DITANET Workshop 08 November 2011 Alessio Bocci, CNA 1

DITANET Workshop 08 November 2011 Alessio Bocci, CNA 1

A silicon strip detector for a novel 2D dosimetric method for complex

radiotherapy treatment verification

Alessio BocciDITANET Experienced Researcher

Complex radiation therapy treatments - IMRTDetectors for dosimetric applications

RADIA2 project : experimental set-upMeasurements and resultsConclusions


Outline

Motivation and Objective

Motivation:Cancer is the second most frequent cause of death in developed countries. At present, although surgery is the most effective way to remove the malignant tissue, when it is combined with radiation therapy improves the cure rate by 40% approximately.

ObjectiveCharacterization of a silicon strip detector dedicated to 2D dose measurements in the axial plane of a phantom for the verification of complex radiation therapy treatment plans (i.e. IMRT)


Many beam directions and entrance points for conformal doses distributions

modulating in space the fluence of each radiation field


Intensity Modulated Radiation Therapy

Dosimetry for radiotherapy verification

Ionization chambers – “gold standard” detectors

Radiographic and radiochromic films

Semiconductor dosimeters: Silicon diodes


2D Detectors dedicated to radiotherapy verification plans

Film dosimetry used as 2D dosimeters : Advantages: high spatial resolution (sub-mm), good uniformity, axial planeDisadvantages: Unusable as on-line detectors and are unreliable and time consuming detectors

Film dosimeters: traditional detectors for verifying treatment plans dose distribution


Dose distribution verification

Axial plane(inferior and

superior)

Coronal plane(ventral and dorsal)

Sagittal plane

IMRTLog (ONCOlog Medical) 2D array silicon diodes spatial resolution

0.5 to 1.0 cm

I’MRT MatriXX (Scanditronix/Wellhöfer)array of ionizations chambers

MapCHECK (Sun Nuclear) silicon diodes

Spatial resolution is still poor with respect to film dosimetersThese devices are useful only for one irradiation angle and they can measure dose distribution

only in the coronal plane of the patient


2D Digital Detectors in radiotherapy treatments

Single silicon detectors and ionization chambers assembled for 2D detectors

New detection systemsTraditional detectors

New detection systems

It is necessary to develop new detection systems that enhance the traditional ones, and that are able to verify in a simple and accurate way complex treatment planning

Radiographic films


Film dosimeters

2D commercial digital

detectors

New detection systems

On-line no

Spatial resolution

poor

2D detectors yesNot monolithic!

Axial plane no

inexpensiveradiation hard

easy to use

Silicon strip detectors mounted @ CNA in a telescope configuration

Silicon strip detectors mounted @ Virgen Macarena Hospital in Seville

silicon tracking detectors silicon detectors for medicalapplications


From Nuclear Physics to Medical ApplicationsMedical applications can benefit from the developments on nuclear and high energy physics

detector technology

• Commercial silicon detector • Low cost

Normally used on particle detection (W1 type from Micron Semiconductor Ltd)• Single sided 16 strips (3.1 mm pitch)• Active area 50 x 50 mm² & 500 µm thick


RADIA2 Project Si-DETECTOR

Advantages:

Linear relationship between photon energy and e/h pairs ;

High sensitivity (Si - 3.6 eV/pair – sensitivity per unit volume: 640 nC Gy-1/mm3 )(about 18000 times the sensitivity per unit volume of an ionization chamber) ;

Small active volume that allows to obtain high spatial resolution ;

Well developed technology for the production of segmented monolithic planar detectors ;

Radiation hard material ;


Why Silicon detectors ?

C. Talamonti et al, NIM-A 658 (2011) 84–89

Signals from each strip are converted to the

absorbed dose

Radiotherapy energies photon interactions :

Compton Effect

Secondary electrons from the phantom interacts centemeters

away


Single-sided-Si-strip detectors (SSSSD)

Water-equivalent phantom

γ photons

Z. Abou-Haidar1, M. A. G. Alvarez1 ,R. Arrans3, A. Bocci1, M. A. Cortes-Giraldo2 , J. M. Espino2 ,M. I. Gallardo2, A. Perez Vega-Leal4

F. J. Perez Nieto5, J. M. Quesada2

1. DITANET group @ National Accelerator Centre (CNA)2. Department of Atomic, Molecular and Nuclear Physics (FAMN),

University of Seville3. Virgen Macarena University Hospital, Seville4. School of Engineering, University of Seville

