ARC THERAPIE FOR INTRACRANIAL STS

Joeri Alberty, G. Pittomvils, Y. Geussens, P. Herregodts, H. Verstraete, R. Weytjens, A. Sprangers, E. Bossuyt, N. Mertens, R. Reymen , P. Huget.

AZ Sint-Augustinus GVA Wilrijk

INTRODUCTION – Since 1996 more then 400 patients are treated at our department with Stereotactic surgery. Since 2003, after a Varian Clinac 2100 EX with 120 MLC was installed and commissioned for circular arc and multileaf static arc. From that moment on, stereotactic surgery is done with both methods.

PURPOSE – Comparing 5 different collimation techniques using Circular Cones in static arc mode and the Brainlab M3 (3-5 mm) and Varian 120 MLC (5 mm) multileaf collimator in static and dynamic arc mode.

METHODS AND MATERIALS – The patient data set is divided on pathology , localization , shape and size. Each patient is retrospectively replanned using circular arc, conformal arc and dynamic arc treatment technique using the Varian 120 MLC and the Brainlab micro-MLC using the BrainSCAN 5.31 planning software. The same fixed, non coplanar beams are used for all plans (Isocenter , number of arcs , start and stop position). The prescribed dose (80 % of the maximum dose) on the CTV and the maximum tolerated doses on the organs at risk are identical as well. The isodose distribution and dose-volume histograms (DVH) are computed and plans are compared in terms of conformity, Dose received to the CTV , organs at risk and the surrounding tissue.

RESULTS – It is feasible to plan with different conformal techniques and choose the best option for individual treatment. Changing a plan from circular cone to conformal arc is time consuming due to software limitations. This makes it necessary to evaluate the size , shape and OAR before planning and choose the best option.

RESULTS INDICATE THE FOLLOWING OUTCOME – The minimum target dose differs significantly where the M3 will be the best, the 120 MLC second, the circular arc technique third due its circular shape. The total brain volume receiving 50 % of the maximum dose is smallest for the circular cones, the largest for the 120 MLC.The conformity index of the M3 treatments will be the best for the M3 dynamic treatments and the 120 MLC treatments will have e better CI then the circular arc treatments.

CONCLUSION – We are convinced that at the end of the study, we will be able to give an answer on the question of the importance of leaf width and treatment mode of MLC against the previous standard. A cutoff value when to use which technique will be found. This should help any reader the determine himself the combination that is the most suited for its centre.

Keywords : stereotactic intracranial treatment, collimation techniques, static/dynamic arcs.

32 – INTENSITY MODULATED PHOTON (IMXT) AND PROTON (IMPT) THERAPY FOR THE TREATMENT OF HEAD AND NECK TUMORS.

Steneker MNJ (1,2); Lomax AJ (1); Schneider U (3);

(1) Dept. of Radiation Medicine, Paul Scherrer Institute, CH

(2) University of Groningen, Groningen, NL

(3) Triemlispital, Zürich, CH

OBJECTIVE – A comparative treatment planning study has been preformed between intensity modulated photon (IMXT) and intenstity modulated proton (IMPT) therapy to investigate the ability of both modalities to spare critical organs in the head and neck region while keeping target dose homogeneous. Additional advantages of variable energy IMPT was also investigated. The treatment planning comparison was extended by varying the number of fields in order to study its effect on the performance of each modality, and risks of secondary cancer induction were also calculated for all plans.

MATERIALS AND METHODS – Five planning CT's were selected for the study. Four different constraints were set to the organs at risk, in order to measure the resulting homogeneity of dose in the target volume. 5 and 9 field plans were made for IMXT and 3, 5 and 9 field plans were made for IMPT and variable energy IMPT. Dose homogeneity as a function of the constraints were visualized using a 'pseudo' Pareto-optimal front approach. Risks of secondary cancer were estimated using the organ equivalent dose model.

RESULTS – Critical organs were best spared using 3-field IMPT and, at least for IMPT, little advantage was seen with increasing field numbers. In contrast, there was a significant advantage in going from 5 to 9 fields for IMXT. Secondary cancer risk was lowest for the IMPT variable energy plans, in which normal tissue received the least integral dose. Interestingly, although integral dose remained the same, increasing the number of IMPT fields increased the secondary cancer risk, due to the increased volume of tissue irradiated to low dose.

CONCLUSION – IMPT has a better ability to spare critical organs than IMXT for the same dose homogeneity. It also significantly reduced the predicted risk of secondary tumour induction, and the use of small numbers of fields further increased this advantage. Given that target homogeneity and normal tissue sparing were equally good with the 3 field IMPT, there appears a clear rationale to deliver small numbers of fields for IMPT.

Keywords : IMRT, protontherapy, secondary cancer

33 – THE MEDICAL RADIATION PHYSICIST TAKING PART IN MEDICOLEGAL PROCEDURES: THE FIRST BELGIAN TENDENCIES AFTER ONE YEAR EXPERIENCE.

P. Vander Henst (1,2,3), H. Bosmans (1,2), G. Marchal (1), T. Harding(3)

(1) Department of Radiology - UZ Gasthuisberg

(2) LUCMFR

(3) Institute for Forensic Medicine – University of Geneva

PURPOSE – The need for qualified radiation experts in medico-legal forensic procedures is high; however the interest and the awareness of finding themselves involved are still low. Being a forensic radiation physicist sounds attractive, but there is a whole world of interesting topics beyond this imaginative idea. Lot’s of victims are surviving an incident, and are in need for qualified persons to evaluate their sustained human damage and to pinpoint the responsible.Wanted: qualified expert! As the past year, 21 cases had been appointed to them by the Belgian courts. They faced quite a lot of questions, as no standard rules existed. The purpose of this study was to highlight a standard procedure.

METHOD & MATERIALSTwo tendencies could be differentiated. The first trend is the so called ‘pure’ medico-legal proceedings. The second one could be categorised into:

claims submitted to the Council of State general radiation protection topics legal entities lodging appeal.

The framework for standard procedures will be set-up together with international experts in other forensic disciplines. Called: multidisciplinary approach!

RESULTS – Adapting to the national context made the creation of the standard procedure complicated. It describes the ways of how an expert could get involved and the expectations towards them. Notwithstanding one general rule came directly out of the study:

NEVER exceed your competence! WATCH out for competence conflicts! NEVER be a judge in your own cause!

CONCLUSION – The standard procedure reduced the threshold for the medical radiation physicist to take up a role in proceedings.

Keywords : forensic radiation science

34 – PHASED ATTENUATION CORRECTION IN RESPIRATION CORRELATED CT/PET

C.C.A. Nagel (1), G. Bosmans (1), A.L.A.J. Dekker (1), M.C. Öllers (1), D.K.M. De Ruysscher (1), P. Lambin (1), A.W.H. Minken (1), N. Lang (2), K.P. Schäfers (2)

(1) Department of Radiation Oncology (MAASTRO), University Hospital Maastricht, Maastricht, The Netherlands

(2) Department of Nuclear Medicine, University Hospital Muenster, Muenster, Germany

PURPOSE – Motion of lung tumors with respiration causes difficulties in the imaging with CT and PET. Since accurate knowledge of the position of the tumor and the surrounding tissues is needed for radiotherapy planning it is important to improve CT/PET image acquisition. The purpose of this study was to evaluate the potential to improve image acquisition using phased attenuation correction in respiration correlated CT/PET, where data of both modalities was binned retrospectively.

METHODS AND MATERIALS – Respiration correlated scans were made on a Siemens Biograph Sensation 16 CT/PET scanner which was modified to make a low pitch CT scan and list-mode PET scan possible. A lollipop phantom was used. The sphere with a diameter of 3.1 cm was filled with approximately 20 MBq 18F-FDG. Three longitudinal movement amplitudes were tested: 2.5, 3.9 and 4.8 cm. After collection of the raw CT data, list-mode PET data, and the respiratory signal CT/PET images were binned to ten phases with help of in house built software. Each PET phase was corrected for attenuation with CT data of the corresponding phase. For comparison, the attenuation correction was also performed with non-respiration correlated (non-RC) CT data. The volume of the sphere and the amplitude of the movement were calculated for every phase of both the CT and PET data (with phased attenuation correction). Maximum and average activity concentrations were compared between the phased and non-phased attenuation corrected PET.

RESULTS – With a standard, non-RC CT/PET scan, the volume was underestimated by as much as 46% in CT and the PET volume was overestimated to 370%. The volumes found with RC-CT/PET scanning had average deviations of 1.9% (± 4.8%) and 1.5% (± 3.4%) from the actual volume, for the CT and PET volumes respectively. Evaluation of the maximum activity concentration showed a clear displacement in the images with non-RC attenuation correction, and activity values were on average 14% (± 12%) lower than with phased attenuation correction. The standard deviation of the maximum activity values found in the different phases was a factor 10 smaller when phased attenuation correction was applied.

CONCLUSION – In this phantom study we have shown that a combination of respiration correlated CT/PET scanning with application of phased attenuation correction can improve the imaging of moving objects and can lead to improved volume estimation and a more accurate localization and quantification of the activity.

