IMRT dosimetry QA

Master's Thesis, 30hp

September 2008 - January 2009

Frida Åstrand

Supervisor: Sean GeogheganDepartment of Medical Engineering and Physics, Royal Perth Hospital

Co-supervisor: Mats IsakssonDepartment of radiation physics, University of Gothenburg

University of Gothenburg

Department of radiation physics

Abstract

The purpose of this project is to investigate if the process behavior charts (PBC) are

useable in the dosimetry quality assurance (QA) process of intensity modulated radiation

therapy (IMRT) plans. The PBCs were used to visualise ion chamber measurements and

�lm measurements (Gafchromicr EBT �lm). To be able to use the �lm measurements

scanner analysis of the Epson perfection V700 photo was done. The scanner was found

to be suitable to digitize the �lms if the �rst scans were not used and the placement of

the �lms were held constant for all measurements. The calibration method of the �lms is

investigated using two di�erent methods, a fraction calibration method and a step-and-

shoot wedge method. The step-and-shoot wedge method is found to be the best method to

use. The �lms were compared to the calculated dose distribution using DoseLab4, which

calculates γ indexes and NAT indexes among other things. From DoseLab γ images, NAT

value images and combined contour images were also derived.

The PBC is found to be a good tool for visualising and observing the variation of the

ion chamber measurements over time. The PBCs use the per cent di�erence between the

calculated dose and the measured dose. The PBCs were constructed both for combined

IMRT plan doses and for measurments of the individual �elds. The number of measure-

ments in this project was not enough to establish accurate process behavior limits. The

PBCs for the ion chamber measurments ended up in State II for both the methods of

constructing the PBC for the head and neck. The n = 1 method for the prostate mea-

surements gave a PBC in State II and the n = 5 method gave a PBC in state IV. The

method using the individual �eld measurements are recommended.

PBCs are also constructed for the �lm measurements using the NAT indexes. The spec-

i�cation limits are not speci�ed for the NAT index and the PBC can not be used to see

if the IMRT plan passes or fails the QA, but the �lm dosimetry part of the IMRT QA

process can be analysed.

To ease the construction of the PBCs a template in excel was constructed. The tem-

plate produces both X̄-charts and R-charts.

Contents

1 Introduction 3

2 Theory 3

2.1 Statistical process control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Dose measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2.1 Non-reference condition measurements . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Analysis tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.3.1 γ index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.3.2 Normalised agreement test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3 Materials and Methods 11

3.1 Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.2 Ion chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.3 Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.3.1 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.3.2 Scanner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.3.3 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.3.4 Calculated dose distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.3.5 DoseLab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.4 Process behavior charts template . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4 Results 29

4.1 Ion chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.1.1 Process behavior charts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.2 Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.2.1 Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.2.2 Dose di�erence comparision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.2.3 Normalised agreement test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4.2.4 Process behavior charts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4.3 Number of segments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5 Discussion 49

5.1 Treatment plans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.2 γ index and NAT index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.3 Dose distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.4 Process behavior charts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Acknowledgement 52

References 53

Appendix 55

1 Introduction

Cancer is a common disease that can be cured with a treatment using ionising radiation

which is called radiation therapy. For tumours situated near sensitive organs and/or with

concave forms a new type of technique has been invented, intensity modulated radiation ther-

apy (IMRT) and its clinical use is more and more common. The IMRT technique makes it

possible to shape the isodose curves through modulating the intensity of each �eld. Because

of the complexity of the IMRT technique, more sophisticated dose checking methods are re-

quired. Con�rmation of the dose has to be performed for all patients treated. In conformal

radiation therapy (CRT) the �elds of the treatment plan have an even intensity and checking

of the calculated doses can be done by independent calculations. The IMRT dose checks are

thorough and time consuming and often have to be done after hours. Research has been done

by Huq et al. 2008 [1] on improving the e�ciency of the veri�cation methods. [2]

The quality assurance (QA) procedure of checking the IMRT plans includes several aspects,

such as multileaf collimator (MLC) QA, measurements of individual patient �uence maps,

calibration of the tools used and procedures to ensure accurate patient positioning [3]. This

project investigates the use of process behavior charts (PBC) in the dosimetry QA of IMRT

treatment. The doses are measured using an ion chamber and �lm dosimetry.

2 Theory

IMRT is a radiation therapy technique that is used to avoid sensitive tissues situated nearby

tumour tissue [3]. The technique can be based on inverse planning where the desired dose

distribution is entered in terms of avoidance, target structures and dose objectives.

There are several di�erent techniques of performing IMRT treatments, two of which are the

dynamic MLC method and the segmented MLC method. Both the dymamic MLC method

and segmented MLC method use the multileaf collimators (MLC) to modulate the intensity of

the radiation. In the dynamic MLC method the radiation is kept on while the MLCs are mov-

ing to create the intensity variation for the �elds. The intensity variation is created because

the leaf speed are varied to give the correct �uence for the di�erent regions. The speed of the

treatment is optimised if the travel speed of the fastest MLC is maximised. The radiation

intensity is then modulated by varying the travel speed of the slower MLCs [3]. The segmented

MLC method, also called step-and-shoot, instead uses segments. Each �eld is divided into a

number of segments with di�erent shapes and sizes created with the MLCs. The radiation is

o� when the MLCs are moving between the positions for the di�erent segments. The di�erent

techniques of delivering IMRT plans has been described by Williams 2003 [4].