5. Instalaciones Inabensa S.A.


RADIA2 Project – Collaboration

A. Bocci et. al., A silicon strip detector for a novel 2D dosimetric method for radiotherapy treatment verification

submitted to NIM-a (October 2011)

A. Bocci et. al., Empirical characterization of a silicon strip detector for a novel 2d mapped method for dosimetric verification of radiotherapy treatments, Radiotherapy and Oncology, Volume 99, Supplement 1, May 2011, Page S172

M. A. Cortes-Giraldo et al. , "Geant4 Simulation to Study the Sensitivity of a MICRON Silicon Strip Detector Irradiated by a SIEMENS PRIMUS Linac",

Progress in Nuclear Science and Technology, in press (2011)

Siemens PRIMUS linac dual energy machine operating at 6 MV photon

mode

University Hospital Virgen Macarena(Seville, Spain)


LINAC accelerator

A treatment planning system TPS (Philips Pinnacle) was used to calculate dosedistributions. Calculations were compared to experimental data

Two phantoms prototypes were designed and built Polyethylene slab material Cylindrical phantom

1. A slab phantom for:detector characterization

(sensitive area perpendicular to the beam direction)

2. A cylindrical phantom for:angular response measurements &

2D treatment plans verification in the axial plane


Experimental Set-up

Set-up 2:The active area of the

detector is parallelto the beam direction (axial plane)

This work is part of a more ambitious project aiming to:

Employ this prototype for obtaining 2D dose maps by an in-house developed algorithm dedicated to verify IMRT treatment plans.This system is patent pending

Since common ways to present dose distributions is a dose map in the patient axial plane, as an innovation the detector was placed in the cylindrical phantom in the axial plane, parallel to the beam axis


Detector in the axial plane

Charge integrators for each strip (electrometers) digitized (12 bits) and analyzed by a Digital signal processor (DSP)

discrete electronics developed in-house

A PC allows to control and to retrieve data via an RS-232 serial bus based on a LabVIEW software


Electronics

School of Engineering, University of Seville School of Engineering, University of Seville

Virgen MacarenaUniversity Hospital,

Seville

Set-up 1:• Linearity• Uniformity• Calibration• Percent Depth Dose

(PDD)• Penumbra

Set-up 2:• TPS and Geant4

Simulations• Angular measurements• Final calibration


MeasurementsDetector

characterization

Angular response

Set-up 1: Linearity • Uniformity• Calibration• Percent Depth Dose

(PDD)• Penumbra


simulations• Angular measurements• Final calibration



characterization

Angular response

Linearity with dose better than 0.1 %for all channels

Set-up 1:SSSSD perpendicular to the beam direction


Linearity

Set-up 1:• Linearity Uniformity• Calibration• Percent Depth Dose

(PDD)• Penumbra





characterization

Angular response

Uniformity better than 0.5 %for all channels



Uniformity

Non-uniformities depend by the different strip efficiency and gain of the electronics

Set-up 1:• Linearity • Uniformity Calibration• Percent Depth Dose

(PDD)• Penumbra





characterization

Angular response

Calibration in standard conditionradiation field 10 × 10 cm2

source-to-surface distance (SSD) = 100 cm 1.5 cm of water-equivalent solid slabs



Calibration

Monitor Units –> Voltage -> cGy

Set-up 1:• Linearity • Uniformity• Calibration Percent Depth Dose

(PDD)• Penumbra





characterization

Angular response

The difference between SSSSD and ionization chamber is: 0.68 % at 10 cm and 0.73 % at 15 cm



Percent Depth Dose

Dose at different depth measured using different water-equivalent solid slabs

Set-up 1:• Linearity • Uniformity• Calibration• Percent Depth Dose

(PDD) Penumbra





characterization

Angular response

SSSSD penumbra value larger than the one obtained when using a single silicon detector This was mainly due to the SSSSD strips pitch of 3.1 mm

Geant4 simulations gave compatible resultsImprovements with a future prototype (next project)



Penumbra

SSSSD: 6.17 ± 0.56 mm - single silicon diode: 3.92 ± 0.20 mmRegion between 20 % and 80 % of the maximum dose levels at 1.5 cm water depth