Keywords : respiration correlation, attenuation correction, CT/PET

35 – IMAGE QUALITY IMPROVEMENTS FOR WHOLE BODY PET WITH TIME-OF-FLIGHT

Stefaan Vandenberghe

Ghent University, ELIS/MEDISIP, Sint-Pietersnieuwstraat 41, B-9000 Ghent, Belgium

Recently there has been a renewed interest and development in Time-of-Flight (TOF) PET based on fast scintillators with a sufficiently high stopping power and high light output like LSO and LaBr3. The measured time difference in 3D TOF PET makes it possible to improve the signal-to-noise ratio of reconstructed images. The improvement in signal-to-noise ratio can be used to reduce imaging time or to improve the quality of scans of eg. obese patients. It is well known that the reduction in imaging time is proportional to the ratio of D/dx with the D the diameter of the object and dx the Full Width at Half Maximum (FWHM) of the TOF resolution. Expected reductions in imaging time for a 30 cm diameter object are 3.3 for 600 ps timing resolution and about 6.6 for 300 ps timing resolution. Results will be presented from a full ring LaBr3 based TOF PET scanner currently under development at the University of Pennsylvania.

Reconstruction algorithms have to be modified to take the timing resolution into account. Forward and back projection is still done along the same lines as in conventional PET but all points along the line are now weighted with a different probability. The maximum probability is given to the point determined by the measured TOF difference and the width of the probability distribution is determined by the expected accuracy of the time difference measurement. Listmode reconstruction seems to be the most obvious choice for TOF-PET, but will result in relatively long reconstruction times. Therefore we have developed a rebinning and mashing algorithm to transform the data to a limited number of 2D datasets. It is shown that images can be reconstructed from less angles when timing information improves. Using these rebinning and mashing algorithms allows the use of fast 2D reconstruction algorithms.

In iterative reconstruction convergence is non linear and depends on the local environment and the object size. To obtain uniform convergence in PET studies one method is to iterate very long and postfilter the images. It is shown with some nice examples that this non-linear behavior improves when TOF information is used. In general convergence improves with better timing resolution and it becomes less dependant on the local environment and the object size. Therefore it can be expected that TOF PET will also help in quantification and diagnosis of iterative reconstructions.

36 – THE “TIBIT”: PRODUCT DESIGN OF A PATIENT POSITIONING SYSTEM FOR TOTAL BODY IRRADIATION

S. Van Boeckel (1), J. Vanregemorter (2), Ch. Baelus (1)

(1) Hoger Instituut voor Integrale Product Ontwikkeling, Hogeschool Antwerpen.

(2) ZNA Middelheim hospital, Department of Radiation Oncology

PURPOSE – This abstract summarises a thesis made in 2005 at the Department of Consumer Goods. The goal was to design a positioning aid for fractionated total body irradiation including transmission blocks for lung shielding and the possibility to make megavolt images using (CR) cassettes.

PRODUCT IDEA – The main drivers for the design were: time reduction, reduction of effort and patient comfort. The main developed items are: A] reproducible, reliable and comfortable body support with fixation of the thoracic region, B] reproducible and reliable shielding of the lungs. Secondary items: system storage and mobility, access to the patient, communication with the patient and connectivity. Extra items: dimensions of the irradiated area, patient position, PMMA radiation diffuser, no radio-opaque material between source, patient and cassette, recognition of patient dependant parameter settings (eg fixation, shielding), patient friendly and easy to clean.

SYSTEM DESIGN – Various existing methodologies were studied and tested. A cost analysis was made as well as a market study.

PRODUCT CONCEPT – The unit consists of a rotating stand on a fixed base plate. The front of the unit supports the PMMA radiation diffuser and an adjustable block system. The patient rests on a reclining surface in carbon to which panels holding the thorax fixation, bottom and head rests are connected. The panels fit in a unique way to the surface. The back of the unit can hold a standard cassette at a selectable height. A number of switches interlocks to the linac to prevent erroneous irradiation. The patient has audio-visual communication with the control room and the progress of the treatment can be followed by the patient on a display in the unit. All cables are integrated in the frame of the unit including connections for in-vivo diodes.The base plate locks into position and is not moved during treatment. For APPA treatment, the stand is rotated maintaining the patient position in the stand, the diffuser, block holder and cassette holder remain steady. The Tibbit unit can be stowed away when not in use and fits through a standard size door.

CONCLUSION – The design tackles most problems that are encountered with Total Body Irradiation. The use of the Tibbit system would add to the quality of the patient’s treatment and reduce handling time by the nursing staff. The poster shows the design with detailed views of the various parts.

37 – SUPER ABSTRACT VAN MARC COGHE (WIP)

Marc en zonen

UZ Gent

xyz

Keywords : xyz

38 – REPEATED (RC)CT IMAGING DURING A COURSE OF RADIOTHERAPY FOR LOCALLY ADVANCED NSCLC PATIENTS.

G. Bosmans, A. van Baardwijk, A. Dekker, M. Öllers, L. Boersma, P. Lambin, A. Minken, D. De Ruysscher

Department of Radiation Oncology (MAASTRO), GROW, University Hospital Maastricht, Maastricht, The Netherlands

PURPOSE – Radiation treatment planning is mostly based on one single CT scan. A respiration correlated (RC) CT scan can improve the imaging of lung cancer patients. The aim of this study was to investigate the changes in tumor volume and tumor mobility during a course of radiotherapy for locally advanced non small cell lung cancer patients and whether repeated imaging is necessary to adapt the treatment plan.

METHODS AND MATERIALS – Images were acquired on a modified Siemens Biograph 16 CT/PET scanner. Twenty-five patients got a conventional CT prior to treatment and one and two weeks after the start of the treatment. For thirteen patients also an RCCT was acquired at the three timepoints. Tumor volumes were drawn for each patient by the same physician, to avoid interobserver variability. Tumor movements were measured in all three orthogonal direction using in-house developed Matlab software. Patients were treated twice a day with 1.8 Gy to ensure a short overall treatment time.

RESULTS – On average the 3D vector of the tumor movement prior to treatment was 7.1 mm ± 4.5 mm wich changed to 7.0 mm ± 5.9 mm and 7.7 mm ± 5.5 mm one and two weeks after the start of the treatment respectively. Breathing frequencies were similar at all three timepoints because the RCCT acquisition was done after the CT/PET scan, so the patients lied down during approximately 2 hours and were completely relaxed. Tumor volumes on CT increased during radiotherapy treatment. The CT volume, relative to the volume prior to treatment increased to about 125% and 107%, one and two weeks after the start of the treatment. This is probably due to the fact that inflammation and atelectases occurs, since after one week already approximately a radiation dose of 20 Gy is given.

CONCLUSION – Tumor mobility did not change during a course of radiotherapy in this patient population, so repeated RCCT is not necessary during treatment. But the tumor volume on CT did change during the treatment, more (biological and molecular) information is needed to investigate whether this is real tumor progression or radiation reaction.

Keywords : respiration correlation, repeated imaging, lung cancer

39 – BREATHING CURVE PRODUCED BY MAGNETIC SENSORS

Michel Destiné (1), Vincent Remouchamps (1), Pierre Ansay (2), Bernard Beckers (2), Dominique P. Huyskens (3), Emile Salamon (1)

(1) Radiotherapy Department, Namur, Belgium

(2) Nomics, Liège, Belgium

(3) 7Sigma, Leuven, Belgium

PURPOSE/OBJECTIVE – To describe the development and the testing of a breathing curve produced by a magnetic distance meter. The aim is to develop a simple, patient friendly, easy to use and not expensive system that will be used to monitor voluntary breath hold.

MATERIALS/METHODS – A magnetic distance meter (Nomics, Belgium) is used to obtain the distance between two resonant circuits with an accuracy below 0.2 mm. This distance meter uses the property of a resonant circuit to excite another circuit, matched on the same frequency through their mutual inductance. The use of resonant circuits instead of stand-alone inductances greatly improves both the performance of the excitation circuit and the sensitivity of the sensor. Resonant circuits are matched to a frequency of the order of 8 kHz and are protected in a 8 x 8 x 27 mm plastic cap. The emission inductance is inserted in a series resonant circuit stimulated at a cadence of 20 Hz. The signal thus consists of a succession of separated, damped sinus waves. The reception circuit restores the signal of the sinus pulses observed on the parallel resonant sensing circuit.

RESULTS – The first resonant circuit is taped on the lateral side of the thorax and the second resonant circuit is taped on the immobilization device or on the treatment table. The baseline distance is typically around 10 cm. The sensors alone were submitted to direct and scattered 6 and 18 MV photons to quantify signal changes by the electro-magnetic field induced by the Varian 2100. During free breathing, the distance variation is in the range of 3 to 4 mm, producing a meaningful breathing curve. Deep inspiration breath hold produces a larger distance difference, 1 to 1.5 cm. Results of the preliminary 1-D analysis correlating the breathing curve and the set up errors on movie portal images will be updated.

CONCLUSIONS – A breathing curve using thoracic magnetic sensors is successfully produced with a user-friendly system. In terms of quality control, the system has the potential to monitor the irradiation window during breath hold treatment. The 3-D reproducibility will be tested in a future protocol.