3

2.1 Statistical process control

Statistical process control (SPC) is commonly used as a tool of quality assurance (QA).

Pawlicki and Whitaker 2008 [5] have recently suggested that this also could be a good tool

in radiation therapy. SPC is used by monitoring the process using process behavior charts

(PBCs), which were invented by Walter A. Shewhart in the 1920's as a way of controling

mass production, the PBC was then called control chart and only recently has become to be

known as PBC. The idea is to look at the variation of the result over time instead of judging

each QA (or sample) one at the time. PBC is one of the seven basic quality tools. The seven

quality tools, or the basic seven as it is also called, are cause-and-e�ect diagrams, check sheets,

PBCs, histograms, Pareto charts, scatter diagrams and �owcharts, which are summarized by

Tauge 2004 [6]. Another quality tool, which is not part of the seven basics, is the failure mode

and e�ect analysis (FMEA) which was introduced by Huq et al. 2008 [1] for use in radiation

therapy quality managment together with cause-and-e�ect diagrams.

The PBC are diagrams plotted over time for the actual QA output, to control how the process

changes over time. An average and process behavior limits (PBLs) are always included in the

chart to visualise the dispersion around the mean and to alert when the limits are crossed.

The mean values and the limits are calculated from historical data and need to come from a

stable process. In the PBC speci�cation limits are also drawn, so the analyse of the new data

can be done on time by comparing it to the speci�cation limits. There are also some out of

control signals which alert for faults even if the points in the PBC are inside the PBLs. For

this reason it can be good to draw the 2σ and the 3σ lines in the chart to visualise the other

out of control signals. σ is the standard deviation of the historical data used for calculating

the PBLs and the average. The implementation of more out of control signals will generate a

greater number of false signals. This is a result from the con�dence interval not being 100 %,

so there will be signals appearing to be out of control which infact are just due to routine

variation.

A process can be categorised into one of four states, see Table 1, depending on the behavior

of the process. The di�erent states require di�erent actions to reach and maintain the ideal

state of State I. A predictable process is also called a stable process, a process in control and

a consistent process. An unpredictable process is also called a process out of control.

For a clinic which never has been using PBCs a process is very likely to be unpredictable, in

state III or IV, Pawlicki et al. 2008 [5] states. The advice is then to calculate conditional

PBLs using the �rst 15− 20 points and when the process is predictable recalculate the limits

with 15− 20 points in a row from the predictable process.

There are a number of di�erent charts that can be used, depending on what one is inter-

ested in displaying. Due to the formation of the measurements there will be a di�erence in

4

Table 1: The four di�erent states that a process is categorised into when using process behaviorcharts and the actions required to reach or maintain the ideal state of State I [7].

State Meaning Action

I Within speci�cations Maintain and continue monitoringand predictable

II Out of speci�cations Change equipment and/or proceduresand predictable

III Within speci�cations Identify and remove systematic errorsand unpredictable

IV Out of speci�cations Identify and remove systematic errorsand unpredictable and/or change equipment and/or procedures

subgroup size. A subgroup is a group of measurements that is expected to result in the same

value. Some of the chart types can not be created for measurements consisting of just one

output parameter. Two types of charts are used in this report. An X̄-chart shows the disper-

sion of the measurements, or the dispersion of the subgroup averages if n > 1 where n is the

subgroup size. An R-chart shows the dispersion of the subgroup ranges, which only can be

done if n > 1.The PBLs and the centerlines are calculated using PBC constants [8], shown in

Table 2, and

PBLX̄ = XGA ±A2RA (1)

PBLRup = D4RA (2)

PBLRlow= D3RA (3)

where XGA is the grand average, which is the average of the 15 − 20 data points used for

calculating the PBLs, and RA is the range of the 15 − 20 data points. A2, A3, D3 and D4

are PBC constants. In Equation 2 up stands for upper limit and in Equation 3 low stands for

lower limit. The PBC constant in Table 2 are calculated using a t value of 3, which means

that the con�dence interval is set to 99.7 %, as emperically determined by Shewhart [5].

The PBLs are used to see in which state the process is in. If the process is producing data

points within the PBLs the process is said to be predictable. A process within speci�cation

has a PBC where the PBLs are within the speci�cation limits. This should not be confused

by the fact that a process out of speci�cation can produce all the data points within the

speci�cation limits.