(PDD)• Penumbra

Set-up 2: Geant4 simulations

and TPS calculations• Angular measurements• Final calibration



characterization

Angular response


Geant4 simulations and TPS calculationsTreatment headGeometry Model Phantom SSSSD Detector

M. A. Cortés Giraldo, Ph. D. Thesis, 2011

The geometry of the Siemens treatment head at 6 MV nominal energy photons, was reproduced in detail

The geometric model of the phantoms and of the SSSSD was also reproduced

The absorbed dose in each strip was calculated

Geant4 Simulations were also performed for calculating the dose-to-water case for comparisons with TPS calculations




(PDD)• Penumbra

Set-up 2:• Geant4 simulations and

TPS calculations Angular measurements• Final calibration



characterization

Angular response

Angular response00

450

3150


gantrydetector

Set-up 2:SSSSD parallel

to the beam direction

Two experimental measurements


Angular ResponseComparison between exp.

data, TPS and Geant4

Set-up 2:SSSSD parallel

to the beam direction

Steps of 450

The agreement between the tendency of experimental data

at different anglesand the TPS is notable

This implies that the new calibration factors are

independent of the irradiation angle

Constant calibration factors


(PDD)• Penumbra

Set-up 2:• Geant4 simulations and

TPS calculations• Angular measurements Final calibration



characterization

Angular response

Calibration factors: Experimental/TPS

Calibration factors independent of angular irradiation and of strip number


Final CalibrationSet-up 2:

SSSSD parallelto the beam direction

Constant calibration factors

gantrydetector

Relative difference between the calibrated dose and TPS calculations are better than 2 %


Final Calibration

Calibrated doseSet-up 2:

SSSSD parallelto the beam direction

gantrydetector

Main Objective: Characterize and benchmark a new detection system based on a Si-strip detector dedicated to 2D dose measurements in the axial plane of a cylindrical phantom

SSSSD characterization: results shows that the prototype is suitable for IMRT verification plans (remarkable linearity, uniformity, PDD)

The angular response of the detector in the axial plane was independent of the irradiation angle and of the strip number

Geant4 simulations gave compatible results with respect to TPS and to experimental data

Final calibration with respect to TPS in the axial plane gives differences smaller than 2 % for all the strips

This system is patent pendingOEMP PATENT number P201101009


Conclusions

Future: work is in progress in order to obtain 2D dose maps from experimental data in the axial plane using an in-house developed algorithm based on the Radon Transform

A new SSSSD prototype and a new experimental set-up has been designed to improve the spatial resolution, coupling the data acquisition system with a reconstruction algorithm and with an on-line graphical interface software

Future developments


Z. Abou-Haidar1, M. A. G. Alvarez1 ,R. Arrans3, A. Bocci1, M. A. Cortes-Giraldo2 , J. M. Espino2 ,M. I. Gallardo2, A. Perez Vega-Leal4

F. J. Perez Nieto5, J. M. Quesada2

1. DITANET group @ National Accelerator Centre (CNA)2. Department of Atomic, Molecular and Nuclear Physics (FAMN),

University of Seville3. Virgen Macarena University Hospital, Seville4. School of Engineering, University of Seville

5. Instalaciones Inabensa S.A.


RADIA2 Project – Collaboration

A. Bocci et. al., A silicon strip detector for a novel 2D dosimetric method for radiotherapy treatment verification

submitted to NIM-a (October 2011)

A. Bocci et. al., Empirical characterization of a silicon strip detector for a novel 2d mapped method for dosimetric verification of radiotherapy treatments, Radiotherapy and Oncology, Volume 99, Supplement 1, May 2011, Page S172



Thank you for your

attention!!!

Single Strip and 2D monolithic silicon detectors

Research is directed towards silicon microstrip technology to improve spatial resolution

Pixellated monolithic silicon detectors such as the 2D array

D. Menichelli et al., Nucl. Instr. and Meth. A, 583, 109 (2007)

Single crystal n-Si128 channels

32 mm x 0.2 mm

128 phosphor implanted n+ strips on a p-type silicon

waferJ. H. D. Wong et al.,

Medical Physics 37 (2010) 427–439 I. Redondo-Fernandez et al,

NIM-a, (2007) 141–144

2. CMRP DMG

441 Si n+p diodes50 µm epi layer growth on MCz p.

Active area: 6.29 x 6.29 cm2.

1. DOSI

3. European project MAESTRO


DITANET Workshop 08 November 2011 Alessio Bocci, CNA 1

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