Keywords : magnetic sensors; voluntary breath hold; breathing curve

40 – RETROSPECTIVE DOSIMETRIC ANALYSIS OF PDR TREATMENT PLANNING IN CERVIX BRACHYTHERAPY BASED ON 3D MR IMAGING

Marisol De Brabandere, An Nulens, Amr Mousa, Erik Van Limbergen

University Hospital Gasthuisberg

In the Radiation Therapy Department of Leuven, PDR treatment planning of cervix cancer brachytherapy is traditionally based on X-ray films. The dose is prescribed to point A; the total prescription dose is as high as possible, limited by the tolerance doses and dose rates to the organs at risk. These tolerance doses are assessed by anatomical points (bladder and rectal points) that are defined and digitized on the films as recommended in ICRU38.

Since 2002, 3D MR images are acquired with the applicator in place in addition to the X-ray films. Before basing our planning fully on this new 3D information, we included a transition period during which we continued to perform treatment planning according to our film approach. The 3D anatomical information was used solely for modest modifications in source and dwell time setup in case of clear underdosage of the target region or overdosage of a critical organ. The aim of this study is to evaluate retrospectively the dose delivered to the target and the critical organs as projected on MRI.

17 consecutive patients with cervical cancer (stage I to IV) treated with PDR during the transition period are selected. Besides bladder, rectum and sigmoid, three target volumes, i.e. GTV, CTV Intermediate Risk (CTV-IR) and CTV High Risk (CTV-HR), are delineated on Plato (Nucletron) following the GYN GEG-ESTRO WG recommendations [1].

Correlations are investigated between the traditional dose prescription at point A and target coverage. The latter is quantified with the dose volume parameters D90, D100 and V100 as proposed by the GYN GEC ESTRO WG, as well as V(60GyEQD2) and V(85GyEQD2) [2]. V(60GyEQD2) plays a role in the evaluation of CTV-IR as an equivalent for the more general 60 Gy reference volume defined for LDR and V(85GyEQD2) reports on a dose which represents more closely the prescription dose to CTV-HR. Total doses of EBT and BT are considered in terms of biologically equivalent dose (EQD2 indicates the isoeffective equivalence to 2Gy/fraction). For evaluation of the OAR, the dose volume parameters D0.1cc, D1cc, D2cc, D5cc are compared to the bladder and rectal ICRU reference points.

This analysis helps us to interpret 3D image based treatment planning and to further optimize the dose distribution in terms of target coverage and dose reduction to the organs at risk. [1] Haie-Meder et al. R&O, 2005;74:235-245[2] Pötter et al. Submitted to R&O, 2005

Keywords : cervix, brachytherapy, 3D planning

41 – COPY TO PHANTOM: A TOOL AND A TOY?

B. De Ost, B. Schaeken, D. Van Gestel, A. Coelmont, D. Van den Weyngaert

ZNA Middelheim, Antwerp, Belgium.

With the ‘copy to phantom’ (CTP) procedure, the beam set-up of an individual patient is copied to a phantom. The dose must be recalculated for the new data set; dosemeasurements can be performed in the phantom and compared with calculations. We use this procedure in routine for all invers IMRT patients (Schaeken et al., submitted for publication in R&O). We found this to be a very powerful tool and started to use it for other purposes as well:

1. Comparison of the alanine measured dose and the Philips (Pinnacle3) calculated dose for a conformal 7-field treatment in a sinus carcinoma (Claus et al., 2001). Despite the large air gaps and the complicated beam set-up, the measurements proved that the calculations are reliable: for a total of 14 alanine measurements the DD (dose difference, ratio calculated and measured dose) is 1.00, 1SD=2.2%.

2. Confirmation of the dose in the match plane of adjacent fields in head and neck patients. A mono isocentric treatment of 2 lateral and 1 supraclavicular beam was copied to the Alderson phantom, 54 TLD’s were positioned in the matching zone, the mean DD was 1.00 (1SD=3.9%). For a ‘classic’ 3 point SSD set-up, with the table rotated to align the supraclavicular field with 2 opposing lateral fields the mean DD in the matching zone is 0.98 (N=30, 1SD=3.8%).

3. Copying an IMRT beam set-up to a new CT scan: during IMRT treatment the anatomy of a head and neck patient changed so much that the radiation oncologist decided for a new CT. To avoid starting the planning procedure all over again, we copied the regions of interest to the new CT data set, saved this as a phantom and copied the original beam set-up to this phantom. Finally we evaluated the dose for the new clinical situation.

4. A forward planning is performed for the irradiation of the breast. Segments are manually created covering the hot spots. In case adjacent lymph nodes must be irradiated, the predicted dose scatter contribution of these fields to the mamma must be correct if we want to compensate for this with a segment. A patient set-up was copied to a rectangular phantom and doses were measured with TLD on different distances and depths. The planning system overestimates the dose, differences become larger with distance from the Co-60 field border, mean ratio predicted dose, measured dose was 1.59 (1SD=79%). For 6MV beams the scatter contribution is calculated correctly: mean ratio predicted and measured dose was 1.00 (1SD=16%).

Keywords : quality assurance, copy to phantom.

42 – IVD IN TBI: PATIENTDOSE, LUNGDOSE AND REPRODUCIBILITY OF THE TREATMENT.

B. De Ost, A. Coelmont, P. Meijnders, J. Vanregemorter, D. Van den Weyngaert

ZNA Middelheim, Antwerp, Belgium.

Total body irradiation (TBI) for ZNA (Ziekenhuis Netwerk Antwerpen) was clinically started in October 2004. Until today 14 patients are treated with a schedule of 6 times 2 Gy in 72 hours using 18MV photons. Transmission blocks are used to reduce the lung dose to 9 Gy. Quality control of the treatment is done with in vivo dosimetry (IVD). A total of 836 measurements were performed in 84 treatments set-ups.Two EDP20 diodes (Scanditronix) are calibrated for the Source Skin Distance and field size used in TBI. The diodes are positioned on the central axis (cax) of the treatment field and under the lung block. In this way the entrance dose and the lung dose is controlled. The measurements are preformed for the anterior and posterior field separately. After calibration of the TLD’s (Thermo 1/8” sq. chips; Harshaw 5500) in treatment conditions the detectors are positioned on the same spots as the diodes; extra TLD’s are placed on the head, sternum and leg. The TLD’s are packed in a plastic bag. Home made water equivalent build-up caps of 10 mm thickness are used for a selected number of patients. Most stable results are found with diodes: the mean entrance dose measured is 1.57 Gy (1SD=3.5%, N=98); mean entrance lung dose is 1.04 Gy (1SD=4.1%, N=98). The standard deviation for TLD measurements with and without build-up is tripled in comparison with diodes. Using no build-up material for the TLD measurements the dose on the lungs is overestimated because of the scatter contribution of the lung blocks. The scatter effect of the lung blocks to the dose in the build up region is analyzed on this moment to declare the differences in value. The mean ratio of the lung dose and the cax dose with diodes is 0.66 (1SD=6.8%, N=133), for TLD without build-up 0.83 (1SD=6.4%, N=83) and TLD with 1cm build-up 0.74 (1SD=6.5%, N=25). Measurements in a rectangular phantom and in the Alderson phantom result in a 75% lung dose in comparison with the dose on the central axis. Reproducibility of the treatment can be traced by interfractional SD of the measurements for 1 patient in one particular measuring spot. The mean SD for these measurements is 6%; in 16 cases the SD is found outside 10% on a total of 129 measuring series. In vivo dosimetry in total body irradiation demands a special calibration set-up. Reproducibility of the treatment is proved, but more details on the scatter contribution of the lung block are needed to analyze the entrance dose measurements correctly.

Keywords : total body irradiation, in vivo dosimetry, entrance dose measurements.

43 – A QUALITY ASSURANCE SOFTWARE NETWORK FOR RADIOLOGY

T Deprez, J Jacobs, G Marchal, H Bosmans

UZ Leuven - Radiology – LUCMFR

PURPOSE – Automated data collection and customizable data analysis for any physical and/or technical quality controls for decentralised radiological centres.

METHOD AND MATERIALS – Two applications QASPR (Quality ASsurance for Physics in Radiology) and QAMPR (Quality Assurance Management for Physics in Radiology) were developed for the need of a global automated QA management. They have been developed in Delphi and Java, respectively. Both applications are set-up in a modular way, to address customization and extension easily. QASPR, used at satellite centres, is the first line providing quality control feedback. It deals with the collection of data from different sources for all performed tests, provides an analysis and communicates with QAMPR in a locally preferred exchange protocol. QAMPR manages the data from all centres and provides more extended means for analyses by our physicists. In addition it handles reporting and functions as the equipment information hub.

RESULTS – We have tested the software tools and in particular their modular structure for several applications in practice: the use of different phantoms, different quality controls, different densitometers, different exchange protocols, and the application of different formulas to calculate the parameters to be supervised in different countries.

CONCLUSION – The software showed to be very stable and easily adjustable to particular applications, such as the different QA mammography phantoms used at our centres, the parameters of the European guidelines for mammography screening versus German DIN norms, data collection from different equipment and other sources like our own software tools concerning QA of viewing stations and digital mammography.

Keywords : QA, software, automation

44 – AN AUTOMATIC PLATFORM TO DETECT ANOMALIES IN TREATMENT PLANNING: OLEANDER, THE GUARDIAN

Séverine Cucchiaro (1), Arnaud Ligot (1), Michel Destiné (1) , Dominique P. Huyskens (2)

(1) Department of Radiotherapy, Namur, Belgium

(2) 7Sigma, Leuven, Belgium

PURPOSE – To develop a platform that detects inconsistencies in treatment plans and sends a warning to the physicist or dosimetrist. These inconsistencies may either be dosimetric errors or erroneous parameter values leading to problems in the further treatment process.