Figure 1 and 2 shows examples of an X̄-chart and an R-chart. The lines drawn are the

grand average of the �rst 20 data points and the PBLs. For the X̄-chart, speci�cation limits

are also drawn. In Figure 1 it can be seen that the PBLs are inside of the speci�cation limits,

5

Table 2: Process behavior chart (PBC) constants for subgroup sizes n and con�dence intervalof 99, 7% [8] (except for n = 1 [7]).

n A2 D3 D4

1 2.660 - -2 1.886 0 3.2683 1.023 0 2.5744 0.729 0 2.2825 0.577 0 2.1146 0.483 0 2.0047 0.419 0.076 1.9248 0.373 0.136 1.8649 0.337 0.184 1.81610 0.308 0.223 1.77711 0.285 0.256 1.74412 0.266 0.283 1.71713 0.249 0.307 1.69314 0.235 0.328 1.67215 0.223 0.347 1.653

which means the process is within speci�cations. It is also a predictable process, while the

data points are within the PBLs. Within speci�cations and predictable means that the process

is in State I.

2.2 Dose measurement

Ion chambers measure the charge collected in the ion chamber during irradiation and to

calculate the dose (Dw,Q) from the collected charge under the reference conditions the IAEA's

TRS-398 formalism [9] is used. Under this formalism the dose is given by

Dw,Q = MQ ·ND,w,Q0 · kQ,Q0 (4)

where MQ is the collected charge, ND,w,Q0 is the calibration factor and kQ,Q0 is a factor

correcting for the di�erence in beam quality. The di�erent beam qualities are denoted with

Q0 for the reference beam, meaning the beam quality used in calibration, and Q for the quality

being used at the actual measurement.

2.2.1 Non-reference condition measurements

As measurement conditions can di�er from the reference conditions de�ned in TRS-398 [9]

must be applied. The correction factor for the temperature and air pressure di�erences (kTp)

is given by

kTp =p0

p· TT0

(5)

where p0 and T0 are the reference pressure and the reference temperature and p and T are the

6

Figure 1: Example X̄-chart for a process in control. The lines drawn in the chart are grandaverage (Average), the process behavior limits (PBL) and the speci�cation limits of 5%.

Figure 2: Example R-chart for a process in control. The lines in the chart are the grandaverage (Average) and the process behavior limits (PBL)

7

pressure and the temperature at the time of dose measurement.

The correction factor for the di�erence in photon energy at calibration (Q0) and measure-

ment (Q), kQ,Q0 , is given by:

kQ,Q0 =kQ

kQ0

=ND,w,Q

ND,w,Q0

=Dw,Q/MQ

Dw,Q0/MQ0

(6)

where kQ is the used beam quality and kQ0 is the calibration beam quality. This method of de-

termining kQ,Q0 requires experimental data. There is a way of calculating kQ,Q0 theoretically,

applying the Bragg-Gray theory using

kQ,Q0 =(sw,air)Q

(sw,air)Q0

(Wair)Q

(Wair)Q0

pQ

pQ0

(7)

where sw,air is the Spencer-Attix water/air stopping-power ratios, Wair is the mean energy

expanded in air per ion pair formed and pQ is a factor representing the combination of all other

perturbation factors. In therapeutic usage the general assumption of (Wair)Q = (Wair)Q0

simpli�es Equation 7 to

kQ,Q0 ≈(sw,air)Q

(sw,air)Q0

pQ

pQ0

. (8)

For non-reference conditions Equation 4 becomes

Dw,Q = MQ ·ND,w,Q0 · kQ,Q0 · ks · kTp · kpol. (9)

2.3 Analysis tools

When analysing the computed and the calculated dose distributions the main goals are to

compare the dose di�erences, ∆Dm, and the distance-to-agreement (DTA), ∆dm, Low et al

1998 [10] states. The dose di�erences are the per cent di�erences between the measured and the

calculated dose distribution, which can be calculated by ordinary subtraction of two images.

Often this is shown as per cent deviation from the reference dose, which in this case is the

measured dose. A signi�cant dose di�erence in high dose gradient regions could be misleading

and therefore the DTA was developed. The DTA is the distance between a measured point

and the nearest point in the calculated dose distribution with the same dose. The analysis

was then performed in two steps, one for the low dose gradient regions and one for the high

dose regions, with di�erent acceptance criteria. One way of using both criteria is to create

an composite distribution and highlight the areas that fails both the dose di�erence and the

DTA criteria.

2.3.1 γ index

Another way of comparing the calculated dose distribution with the measured dose distribution

is to use a method introduced by Low et al. 1998 [10] which uses both the dose-di�erence

criterion and the DTA. The combined comparison can be illustrated schematically as an

8

Figure 3: Schematic two dimensional representation of the combined gamma evaluation [11].The calculated dose distribution is denoted with c and the measured dose distribution isdenoted with m.

ellipsoid, as shown in Figure 3. The surface of the ellipsoid is described by

1 =

√r2(rm, r)

∆d2m

+δ2(rm, r)

∆D2m

(10)

where

r(rm, r) = |r− rm| (11)

and

δ(rm, r) = D(r)−Dm(rm) (12)

where δ(rm, r) is the dose di�erence at position rm.

The γ function also consists of an γ index, which is given by

γ(rm) = min{Γ(rm, rc)}∀{rc} (13)

9

where

Γ(rm, rc) =

√r2(rm, rc)

∆d2m

+δ2(rm, rc)

∆D2m

, (14)

r(rm, rc) = |rc − rm| (15)

and

δ(rm, rc) = Dc(rc)−Dm(rm) (16)

where δ(rm, rc) is the di�erence between dose values of the calculated and the measured dis-

tribution.

This method can be used as a pass/fail criterion, which then becomes:

γ(rm) ≤ 1, calculation passes

γ(rm) > 1, calculation fails

The γ index is also used to discover dose discrepancies in two dimensional measurements. Pix-

els that fail the γ index are highlighted [11]. Childress et al. [12] used a preliminary tolerance

limit that allows a maximum 20% of the pixels to fail the γ index.

2.3.2 Normalised agreement test

The normalised agreement test (NAT) was introduced by Childress and Rosen 2003 [13] be-

cause the lack of an analysis index which gives an output of a number which is clinically

signi�cant. The creation of the NAT is made by looking on the qualities an ideal analysis pa-

rameter would have. An ideal analysis parameter should be biologically signi�cant, meaning

that the change of the parameter is proportional to the e�ect on the patient of the change of

the dose distribution. The parameter should also be physically signi�cant so that it is easy to

understand the change of the parameter. An ideal analysis parameter should also be quickly

computed, consistent over time and comparable between di�erent institutions without any

changes. Independency of image size, representation of dose range and measurement tech-

niques are three other qualities that are desired. All these qualitys are desirable, but it is

almost impossible to include all these qualities into one parameter. The NAT is an approxi-

mate of the ideal parameter. [13]

The NAT consists of two dimensional NAT values and a single valued NAT index. The

NAT value is set to 0 when the dose criteria ∆Dm and ∆dm are ful�led. If the dose criteria

are not ful�led but the computed dose is higher than the measured dose and the pixel of

interest is ouside of the planned target volume (PTV) the NAT value is set to 0 anyway. In

all other cases the NAT value is given by

NAT value = Dscale · (δ − 1) (17)

10

where Dscale is the greater one of the measured and the calculated dose at the pixel of interest

divided with the maximum dose and δ is the lesser of Abs(∆D/∆Dm) and ∆d/∆dm.

The 2D NAT indexes can be converted to a single value NAT index using

NAT index =Average NAT value

Average of the Dscale matrix· 100. (18)

NAT index equal to 0 means that all the pixels pass the NAT criteria, which means that they

pass the dose di�erence, the DTA or are cold areas outside the PTV. NAT indexes greater

than 0 shows that there are pixels that do not pass the NAT criteria. Childress et al. 2005 [12]

used a prelimenary tolerance limit of 45 for the NAT index. The NAT index are dependent on

the methods and dose criteria and DTA set by the user, and it is therefore not recommended

to use the same value.

The NAT values are not recommended to be the only way of analysing the IMRT dose distri-

bution measurements according to Childress and Rosen 2003 [13]. The parameter is a good

way of comparing the analysis of the plane measurements in IMRT plans over time.

3 Materials and Methods

The treatment plans used in this project are created in the tratment planning system (TPS)

Pinnacle version 8.0m (RaySearch Laboratories AB, Royal Philips Electronics). In Pinnacle a

plan consists of several di�erent trials. Each trial is a plan, and this allows the user to create

a number of plans for each patient. These trials are compared with each other to identify

the optimal treatment plan. In this project the trial function was used to create six almost

identical trials for each plan. The trials only di�ered in the maximum allowed number of seg-

ments, which is to be selected for the IMRT calculations. The I'mRT phantom (Scanditronix

Wellhöfer) was scanned and imported into the TPS. There are di�erent possibilities of using

the I'mRT phantom. Figure 4 shows the di�erent parts of the phantom. Part (a) and (c)

of Figure 4 shows the head and neck part with the white shoulders on, which is used for ion

chamber measurements for the plans performed in the prostate part of the I'mRT phantom.

The prostate part of the I'mRT phantom can be seen in part (b), this part is CT scanned

and the prostate plans are planned on the resulting CT images and the �lm measurements is

performed in that part of the I'mRT phantom. The head and neck part of the I'mRT phantom

without the shoulders was also CT scanned and the head and neck plans are planned using the

resulting CT images. The head and neck part of the I'mRT phantom have a lenght of 18 cm

and the prostate part of the I'mRT phantom has an length of 15.5 cm. To get around the

di�erence in length of the two I'mRT phantom parts used for measurements of the prostate

plans the middle of the phantom part is used as the reference point for placements. The

prostate part of the I'mRT phantom has needles in all slices, so that it is easily seen where

11

the �lm was placed during exposure. In the rest of this report the I'mRT phantom parts are

called after the plan type that is measured in them (head and neck or prostate).

Figure 4: Parts of the I'mRT phantom (Scanditronix Wellhöfer). Part a) shows the prostatepart of the I'mRT phantom used for the ion chamber measurements. The white parts on theedges can be taken o� and that is the head and neck part of the I'mRT phantom used forboth �lm and ion chamber measurements. Part b) of the image is the same part of the I'mRTphantom from another angle and part c) shows the prostate part of the phantom used for �lmmeasurements.