MATERIAL/METHODS – The software program Oleander, written in C++, accesses directly the database of Vision Generation 7 (Varian Medical System) of the Radiotherapy Department of Namur. The program consists of a cohort of tests meant to detect anomalies in the treatment planning.

The following categories of tests are available:

tests on treatment prescription: i.e. total dose, dose per fraction, energy tests on treatment fields: i.e. monitor units, wedge, gantry and collimator

angles, tolerance table, etc. tests on plans: sum of field weights, plan approval, etc.

The number and type of tests will increase as the platform evolves.Oleander consists of two parts:

a configuration module: this is the initialization part of Oleander. The user determines which tests will be activated for a given treatment site. For each site, the user should enter a list of patients that will form the reference population for that treatment site. If the nature of the test requires so, statistics are established with the reference population.

the platform: new plans are sent to the platform where the tests are executed; a report on the results of the tests is generated.

RESULTS/DISCUSSION – The first prototype of Oleander was developed. It has been tested for breast treatments. About 30 tests have been developed. Although further improvements are needed in terms of interface, Oleander has already proven useful in detecting inconsistencies in treatment plans. Preliminary results will be presented.

CONCLUSION – Oleander, the guardian, will not only help to detect errors in treatment proposals, but it will also improve the efficiency of the process by detecting inconsistencies up front.

Keywords : quality control; automation; radiotherapy

45 – QUALITY CONTROL OF PET AND PET/CT

Immesoete P, begeleider: Bacher K., promotor: Thierens H.

AV Controlatom

INTRODUCTION – According to the Belgian law, the criteria in the publication Radiation Protection 91 for quality control of a gamma camera and isotope calibrators are to be used. However, there are no recommendations given neither for PET nor for PET/CT. As quality control of the equipment is one of the main tasks of a medical physicist, the aim of this work was to look which tests were relevant for image quality, which tests could easily be performed in routine and to establish a camera independant tool to check the quality of the PET/CT-scanner. We will focus on PET as the criteria for quality control of CT are well described in Radiation Protection 91 and need to be used.

METHODS AND MATERIALS – The tests proposed are based on the NEMA NU 2-2001 publication where several tests for quality control for PET are mentioned. For these tests specific phantoms are needed, a huge amount of activity is required and the tests + analysis take about 24h. Therefore a more user-friendly way to check the quality of the camera under circumstances used in daily clinical routine is proposed. The Jasczackphantom was used for spatial resolution and uniformity (as for SPECT-systems) as well as for sensitivity and scatter fraction. The difference with the NEMA-tests are that NEMA looks after the best possible results where we performed the tests under clinical circumstances by using the routine reconstruction algorithm, scatter correction method and randoms correction. As scatter material we used water mixed with F-18. The spatial alignment phantom with 6 Na-22 point sources is used to verify the alignment between PET and CT.

RESULTS – The tests and criteria that will be presented are only for PET except one test that verifies the alignment between PET and CT:

alignment between PET and CT: max. 5 mm misalignment (visual control) peak settings transmission sinogram: no non-uniformities emission sinogram: no large gaps, no fluid lines spatial resolution: min. diameter of 7,9 mm need to be clearly visible sensitivity: use baseline-value scatter fraction: use baseline-value integral uniformity: <20%

CONCLUSION: To establish a program for quality control of PET and PET/CT is necesarry. Progress has been made but this work needs to be continued. The proposed tests need to be performed on different types of scanners and need to be compared to make a general Belgian proposal.

Keywords : quality control, PET/CT

46 – MONIQA, MONITOR QUALITY ASSURANCE: VALIDATION AND INITIAL RESULTS OF A DYNAMIC PATTERN FOR DAILY QUALITY CONTROL OF MEDICAL SCREEN DEVICES

Jurgen Jacobs, Tom Deprez, Frank Rogge, Hilde Bosmans

UZ Leuven, GasthuisbergLUCMFR

PURPOSE – We have developed a complete environment (MoniQA) for quality control of all types of radiological screen devices, including specific viewing stations for digital mammography. The European Protocol for quality control of digital mammography systems (EUREF) prescribes tests for viewing stations, including daily tests. Because these proposed tests have limitations (static patterns, learning effect, difficult to use), we decided to develop a new dedicated pattern for daily quality control. We report on the practical workout and on the first validation of this pattern against the patterns of well-known protocols like the ones proposed by the AAPMtg18 and the DIN.

METHODS AND MATERIALS – MoniQA was developed in JAVA. Besides a main application to show patterns and to collect user data, it also consists out of a framework to simplify the creation of new patterns. We implemented all patterns needed by the AAPMtg18 protocol, by the DIN protocol and also a new dedicated pattern which we call the MoniQA pattern. This pattern contains all elements necessary to perform a full daily quality control: luminance, resolution, geometric and general image quality checks. This is a dynamic pattern. Every time another pattern is created according certain rules. This makes sure that there is no learning effect for the observer. We compared the results of the different elements in the protocols and the needed time. Four observers (both expert and novice) performed the three already mentioned protocols on twelve different monitors. We included high end, 5MP monitors for mammography as well as screen devices for general radiology in our test. The observations were made under clinical conditions.

RESULTS – We tested our software tool in a medical environment and it proved to be stable. The comparison of the different results indicated that our pattern is able to show at least the same possible malfunctions. In overall we saw that an evaluation of the MoniQA pattern only took 50% of the time of the AAPMtg18 protocol and resulted in a time gain of 15% compared with the DIN protocol.

CONCLUSION – Our preliminary results demonstrate that with our new pattern we can easily and quickly monitor the quality of medical screens on a daily basis. The results of our tests show that our approach gives at least the same results as the well-known protocols. Combined with our software environment, we implemented a powerful framework to perform the quality control of screen devices.

Keywords : monitor quality assurance

47 – IMAGE QUALITY PERFORMANCE OF LCD DEVICES: INFLUENCE OF DISPLAY RESOLUTION, MAGNIFICATION AND WINDOW SETTINGS ON CONTRAST-DETAIL DETECTION

Klaus Bacher (1), An De Hauwere (1), Peter Smeets (2), Philippe Duyck (2), Koenraad Verstraete (2), Hubert Thierens (1)

(1) Ghent University, Department of Medical Physics and Radiation Protection

(2) Ghent University Hospital, Department of Radiology

The aim of this study was to investigate the combined effects of liquid crystal display (LCD) resolution, image magnification and window/level adjustment on the low contrast performance in soft-copy image interpretation in digital radiography and digital mammography. In addition, the effect of a new LCD noise reduction mechanism on the low-contrast detectability was studied.

Digital radiographs and mammograms of two contrast-detail phantoms (CDRAD 2.0 and CDMAM 3.4) were scored by a group of fifteen observers on five LCD devices with varying resolutions (1-, 2- , 3- and 5-megapixel) and one dedicated 5-megapixel cathode ray tube monitor. Two 5-megapixel LCDs were included. The first one was a standard 5-megapixel LCD, the second had a new (Per Pixel Uniformity) noise reduction mechanism. The contrast-detail images were analyzed in four different interpretation sessions. In the first session, images were visualized using a standard representation. Secondly, the observers were forced to use the interactive window/level adjustment. In the next session, the images were presented at full resolution but window/level adjustment was not allowed. Finally, observers scored the images at full resolution together with the use of window/level adjustment. For each image analysis, the interpretation times were registered.

A multivariate analysis of variance revealed a significant influence of LCD resolution (p=0.01), image magnification (p=0.002) and window/level adjustment (p=0.001) on the low-contrast image quality performance. The interactive adjustment of brightness and contrast of digital images did not affect the reading time (p=0.10), whereas magnification to full resolution resulted in a significantly slower softcopy interpretation (p=0.008).

For digital radiography applications, a 3-megapixel LCD is comparable with a 5-megapixel CRT monitor in terms of low-contrast performance as well as in reading time. The use of a 2-megapixel LCD is only warranted when radiographs are analysed in full resolution and when using the interactive window/level adjustment.In the digital mammography setting, a 5-megapixel monitor should be the first choice. In addition, the new PPU noise reduction system in the 5-megapixel LCD devices provides significantly better results for mammography reading as compared to a standard 5-magapixel LCD or CRT. If a 3-megapixel LCD is used in mammography setting, a very time-consuming magnification of the digital mammograms would be necessary.

Keywords : soft-copy display, digital radiography, digital mammography

48 – IMAGE QUALITY AND RADIATION DOSE IN DIGITAL CHEST IMAGING: COMPARISON OF AN AMORPHOUS SILICON AND AN AMORPHOUS SELENIUM FLAT-PANEL SYSTEM

Klaus Bacher (1), Ludo Vereecken (2), An De Hauwere (1), Peter Smeets (3), Philippe Duyck (3), Robert De Man (2), Koenraad Verstraete (3), Hubert Thierens (1)

(1) Ghent University, Dept of Medical Physics and Radiation Protection

(2) Heilig Hart Hospital Roeselare

(3) Ghent University Hospital

The aim of this study was to compare the image quality and the radiation dose in chest imaging using an amorphous silicon and an amorphous selenium flat-panel detector system. In addition, the low contrast performance of both systems with standard and low radiation dose was compared.