The exposures were carried out with an Elekta Synergy linac located at Perth Radiation

Oncology, Wembley, Perth (PRO). The machine uses the step-and-shoot technique to deliver

the doses.

3.1 Planning

Pinnacle consists of several institutions, which divides the program into di�erent parts. The

division into institutions prevent for example research and clinical work to interfere. The

institutions are created by the user and in this project the institution named Frida is used.

When the institution is chosen computed tomography (CT) images are imported and two

di�erent patients are created; IMRT_prostate and IMRT_head&neck. For each patient a

number of plans can be created. In this project two plans are created for each patient. The

IMRT_prostate plans are 01sym and 02sym and the IMRT_head&neck plans are 01plan and

02plan. For each plan six trials are created (05seg, 15seg, 25seg, 35seg, 45seg and 55seg).

The planning procedure begins by de�ning the couch of the CT in the setup part of Pin-

12

nacle. Contours are drawn which represents the target and sensitive areas, the geometry for

the four plans can be seen in Figure 5 to 8. The isocentre is placed automatically within the

center of the ROI. Five beams are added to the plans, for plan 01sym the angles are 0, 60,

150, 260 and 300 and for the three other plans the angles are 0, 45, 105, 255 and 315 (see

Figure 9). All beams are given the energy of 6 MV. A dose grid is drawn, that must cover the

volume in which the dose is to be calculated. For this project the whole phantom is covered

with the dose grid. In the evaluation part of Pinnacle the values of the isodose curve that

is to be displayed are chosen. The dose calculations are done in the IMRT part of Pinnacle,

but before the calculations are performed the treatment objectives and constraints, shown in

Table 3, are added for the created plans. The �nal adjustment is done for each trial in the

IMRT parameters window choosing the direct machine parameter optimisation (DMPO) and

the maximum number of segments, which is varied for the six trials in each plan (5, 15, 25,

35, 45 and 55). In the IMRT parameter window the number of iterations are also chosen. The

optimisation of the trials can then be started.

The plans are exported to Mosaiq from where the plans can be reached and used at the

linac to perform the �lm and ion chamber exposures.

Table 3: The dose objectives for the four plans (01plan, 02plan, 01sym and 02sym) createdin Pinnacle version 8.0m.

Plan Volume of interest Constraint Prescription Weight

01plan Target Uniform dose 70 Gy 20Sens Max dose 30 Gy 5Top Max DVH 10 Gy 30% 5Back Max DVH 20 Gy 20% 10

02plan Target Uniform dose 70 Gy 20Suround Max dose 30 Gy 10

Max DVH 25 Gy 30% 5Top Max DVH 20 Gy 50% 5

01sym Target Uniform dose 70 Gy 20Max DVH 75 Gy 50% 1

Square Max dose 35 Gy 10Sens Max Dose 20 Gy 5

02sym Target Uniform dose 70 Gy 20Back Max dose 40 Gy 20

Max DVH 10 Gy 70% 1Sense Max DVH 20 Gy 30% 5

3.2 Ion chamber

To control the consistency of the calculated dose compared to the measured dose an absorbed

absolute dose is measured for one point. To reduce uncertainty in the measured dose, the

13

(a) (b)

(c) (d)

(e) (f)

Figure 5: The ROIs for plan 01plan in two di�erent planes for three di�erent viewing angles,(a) and (b) inferior, (c) and (d) left and (e) and (f) anterior. The red volume of interest (VOI)is the target. Images taken from Pinnacles version 8.0m

14

(a) (b)

(c) (d)

(e) (f)

Figure 6: The ROIs for plan 02plan in two di�erent planes for three di�erent viewing angles,(a) and (b) inferior, (c) and (d) left and (e) and (f) anterior. The red volume of interest (VOI)is the target. Images taken from Pinnacles version 8.0m

15

(a) (b)

(c) (d)

(e) (f)

Figure 7: The ROIs for plan 01sym in two di�erent planes for three di�erent viewing angles,(a) and (b) inferior, (c) and (d) left and (e) and (f) anterior. The red volume of interest (VOI)is the target. Images taken from Pinnacles version 8.0m

16

(a) (b)

(c) (d)

(e) (f)

Figure 8: The ROIs for plan 02sym in two di�erent planes for three di�erent viewing angles,(a) and (b) inferior, (c) and (d) left and (e) and (f) anterior. The red volume of interest (VOI)is the target. Images taken from Pinnacles version 8.0m

17

(a) (b)

Figure 9: The two di�erent beam geometries used. Part (a) showing the geometry used forplan 01sym and part (b) showing the geometry used for plans 02sym, 01plan and 02plan.

point has to be situated in an low dose gradient region. This is commonly found in the target,

because a uniform dose distribution was the highest weighted objective for optimisation of

the treatment plans (see Table 3). The measurements are performed with an ion chamber

(CC13/TNC, IBA, Scanditronix Wellhöfer, serial number 6689), that is positioned at the

chosen measurement point in the I'mRT phantom. The charge is collected with an electrometer

(NE 2570/1 Farmer dosemeter, NE Technology, serial number 1212). The ion chamber is

exposed to three 10 × 10 cm2 calibration �elds of 100 MUs in the I'mRT phantom without

moving the chamber or phantom. With a knowledge of the per cent depth dose (PDD), the

output factor (OF) and that the linac is calibrated to deliver 1 cGy/MU at a given reference

point under standard conditions, the dose per MU (D/MU) are given by

D/MU =OF

PDD·SDD2

ref

SDD2used

(19)

where SDDref is the source-to-detector distance if the measurements were performed with a

source-to-surface distance of 100 cm and the SDDused is the source-to-detector distance used

in the calibration exposures.

The electrometer is kept in the measurement setting for all the �ve beams in the IMRT plan,

which are delivered with their planed gantry and collimator angles. The collected charge is

noted after each beam.

The measured doses (Dm) are compared to the calculated doses (Dc) for all the points using

[7]

18

P =Dm −Dc

Dm· 100. (20)

The comparison is performed for the total measurement and for the individual beams for all

four plans.

3.3 Film

IMRT planar doses can be measured and compared to the calculated doses in the same plane.

The comparison between the measured dose and the calculated dose can be performed in

di�erent ways, in this project �lm dosimetry is used. There are a number of di�erent �lm

types to choose from and in this project Gafchromicr EBT �lm was chosen. EBT �lm is

radiochromic �lm, which is easy to handle because it is self developing. The dose range for

the �lm is 1− 800 cGy [14]. The �lm batch used in this project is number 47277-06I.

The EBT �lm is considered a good dose integrator as tests by International Specialty Products

(ISP) by comparing the darkening of �lms exposed to 5 Gy in one fraction or in 1 Gy steps

over intervals of half an hour have shown [14]. The EBT �lm is used in this project to measure

IMRT plans that consists of beams of approximately 40 cGy each under the prescription 200

cGy and the number of beams are �ve. Each beam consist of several segments, so that the

dose is delivered in very small fractions within a short time interval. The question is then if

the EBT �lm is an good integrator in this case as well, meaning smaller dose steps and smaller

time intervalls between the exposures.

3.3.1 Calibration

The Gafchromicr EBT �lm must be calibrated, so the darkening of the �lm can be related to

a dose. The calibration was done in two di�erent ways, one step-and-shoot wedge calibration

and one fraction calibration.

A step-and-shoot wedge was created in Pinnacle. A step-and-shoot wedge is a staircase pat-

terned wedge, which is created by starting with a long �eld (5× 20 cm2) which is shorten in

steps of 2 cm. An even staircase pattern is created by assigning all the �elds the same number

of MUs, in this case 25 MU. A solid water phantom is positioned under the gantry and a

Farmer chamber (PTW 30013, serial number 1677) connected to an electrometer (NE 2570/1

Farmer dosemeter, NE Technology, serial number 1212) is placed at the depth of 29.5 mm

and with back scatter material of approximately 10 cm. Each dose step in the wedge was

measured, exposing the whole wedge for each measurement. Two measurements are also per-

formed just outside of the lowest and the highest dose steps. Three strips of �lm from three

di�erent sheets of �lms were exposed to the step-and-shoot wedge, to investigate if each sheet

of �lm has to be calibrated or if it is su�cient to calibrate one piece of �lm per batch of �lms.

The three strips of �lm gives the same relation between the darkening of the �lm and given

dose, as shown in Figure 10.

19

Figure 10: The mean pixel value as a function of dose for three strips of �lm exposed to thestep-and-shoot wedge.

Another plan was created in Pinnacle consisting of 20 �elds in pair given the same number of

MUs as the di�erent steps in the wedge calibration. Ten of the �elds are delivered all MUs

in one exposure and the other ten are given their MUs in small segments of �ve MUs. Three

of the �elds were measured with an ion chamber (PTW 30013, serial number 1676) and an

electrometer (NE 2570/1 Farmer dosemeter, NE Technology, serial number 1087) and those

are compared to the doses calculated in Pinnacles. The time consumed to do one exposure of

all twenty �elds are approximately 20 min, and to measure each �eld would take more time

than was available. One sheet of �lm was exposed to the fraction calibration plan.

The two methods of calibration were made to investigate how to do the calibration expo-

sure and analysis as accuratly, fast and simply as possible. The two di�erent calibration

methods also investigate how the darkening of the �lm depends on fractionation.

The di�erent calibration methods was found to give the same dose for the pixel values (PV),

as illustrated in Figure 11. The step-and-shoot wedge is chosen to be used because it is much

more time e�cient. The step-and-shoot wedge calibration method is also more inexpensive,

while less �lm is used. The analysis could be done simply by rotating the image of the strip

of �lm in Image J version 1.5 and take out a pro�le. To make it consistent with the analysis

in DoseLab4 the TIFF image of the calibration strip is splitted into the red, green and blue

parts and the red image is saved and used.