In two groups of 100 patients each, digital chest radiographs were acquired either with the amorphous silicon or the amorphous selenium flat-panel system. The effective dose of the examination was measured using thermoluminescent dosimeters placed in an anthropomorphic Rando phantom. The image quality of the digital chest radiographs was assessed by five experienced radiologists using the European Guidelines on Quality Criteria for Diagnostic Radiographic Images. In addition, a contrast-detail phantom study was set up to assess the low contrast performance of both systems at different radiation dose levels. Differences between two groups were tested for significance using the two-tailed Mann-Whitney test.

The amorphous silicon flat-panel system allows an important and significant reduction in effective dose in comparison with the amorphous selenium flat-panel system (p<0.0001) for both the PA and lateral views. In addition, clinical image quality analysis showed that the dose reduction was not detrimental to image quality. Compared to the amorphous selenium flat-panel detector system, a significantly better low-contrast phantom performance of the amorphous silicon detector system was shown for phantom entrance dose values up to 135 µGy.

Chest radiographs using the amorphous silicon flat-panel system can be acquired with a significantly lower patient dose compared to those made with the amorphous selenium system, thereby producing an image quality that is equal to or even superior to that of the amorphous selenium flat-panel detector system.

Keywords : digital radiography, flat-panel detectors, contrast-detail

49 – COMPARISON OF OPTIMISATION METHODS FOR INVERSE IMRT PLANNING

Z.Beelen, E. Bressers, K.Bamps, P.Bulens

Virga Jesseziekenhuis / Limburgs Oncologisch Centrum (LOC)

In radiotherapy there are different techniques for applying a dose to the patient. One of the advanced techniques used nowadays is Intensity Modulated Radiotherapy (IMRT). During IMRT, a number of beams irradiate a target volume. Each beam is divided in a series of subsequent segments defined by a multileaf collimator (MLC).

Before planning starts a number of objectives are assigned to the different volumes of interest (VOI). For the organs at risk (OAR) these objectives are defined in terms of maximum dose or as a dose volume constraint. For the target volumes a prescribed dose is defined together with a maximum and/or a minimum dose.

To create an optimal IMRT plan it is necessary to make use of an inverse treatment planning system (TPS). This system optimises the dose distribution until it fulfils the most of the objectives and constraints.

To optimise the dose distribution the objectives and constraints are all incorporated in an objective function. This function will have to be minimised (or maximised in some cases) to find an optimal plan. At the moment there are several techniques implemented in commercially available systems to find this optimum.

In this study a number of optimisation methods were compared: the gradient descend method, the DMPO method in which the beams are immediately segmented and the MLEM optimisation used in IMRS. The raw data of this comparison was examined using ANOVA and MANOVA statistics. The results show significant differences between the investigated methods at a 95% confidence level. The differences occur as well in the number of monitor units (MU) needed to deliver a specific dose as in the quality of the plans. The quality of the plans was expressed using a parameter that incorporates the objectives and constraints for the Volumes of interest (VOI).

Keywords : inverse dose planning, IMRT, dose optimalisation

50 – ACCEPTANCE TESTING OF A TREATMENT PLANNIG SYSTEM

Belge D., Infantino S.

CHU Tivoli - Radiotherapy Dpt

PURPOSE - To present our acceptance tests with Eclipse TPS. Tangentiel fields : work in progress !!

MATERIAL AND METHOD – Eclipse TPS, PBC algorithm V. 7.3.10 from Varian Medical Systems. Measurements : Blue Phantom, ionization chambers IC13, Dose1 electrometer from Wellhöfer; polystyrene phantom; Kodak EDR2 films; Vidar VXR-16 DosimetryPro scanner, NE 2571 0.6 cc carbon chamber. Analysis softwares : DoseLab 4, OmniPro-Accept 6.4. Beam : cobalt, open and wedged fields (30° and 45°). SSD ref. cond. = Varian recommendation (calculation algorithm optimized for SSD basic measuments). Protocols : Estro booklet n°7 and NCS draft publication "Quality assurance of 3D TPS".

RESULTS - PDDs OK for open and wedged beams (diff. < 0.1 %, tolerance 2 %).Beams profiles :

Fields dimensions : OK for all open and wedged beams (diff. < 0.2 mm, tol. 2 mm).

High dose - high gradient areas : OK for open square fields from 4 to 20 cm² and up until 10 cm depth. Larger sizes and depth : penumbras out the 2 mm shift tolerance (20 % iso. shift : up to 3.1 mm 20 %; 80 % iso. shift : 3.6 to 9.5 mm for fields > 25 cm²). Wedges : 20 % iso. shifts increases with fields size and depth (3 to 11 mm, thin edge only).

High dose - low gradients areas : OK for open fields, sizes > 6 cm² and 0.5 to 10 cm depth. Out of the 2 % tolerance for other cases (2.1 to 8.5 %) leading to a shrinking of 90 % and 95 % isodoses. Wedges OK up to 5 cm depth, out of tol. for larger depths (2.5 to 7.8%).

Low dose - low gradients areas : out of tolerance in all cases due to collimator screws.

Special cases : tangential fields. Eclipse calculated doses in the "thin area" seem very low, we are studying these cases with films. Inhomogeheities still have to be studied. Treatment times : OK.

CONCLUSIONS – We are confident in Eclipse for calculations up to 10-15 cm depth. For larger depths (unusual for a cobalt unit) we know we have to be careful due to 90 % and 95 % isodoses shrinking. Doses underestimation outside beam edges is problematic in junctions (collimator screws). Still much work in progress for tangentiel fields and inhomogeneities!!! Comparision of calculations with SSD and SAD basic measurements is foreseen.

Keywords : TPS, dosimetry, acceptance

51 – THE USE OF ELECTRONIC COMPENSATORS TO OPTIMIZE DOSE DISTRIBUTIONS IN BREAST TREATMENTS: A FEASIBILITY STUDY AND FIRST TREATMENTS

Sabine Bernard (1), Ann Van Esch (2), Roxanne Delforge (1), Catherine Weber (1), Emile Salamon (1)

(1) Department of Radiotherapy, Namur, Belgium

(2) 7Sigma, Leuven, Belgium

PURPOSE – To replace the use of conventional wedges in breast treatments with electronic compensators generated by dynamic multileaf collimation in an attempt to improve dose homogeneity

BACKGROUND – Breast patients in the Radiotherapy Department of Namur are routinely planned –by using a 3D CT-scan- with conventional (hard and dynamic) wedges in the medio-lateral direction combined with –in some cases- in the cranio-caudal direction. The development of IMRT has opened the possibility to “redesign” the concept of missing tissue compensators to further improve the dose inhomogeneity. Varian has recently implemented such electronic compensators (“irregular surface compensators”) into the Eclipse TPS. Standard IMRT requires contoured volumes for the planner to run the optimization on. Electronic compensator modules, on the contrary, require no other contouring than the body outline and automatically generate the surface on which the dose volume optimizer aims to deliver a homogenous dose. The optimal fluence produced as such is converted into dynamic leaf motion files for delivery. Hence, the former mechanical compensators are mimicked by the dynamic multileaf collimator.

MATERIAL AND METHODS – For a cohort of breast patients (N> 40), electronic compensator plans were compared with optimized conventional wedged plans with regard to: DVH (breast and lung) and planning time. If the electronic compensator plans were deemed dosimetrically superior to the conventional wedged plans, obviously the patients were treated with the electronic compensators.

RESULTS – Especially for large breast and/or asymmetric breasts, the electronic compensation leads to better target coverage. Also, energy mixing (6MV and 18 MV) was no longer necessary for many of these cases. Planning time was also substantially reduced. For small breasts with a single wedge in the medio-lateral direction, no substantial gain was observed with the electronic compensation.

CONCLUSION – The new modules for the irregular surface compensators (Eclipse, VMS) allow fast and flexible planning and delivery of a missing tissue compensator with the dynamic MLC. This is a natural extension of using IMRT tools to improve dosimetry of conventional 3D plans.

Keywords : electronic/irregular surface compensators; breast treatments; dMLC

52 – MEASUREMENT STRATEGIES FOR SMALL FIELD DOSIMETRY DURING THE COMMISSIONING OF THE MODULEAF MLC

G. Pittomvils (1), W. De Gersem (1), M. Coghe (1), F. Crop (1,2), B. Van Duyse (1), F. Jacobs (1), C. De Wagter (1), W. De Neve (1)

(1) Dep.Radiotherapy, Ghent University Hospital, De Pintelaan 185, 9000 Gent

(2) Dep.Medical Physics, Ghent University,Proeftuinstraat 86, 9000 Gent

Intracranial stereotactic radiosurgery treatments are delivered at the U.Z. Gent using a mini-MLC (Moduleaf MLC) with 40 leaf pairs of 2.5 mm width in the isocenter plane allowing maximum field sizes of 100x120 mm². The mini-MLC is mounted as an add-on collimator on the Elekta SL25 linear accelerator.

A literature study revealed that no ideal detector is available. The diamond detector has an optimal spatial resolution and negligible energy dependence but is dose rate dependent. The smallest cylindrical chambers (0.015cc) are limited to field sizes between 15 mm and 50 mm, have acceptable spatial resolution and no energy nor dose rate dependence and larger cylindrical chambers (0.125 cc) have limited spatial resolution.