20

Figure 11: The mean pixel values of the RGB-image as a function of dose for the step-and-shoot wedge calibration method, wedge, and for the fractioning calibration method, includingthe fractionated �elds, frac, and the �elds exposed in one exposure, whole. The error bars arein most cases smaller than the points.

Figure 12: A pro�le over a strip of �lm exposed to the step-and-shoot wedge. The pro�le wasdone in Image J version 1.5 for the red part of the image.

21

Table 4: The mean pixel value for all dose steps taken from the dose pro�le given by Image Jversion 1.5 in Figure 12.

Dose (cGy) Mean Pixel Value

2.448 420572.235 383351.994 354931.745 331731.480 310551.236 293630.997 275750.751 263860.508 253400.267 24384

3.3.2 Scanner

The �lms are digitized in the Epson perfection V700 photo scanner. This particular �at bed

scanner at PRO Wembley had not been used for this purpose before and a thorough analysis

is therefore performed. This analysis should include examination of the time, placement and

scanner procedure dependency, while the PV can vary signi�cantly with these parameters

according to Martisikova et al. 2008 [15] and Paelinck et al. 2007 [16]. A number of di�erent

scans were performed on the scanner, using a small piece of unexposed �lm, from the same

batch of �lm as the IMRT exposed �lms.

In Image J a region of interest (ROI) was placed over the di�erent images and the mean

pixel value and the standard deviation in the ROI was noted. The ROI used was almost as

big as the �lm piece and placed central to the �lm.

The procedure of scanning can di�er from person to person and from time to time. One

thing that is hard to get exactly the same is the margin size, which is the margin set in the

scanner program between the �lm and the edge of the scan, see Figure 13. The �lm was

therefore scanned in the same position using di�erent sizes of the margins, results are shown

in Figure 14.

It was found that the margin size does not in�uence the scanner output more than the pixel

variation, but to be on the safe side the scans were performed with the same margin size. For

the prostate measurements a whole sheet of �lm is used. The sheets of �lm are just some

smaller than the bed area and therefore the scan area was set equal to the bed area. For the

head and neck measurements 16× 16 cm2 pieces of �lms is used. As there is not an easy way

of getting the margin sizes equal from one scan occasion to another the scan area is set equal

to the bed area.

22

Figure 13: Figure showing the geometry for scanning and analsying the �lms, (a) bed area,(b) scan area, (c) �lm area and (d) ROI.

Figure 14: Scanner output variations for scans of an small unirradiated piece of �lm wherethe margin scan has been varied, (1) no margin, (2) small margin and (3) the whole scannerarea scanned. The change in mean pixel value is not signi�cant for the di�erent margin sizes.

23

The scanner can have a warm-up time during which the scans can have a greater varia-

tion than due to routine variation. This was investigated through multiple identical scans

over a period of time, see Figure 15. After pauses of more than 13 min between scans the

scanner turned itself o� and a pause with automatic warm-up was done before the next scan.

Warm-up pauses happened before the last two scans in the scan series. Another series of scans

was performed, scanning a sheet of �lm consecutively 30 times, the data for which are shown

in Figure 16. The thirty scans took 13 min to perform. To exclude any other dependency the

piece of �lm was not moved between the scans.

Figure 15: Scanner output variations for scans of a small unirradiated piece of �lm, placed inthe middle and with a small margin. The two latest scans are done with so long pause thatthe scanner had to warm up again. The result is not signi�cantly dependent on the time,except for the �rst scans. The �lm is darkened by the lamp of the scanner.

The time dependency of the �lm is found to be important just for the �rst scans and by doing

some other scans �rst the problem is avoided. Martisikova et al. 2008 [15] scans all �lms three

times and use the third �lm for analysis and Paelinck et al. 2007 [16] do two dummy scans

and then use an average of the next three scans. In this project all �lms are scanned at the

same time and the scanner was used for other scans just before the scans were made with a

time gap of approximately 4 min, which is lesser than the 13 min it was found to take the

scanner to turn itself o�. When the scanner has been turned o� it has to warm up again. It

is not recommended to use the �rst scans after the scanner has turned itself o�.

The scanner can also have varying output depending on where on the scanner bed the �lm is

placed. To investigate the placement dependency of the scanner a piece of �lm was scanned

on di�erent positions on the bed area, the di�erent positions are shown in Table 5.

The output of the scanner varies depending on the placement on the bed area and therefore

24

(a) (b)

(c) (d)

Figure 16: Scanner output variations for 30 conscutive scans of an unirradiated sheet of �lm.The analysis was done with three di�erent ROI sizes, (a) small sized ROI (384 pixels), (b)medium sized ROI (10912 pixels) and (c) ROI covering the whole �lm sheet (377024 pixels).Part (d) shows all the di�erent ROI sizes (a), (b) and (c) drawn in the same diagram withouterror bars. For all the scans the scan area was equal to the bed area. The change is mostlydue to the lamp of scanners in�uence of the darkening of the �lm. The result is not signi�cantdependent except for the �rst number of scans.