A large set of measurements was recorded and compared. Profiles were acquired using the diamond detector, depth dose curves were measured with the pin-point chamber and output factors were obtained using the three different detectors. Measurements of a standard field (10x10 cm2) at standard depth (6 MV, 5 cm) for each detector were used to scale the results to absolute values in cGy/MU. This enabled the comparison of the points of each percentage depth dose curves to the central points of the four measured profiles.

Dose rate dependence of the diamond detector was observed in the dose rate range 25-400 cGy/min. The experimental data were fitted using the known theoretical approximation. After this correction, a good correspondence (average differences < 1%) between the small cylindrical chambers recordings and the diamond detector recordings is observed for the total set of measured fields, ranging from 15x15 mm² to 100x120 mm² in size and for measuring depths ranging from 15 mm to 200 mm.Below that field size an increasing absolute dose difference with decreasing field size is observed between both detectors probably due to the volume effect of the cylindrical chambers. The amplitude of this difference however is found to be insensitive to the measurement depth and therefore relative profile depth measurements recorded with small cylindrical chambers can be used safely for field sizes of at least 5x5 mm².

The output factors measured with the three different detectors confirm the conclusions of the profile, percentage depth dose recordings. The required accuracy for reproducible output factors using small sized cylindrical chambers was found to be one tenth of a millimeter for field sizes smaller then 10 mm.

Keywords : small field dosimetry, diamond dectector, pinpoint

53 – CLINICAL INTRODUCTION OF RESPIRATION CORRELATED (RC) CT/PET

I. Potargent (1), G. Bosmans (2), H. Janssens (1), A. Dekker (2), M. Öllers (2), D. De Ruysscher (2), P. Lambin (2), A. Minken (2)

(1) Xios Hogeschool Limburg, Diepenbeek, Belgium

(2) Department of Radiation Oncology (MAASTRO), GROW, University Hospital Maastricht, The Netherlands

PURPOSE – Respiratory motion may reduce the accuracy in imaging lung cancer patients. Especially with a combined CT/PET scanner differences occurs since the acquisition time is different for both modalities. The purpose of this study was to commission and implement respiration correlated software for the PET modality clinically.

METHOD AND MATERIALS – The Siemens Biograph Sensation 16 CT/PET scanner was used for the respiration correlated scans. RCPET images were acquired with the listmode PET software from Siemens and attenuation and scatter correction was performed with the free breathing CT. Respiration correlated CT (RCCT) images were retrospectively binned and reconstructed with in house built software. The respiration signal which was recorded with a pressure sensor in a chest belt (AZ-733 V, Anzai Medical Corporation, Tokyo, Japan) and the software generated a ‘gate’ signal at maximum expiration which was sent to the PET system, this was necessary for binning the PET images. Both image modalities were binned into ten phases of the respiration. Phantom testing was performed with a lollipop phantom which was filled with approximately 20 MBq 18F-FDG. Different movement amplitudes, frequencies and volumes were imaged to investigate the accuracy of the phase binning reconstruction for RCCT as well as for RCPET. The first patients underwent a normal free breathing CT/PET, an RCCT and an RCPET. The volumes and the movements of the tumor in all these modalities were compared to each other.

RESULTS – With RCCT and RCPET we could accurately measure the shape, volume and movement of the lollipop phantom within acceptable tolerance level.Both RC image modalities were capable of determine the average tumor position and tumor movement in all three orthogonal directions for all patients. There were clear under and overestimates of the tumor volume between the RC image modalities and the free breathing CT/PET scan.

CONCLUSION – The RCPET software and the in house built RCCT software were both able to accurately determine the actual shape of the lollipop phantom. In patients both modalities could improve the imaging of lung tumors and activity concentration could be better defined with the RCPET, which will improve automatic delineation techniques. Further investigation of using phased attenuation correction still has to be done in patients.

Keywords : respiration correlation, CT/PET, lung cancer

54 – DEVELOPMENT OF A BODY SHAPE CONTOURING SYSTEM BASED ON THE USE OF A LASER TELEMETER AND DEDICATED TO PATIENTS TREATED FOR BREAST CANCER .

Van Dycke Michel (1), Duchateau Michel (2), Delmoitié Eric (1), Van Aelst Ronnie (1), Debusscher Christophe (2)

(1) Clinique Saint Jean, Clinique Saint Jean , Service de Radiothérapie, 32 Bvd du Jardin Botanique , B-1000 Bruxelles

(2) Altasys (ULB)

For radiotherapy conservative breast treatment more and more irradiation techniques to assume a homogenous dose repartition in the whole breast without excessive hot spots are developped. To achieve this objective it is necessary to use enough information about the shape of the patient but in the same time to garantee that the position of the patient does'nt change between the simulation or treatment and the body shape data acquisition.

Due to the special position of the patient on an inclined device it is very often difficult to match the previous statement on a CT system without to introduce limitations at the level of the positionning during the simulation. For this reason we have decided to developp a system able to acquire the patient shape information during the simulation in the real treatment position. Our system is based on data acquisition of the patient by a laser telemeter fixed on the simulator head and rotating by 180° around the patient. The data sended by the laser are captured on a PC and the developped software transform these data from polar coordinates to cartesian coordinates and finally export these data directly in the right format to the planning system. The acquisition time for 1 slice is directly dependent of the speed rotation of the simulator gantry and takes 27 sec. A whole patient time acquisition for 10 slices takes about 6 minutes. This application gives also the possibility to enter reference points and to export the measured data in different formats in order to be compatible with different treatment planning systems. At the beginning measurements on a phantom were performed to evaluate the whole precision of the system and both CT images and external shape acquisition were used for the first 20 patients to compare results of the 2 modalities. The presentation will cover the differents parts composing this application : electronic devices , sofware development , formats used and also the evaluation of the estimated precision of the system.

Keywords : laser telemeter , breast treatment dedicated contouring system

55 – MONITOR CHAMBER BACKSCATTER IN A VARIAN CLINAC 2100C/D LINEAR ACCELERATOR: INFLUENCE ON THE OUTPUT FACTORS OF A 6MV BEAM

Vanhoutte, Frederik

Heilig-Hartziekenhuis Roeselare-Menen, dienst radiotherapie

Most common systems of dosimetric calculations separate dose, in transient and lateral electron equilibrium conditions, in several contributions proportional to the focal energy fluence, taken to be field size independent. Since the fluence itself is an unwieldy unit in a clinical environment, it is represented by a machine parameter, the monitor unit, which by definition should also be independent of the downstream collimator settings of the accelerator. In practice, interaction of the primary photon beam with the collimators and other structural elements results in backscattered radiation into the monitor chambers. Depending on the accelator design this can lead to a doseless contribution to the recorded monitor units, directly affecting the feedback driven dose rate servo.

This is the case for the Varian 2100C/D Clinac linear accelerator and similar designs. It uses a single fixed set of monitor chambers for both photon and electron beams. This necessitates a thin-walled chamber for optimal electron transmission. As a side effect, the low energy backscattered photons and electrons can easily penetrate. The amount of backscattered radiation is dependent on the jaw settings, in effect disturbing the proportionality of the focal fluence and the monitor units.

We investigate the backscatter effect for the 6MV photon beam of this type of linear accelerator. An open-loop measurement technique is employed in which the dose rate servo is decoupled from the primary monitor chamber signal. The secondary dosimetry circuit is modified so the output of the secondary chamber can be directly recorded with an electrometer. The measured signal decreases by 3.5% ranging from 0.5cm to 40cm square fields.

However, since the dose rate servo operates on the primary dosimetry circuit, these results need to be corrected for response differences between the two monitor chambers. The difference is obtained by comparing the secondary monitor unit readout with the primary one for different field sizes in the normal operating mode of the linac. The corrected effect amounts to an additional 2.2% increase in output factor over the entire range of field sizes.

The overall size-dependence of the effect and the relative contributions of the different jaws can be reproduced with a simple geometric model correlating the backscatter with the projected exposed collimator area into the chamber.

Keywords : monitor chamber backscatter, output factor, dosimetry

56 – ICRU 72, HISTORY AND FACTS.

S. Vynckier ( and the other members of the ICRU 72 commission )

UCL, Cliniques univ. St-Luc, Radiotherapy Department

In 1997 a new ICRU commission was established with the task to prepare a report on dosimetry of beta-ray sources. The report was originally intended to provide guidance on the specification and calibration of beta-ray sealed sources as used for Brachytherapy applications. In the beginning the main interest was focussed on ophthalmic application, however during drafting, the scope of the report was adjusted to the use of beta-ray sources for intravascular applications due to rapid expansions of these applications at that time. Moreover, as these beta-ray sources exhibit rather similar dose distributions as those of low energy photon seed sources and as for a number of applications the latter are also utilized, corresponding guidance and recommendations of low energy photon sources was also included in the report. The report was finally published in 2005.

The presentation will give an overview of the different chapters and will summarize the recommendations of the report. Moreover, within the scope of the report the commission members performed also intercomparison measurements and Monte Carlo calculations for clinical beta-ray sources. The results of these measurements and calculations will also be presented.

Keywords : ICRU, brachytherapy, beta- and low energy photon sources

57 – TESTING OF THE ANALYTICAL ANISOTROPIC ALGORITHM (AAA) FOR PHOTON DOSE DISTRIBUTION

Martin Morelle (1), Ann Van Esch (2), Hannu Helminen (3), Laura Tillikainen (3), Sami Siljamäki (3), Dominique P. Huyskens (2)

(1) Clinique Ste-Elisabeth Namur, Radiotherapy Department, Belgium

(2) 7Sigma, Belgium

(3) Varian Medical Systems, Finland

PURPOSE – To test the accuracy of the new algorithm AAA implemented in the TPS Eclipse (Varian Medical Systems). AAA was developed to replace the single pencil beam (SPB) algorithm for photon dose distributions and to improve the dose calculation accuracy in heterogenous media. Tests were performed for 6 MV and 18 MV photon beams (Clinac 21EX VMS)

MATERIAL & METHODS – The AAA algorithm consists of two modules: a configuration module and a dose calculation engine. The configuration module: this part of the program constructs the phase space of the treatment unit by optimizing the agreement between the calculated and measured depth dose curves and profiles. The phase space is described by a primary photon source, a secondary finite-size photon source and an electron contamination source. The dose calculation engine: the dose in the patient is calculated as the superposition of the dose deposited by the two photon sources and by the electron contamination source. The photon dose is calculated by a 3D convolution of Monte-Carlo pre-calculated scatter kernels. The interacting scatter kernels are scaled using the electron density matrix.The accuracy of AAA was tested for different field sizes and under the following conditions: open fields and MLC fields (depth dose curves, profiles, monitor units); IMRT fields (depth dose curves and profiles); cork inhomogeneities (depth dose curves and profiles). An intercomparison was made between measurements, AAA calculations and SPB calculations.

RESULTS/DISCUSSION – The calculated phase space reconstructs the depth dose curves, profiles and monitor units of open beams with good accuracy. Especially the second photon source improves the penumbra modeling. Depth dose curves of MLC fields and IMRT fields are adequately modeled. Depth dose curves in the cork are substantially better with AAA than with the single pencil beam, but not in the solid water behind the cork. Profiles at the interface between solid water and cork show in general a better agreement with AAA.

CONCLUSION – Compared to SPB, AAA improved in most cases the accuracy of dose calculations. Splitting up the algorithm in a configuration module and a calculation engine offers the possibility to progressively fine-tune the calculation engine.

Keywords : TPS testing, photon dose algorithms, phase space

58 – EVALUATION AND VALIDATION OF GYN GEC-ESTRO RECOMMENDED TARGET CONCEPTS AND DOSE VOLUME PARAMETERS OF MR BASED BRACHYTHERAPY FOR CERVIX CANCER.

A. Nulens (1), S. Lang (2), E. Briot (3), M. De Brabandere (1), C. Kirisits (2), J. Dimopoulos (2), C. Haie-Meder (3), R. Pötter (2), E. Van Limbergen (1)

(1) University Hospital Gasthuisberg, Leuven, Belgium

(2) Medical University of Vienna, Vienna, Austria.

(3) Institut Gustave Roussy, Villejuif, France

The GYN GEC-ESTRO group has recently defined recommendations for target delineation (Haie-Meder et al.) and reporting 3D image based brachytherapy of cervical cancer (Pötter et al.). The recommended concepts and parameters were validated during delineation workshops carried out in the 3 participating centres. In this study we present data of 6 patients, 2 patients from each centre with good and poor remission after external beam therapy and before brachytherapy.

METHODS – For each patient 3 target volumes GTV, High Risk CTV (HR CTV) and Intermediate Risk CTV (IR CTV) were delineated on axial MR images at time of brachytherapy with the applicator in place. Contouring was performed independently by 3 physicians of the participating centres according to the target delineation protocol defined by our group. The patients received a brachytherapy plan according to the treatment approach of the hospital of actual treatment. Treatment approaches differ in dose rate (HDR,LDR,PDR), type of applicators, dose specification methods and dose prescription. Absolute values and localisation of target volumes were compared among the 3 physicians, as well as dose volume parameters D100 and D90. In order to compare doses of different dose rate and fractionation schedules, all doses were biologically normalized to 2Gy per fraction external beam therapy (EQD2) equivalent to 50cGy/h LDR using the linear quadratic model of monoexponential sublethal damage repair.

RESULTS – The average standard deviation in percent for the 6 patients with regard to the average volume within each patient was 48.7%, 20.3% and 14.1% for GTV, HR CTV and IR CTV respectively. Differences in absolute volumes and localisation of target volumes were due to differences in interpretation of radiological images of the anatomy, differences in integration and translation of clinical information onto MR images, inherent differences present in the delineation process itself and specifically for GTV due to the very small absolute volume at time of brachytherapy. The average standard deviation in percent for the 6 patients with regard to average doses D100 and D90 within each patient was less then 5% for HR CTV and IR CTV. Dose differences for GTV were higher (average SD 11%) due to larger differences in absolute target volume.

CONCLUSION – Definitions of the target concepts as recommended by the GYN GEC-ESTRO group proved to be unequivocally applicable and clear in a small group of three experienced institutions.

Keywords : cervix brachytherapy, target concept, dose volume parameters

59 – LEAF AND JAW POSITION OPTIMIZATION FOR INTENSITY-MODULATED ARC THERAPY (IMAT) APPLIED FOR A RECTAL CANCER CASE

Luiza A.M. Olteanu, Ir.,Werner De Gersem, Ir., Wim Duthoy, M.D., Marc Coghe, Lic., Wilfried De Neve, M.D., Ph.D.

Division of Radiotherapy, Ghent University Hospital, Ghent, Belgium

PURPOSE – Our in-house developed optimization software [1] was adapted in order to take into account the dynamic constraints imposed by the delivery of the Intensity-Modulated Arc Therapy (IMAT, [2]) plan on an Elekta delivery system. The optimization of the X jaw positions was included in the optimization software along with a technique that handles the speed constraints of X jaws and multileaf collimator leaves. The purpose of this presentation is to show the advantages and disadvantages of these adaptations.

METHODS AND MATERIALS – The IMAT optimization cycle is divided into three parts: (1) monitor unit optimization, (2) leaf and jaw position optimization and (3) leaf and jaw position adaptation according to their speed constraints (there is no optimization made in this part). This cycle is repeated for a predefined number of times and the quality of the plan is reflected in the value of the biophysical objective function [1]. We applied this procedure on a rectal cancer case for which the beam shapes were collimated around the Planning Target Volume (PTV). Hereafter we expanded the PTV volume with 1 cm and three plans were made. For one plan, the jaw positions were set manually at 8 mm from the expanded PTV and leaf positions were optimized. For the other two plans, the beam shapes were left unchanged and we optimized one plan with automatic jaw position optimization and the other without. The intermediate values of the objective function during the optimization were recorded. The final Dose-Volume Histograms (DVHs) of all targets and critical organs were computed after all optimizations.

RESULTS – At the end of some of the optimization cycles, the value of the objective function became worse after the application of speed constraints; however this decrease in the quality of the plan was compensated by the subsequent optimization cycles. There were no important differences between the DVHs of the critical organs obtained after all three optimizations. The partial volume of the PTV that was underdosed, was smaller for both plans in which the jaw positions were changed.

CONCLUSION – Manual adaptation of jaw positions could be replaced by their optimization implemented inside the optimization software. The final positions of both leaves and jaws met the speed constraints imposed by the delivery system.

[1] De Gersem W. et al. IJROBP 2001; 51; 5; 1371-1388 [2] Duthoy W. et al. IJROBP 2004; 60; 3; 794-806

Keywords : IMAT, leaf and jaw position optimization, rectal cancer

60 – MODELLING THE MODULEAF MLC IN PINNACLE

G. Pittomvils (1), M. Coghe (1), W. De Gersem (1), F. Crop (1,2), B. Van Duyse (1), F. Jacobs (1), C. De Wagter (1), W. De Neve (1)

(1) Dep.Radiotherapy, Ghent University Hospital, De Pintelaan 185, 9000 Gent

(2) Dep.Medical Physics, Ghent University,Proeftuinstraat 86, 9000 Gent

The stereotactic module of Pinnacle 7.4 does not support a mini MLC. The standard module was used to implement the moduleaf MLC of MRC for treatment planning of stereotatic intracranial applications. The auto-modelling software however proved to be unsuitable for small fields below 20x20 mm² and differences of more then 10 % were observed between modelling and measured data especially for the depth dose profiles.

Manual modelling was started, using the percentage depth dose curves of field sizes of 10x10 mm² and 20x20 mm² instead of the standard 100x100 mm² field size. Using a manual fit of the energy spectrum, source size, Gaussian height and Gaussian width and the off axis softening factor a good agreement between the measured and calculated profiles and depth dose curves was obtained for field sizes from 5x5 mm² to 100x100 mm².

The leaf-tips are modelled using the rounded leaf end model available in the software. The checking of this manual beam modelling was done on different levels

1. The calculated output measurements were checked for square fields ranging from 5x5 mm² to 100x100 mm² using the finest applicable calculation grid (0.1mm).

2. Half beams parallel and orthogonal to the leaf settings were compared with measured data at four different depths 15, 50, 100 and 200 mm.

3. Odd shaped fields using planes with hinge angles of 30,45 and 60 degrees to the leaf orientation were used to check the penumbra shaping of Pinnacle 7.4 at the same four depths.

4. Finally the penumbra in the edge of an angle of 105 degrees is checked to the measured data at the same four depths.

The obtained results are within the acceptance levels for single fraction stereotactic treatments.

1. A maximal deviation of 0.5 % on the output factors is observed.2. For standard field settings, a maximal deviation of 1 mm between the

measurements and the calculations in the field profiles is observed.3. Only for a 60 degree plane, a deviation exceeding 1 mm but inferior to 2 mm

is recorded around the 20 % and 90 % isodoses.

The Collapsed Cone algorithm used by the Pinnacle planning system is well adapted for the modelling of field sizes used for stereotactic treatments when a separate machine is modelled for those small field sizes using a 10x10 mm² or 20x20 mm² as the standard field size for modelling the machine parameters. Keywords : Pinnacle, modelling, stereotactic dosimetry

61 – QUALITY ASSURANCE IN RADIOTHERAPY BY A MAILED THERMOLUMINESCENCE DOSIMETRY PROCEDURE AT THE PATIENT LEVEL

Ans Swinnen, Jan Van Dam, Walter Van den Bogaert

UZ Gasthuisberg

PURPOSE – During transfer of large amounts of information between the steps in the radiotherapy process, errors can occur that can result in systematic or random errors in individuals or in a group of patients. QA programs are therefore needed and should not only include regular internal checks performed by the centres’ physicists, but also external audits made by an independent organisation. Latter programs relying on postal dosimetry systems are generally limited to beam output checks by thermoluminescence dosimeters (TLDs) in a water phantom. Phantom measurements however do not reflect by definition the total accuracy of patient treatment delivery. Hence, in vivo dosimetry should be a complementary part of the QA methods to assess the overall accuracy from prescription to delivery.

METHODS – For application of TLDs in mailed in vivo entrance dosimetry studies, cylindrical build-up caps in Al, Cu and Ta are developed with thickness corresponding to the depth of maximum dose for photon energies between respectively 60Co gamma rays, 4-10MV and 10-18 MV X-rays. The dose attenuation by the detectors (i.e. TLD in build-up cap) is investigated by film dosimetry. Next, the feasibility of the detectors is verified by placing them on patients treated in the Leuven radiotherapy department. Further, the detectors are mailed to 22 European radiotherapy centres that select a cohort of patients and calculate the entrance doses by their TPS.

RESULTS – Phantom measurements reveal that the caps have an adequate shape and thickness. The detectors show a clinically acceptable local perturbation. Two in vivo measurements per patient demonstrate that the reproducibility of the dose measurement is on average better than 2%. A tolerance level of ±5% is applied. The mean ratio of measured to calculated entrance doses for the pilot in vivo study is 0.994±0.021(N=26). For the mailed in vivo study, the mean ratio is 0.999±0.087(N=216). Some significant systematic errors are traced. The majority of discrepancies between measured and calculated entrance doses are random variations in patient setup. An adjusted tolerance level for certain clinical indications could be desirable, for instance for breast cancer patients with wedged treatment fields.

CONCLUSION – We have been successful in our attempt to provide a validated methodology which extends the basic mailed dosimetry audits to the level of patients with the more clinically related checks by mailed in vivo dosimetry with TLDs

Keywords : thermoluminescence dosimetry, mailed, in vivo

62 – TOTAL BODY IRRADIATION: THE MIDDELHEIM PARADIGM

J. Vanregemorter, P. Meijnders, D. Van den Weyngaert

ZNA Middelheim hospital, Department of Radiation OncologyLindendreef 1, B2020 Antwerp, Belgium

PURPOSE – In this study we describe a technique used for fractionated total body irradiation. The patient is treated with 12 Gy in 6 fractions over 72 hours with the dose to the lungs limited to 9 Gy.

METHODS AND MATERIALS – After comparison of several techniques for positioning the patient (antero-posterior irradiation with patient laying on his side, free standing, sitting or crouching, left-right irradiation with patient in supine position) and positioning the lung shields, we have opted for a standing position of the patient, fully stretched with adjustable crouch and armpit supports and hand grips. A reclining back support at 15 degrees gives the effect of a resting position. The ”treatment cabin” is positioned at 3 meters from the linac’s isocenter. For the lung shields we use transmission blocks cut out of lead sheets. The blocks are glued on a plastic foil which is fixed by two clamps to the 2 cm thick PMMA screen used to homogenise the surface dose. To track the position of the blocks and make it reproducible, during simulation a 8mm steel ball bearing is put on the patient on the projection of a central mark on the PMMA screen. This mark (or rather the ball bearing) shows up on an “x-ray” taken with 12 MU (18 MeV) on a CR screen. The magnification known, a reduced copy is printed to give the size of the lung blocks and their position relative to the central mark. Once the blocks are cut, they are glued on the foil relative to the mark. Positioning the mark on the foil onto the mark on the PMMA screen gives a reliable reproducible position of the blocks relative to the patient. During treatment, each session starts with a verification “x-ray” of the blocks and the lungs.

RESULTS – Only small (1-4 mm) adjustments of the position of the block foil are necessary depending on the general status of the patient. The reclined standing position combined with the support and individual head rest gives a very reproducible position for both anterior and posterior irradiation. Patients experience the position as rather comfortable.

MEASUREMENTS AND CALCULATIONS – Depth dose and transmission were measured at the effective distance of the treatment. Lung dose relative to central dose is measured on line by diodes every treatment fraction. TLD measurements track central dose and dose to extremities.

CONCLUSION – After a series of 15 patients we can conclude that the described paradigm allows for routine TBI treatment. The specially designed treatment cabin is easy

Keywords : total body irradiation

63 - STUDY RADIOLOGICAL PHYSICS ON DENTAL CONES

Jean-Louis Greffe

CHU de Charleroi

This study makes it possible to check the validity of the standards of the documents RP91, in particular from the point of view:

output of the tube with X-ray (microGy/mAs à 1 m). reproducibility of the output of the tube with X-ray. precision of the tension of the tube with X-ray. reproducibility of the tension of the tube with X-ray. precision of time of the irradiation. reproducibility of time of the irradiation. measurement of the layer of half attenuation

Conditions of measurements :

numbers of tube with X-ray 33 type of the device dental cone measuring apparatus Baracuda probe detection MPD

Preliminary results

output of the tube with X-ray (microGy/mAs à 1 m), for tensions of 50 to 70 kV.o 42,0 (sd = 54,4 %), the norm 30<x<80.o tube with X except limit, 2 dental cones with results too light and 1 dental

cone with results too high.

tension of the tube with X-ray, variation with the nominal voltage.o - 4,6 (sd = 107 %), the norm <10%.o tube with X except limit, 4 dental cones with results too light.

time of the irradiation (%), variation at the irradiation time.o 1 (sd = 1700 %), the norm <10%.o tube with X except limit, 1 dental cones with results too light and 6 dental

cone with results too high.

Conclusion

There can be strong variation so much from the point of view of precision of the tension of the tube (tension too weak) that precision of the timer (irradiation time too high).

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65 – AUTOMATED TUMOR DELINEATION IN FDG PET IMAGES

F. Jacobs, B. Vanderstraeten, W. De Gersem, G. Pittomvils, M. Coghe, B. Van Duyse, C. De Wagter, W. De Neve

Dep.Radiotherapy, Ghent University Hospital, De Pintelaan 185, 9000 Gent

PURPOSE – Current research on the automated delineation of FDG PET tumours is usually based on thresholds, where the authors try to optimize this threshold based on the loco-regional signal-to-noise ratio and recovery coefficients. Such methods require the acquisition of calibration curves using phantoms. We present an algorithm based on a gradient filter, which does not use a threshold, and does not require measurements. We investigated the effect of the applied pixel neighbourhood – usually not mentioned in any article – and the applied gradient filter on the derived tumour volume.

METHODS – The PET image is first filtered with a low pass average filter to reduce image noise. Then a gradient filter is applied which replaces each pixel value by the local slope. Finally a region growing is applied on the gradient image using a predefined pixel neighbourhood, starting from a predefined seed point. Region growing is performed until each adjacent pixel holds a smaller slope. The maximum pixel value in the delineated region is determined and used as the new seeding point. The process stops when the seeding point becomes constant. The volumes of sixteen head and neck tumours have been determined. Three gradient kernels (G1, G2, G3) and three pixel neighbourhoods (6, 18, 26 pixels) were investigated. Differences were interpret with a mixed model ANOVA technique, with the gradient kernel and the neighbourhood as fixed factors and the patient number as a random factor. Post-hoc Scheffé tests were performed for gradient kernel and pixel neighbourhood. No cross products were taken into account.

RESULTS – All tumours were delineated without problems. Results were independent of the initial seeding point, as long as this point was not too far downhill. No significant differences were found with respect to the applied gradient (p = 0.939). The pixel neighbourhood, however, did have a significant influence (p < 0.001). After visual inspection, we decided to use a neighbourhood of 18 pixels for future studies. Aside from the 16 investigated tumours, we also investigated the robustness of this method for other types of tumours and other locations. All investigated tumours rendered robust results, i.e., independent of the initial seeding point and not “running away” into regions with high uptake, e.g., the brain or the heart.

CONCLUSION – An algorithm for automated tumour delineation in FDG PET images has been proposed. The algorithm yields robust results. The applied pixel neighbourhood has a significant influence on the derived volumes.

Keywords : delineation, FDG, PET

66 – Voxel phantom ...

Jessica Pages, Filip Vanhavere