Table 5: The six di�erent placements on the �atbed scanner used to investigate the placementdependency. The speci�ed distances are from the �lm to the right edge of the scanner and tothe bottom edge of the scanner, if standing in front of the scanner.

Number Distance from right (cm) Distance from bottom (cm)

1 7 122 7 183 7 74 11 125 2 126 11 21.5

25

Figure 17: Scanner output variations for scans of an small unirradiated piece of �lm wherethe placement of the �lm has been varied, the six di�erent placements can be seen in Table5. The result is signi�cant dependent on the placement on the bed area.

the �lm is placed in the same postion for all measurements.

The analysis of the scanner showed that it is possible to use it for digitizing �lms. The

analysis program to be used (DoseLab4) require TIFF �les which is selected in the scanning

software. The EBT �lm has an absorption maxima at about 635 nm [14], which is in the red

part of the spectra. To be able to extract the red part of the image the EBT has to be scanned

in colour. All the settings were made as recommended by ISP [17].

3.3.3 Measurements

A plane in the phantoms is measured for two of the treatment plans, one in the head and neck

part of the phantom (01plan) and one in the prostate part of the phantom (02sym). The �lm

was placed in the chosen plane in the phantom and irradiated with all the �elds. This was

done for all six trials for both of the plans. The �lms are scanned placed on the bed area as

shown in Table 6.

Table 6: The placement on the bed area for the �lms used for measuring the dose distribution.

I'mRT phantom part Distance from right (cm) Distance from bottom (cm)

Head and neck 3 6Prostate 0.5 3

26

3.3.4 Calculated dose distributions

The calculated dose distributions are exported from Pinnacle as planar dose maps for the plane

which were measured with the EBT �lm. The planar dose maps from Pinnacle are saved as

text documents. The doses given by Pinnacle are for the whole treatment, which consists of

35 fractions. The comparison between the two dose distributions is to be compared for just

one fraction and therefore the doses are scaled by 1/35.

3.3.5 DoseLab

Before doing the analysis in DoseLab the preference �les are edited to make the program �t

the project and the departments needs. In Patient information.txt the department speci�c

information was entered. Dose comparison.txt is the largest preference �le and while just one

type of comparison is to be used, the �le was just changed for this type of comparison, Image

to Image - needs registration. The pixel sizes used for the calculated dose distribution and the

measured dose calculation was entered. The dose conversion factor was set to 0.1 for both

dose distributions. In Default values.txt the used pixel sizes are set to default and the dose

criteria and DTA are set to the value investigated to be the most suitable.

Before the comparison of the calculated dose and the measured dose some processing of the

�les has to be done. In DoseLab there are a number of auxiliary tools which can be used to

process the �les. The pixel size for the calculated dose distribution is 4 mm, which was con-

verted to resolution using the Resolution converter auxiliary tool giving 6.35 dpi. The scaling

factor (1/35) and the resolution was entered in the Batch TPS to TIFF conversion auxiliary

tool that converts the text document to a TIFF image which can be analysed in DoseLab.

The scanned �lm images are rotated to be in the same con�guration as the calculated dose

distribution. The head and neck images are rotated 90◦ to the right and �iped horisontal and

the prostate images are �ipped vertical in Image J. The images are splitted into the red, green

and the blue part in Image J and the red part of the image is saved. The pixel values in the

red part of the image are converted to dose using the Film to dose auxiliary tool. The Film

to dose auxiliary tool uses a calibration �le, which need to be created manually. This is done

using the pixel values and doses from the step-and-shoot wedge calibration. The calibration

�le is a CSV �le, which consists of three rows. The �rst row is the PV for each dose step,

the second row is the measured doses for each dose step and the �rst number in the third row

decides which type of �t should be applied on the data. In this project the third order �t is

used. The third row have to consist of the same number of columns as row one and two and

the remainder is �lled with zeros.

The analysis are performed in the main window of DoseLab choosing the Image to Image

- needs registration dose comparison type. The caculated dose distribution and the measured

dose distribution are uploaded and the output �les are named appropriately after the phantom

27

type and number of segments. The process button is pushed and the data for the trial are

entered into the sheet which is shown. In the sheet the maximum dose is to be entered, this

is the maximum dose for the measured plane and is found in the measured dose distribution

using the maximum formula in excel. The two dose distributions are in the same position

and the rotation are not performed in DoseLab but the measured dose distribution must be

cropped. The analysis can only be performed if the measured dose distribution is smaller

in both x and y direction compared with the calculated dose distribution. DoseLab has an

autopositioning tool which places the dose distributions at the same position. The position of

the autopositioning tool has been used in the project. The contourlines that is to be shown in

the output images are chosen to 55 cGy, 105 cGy, 125 cGy, 145 cGy, 165 cGy, 185 cGy. Dose-

Lab then computes a number of comparison parameters and displays a number of diagrams

and �gures. The di�erent �gures and diagrams are all highlighting di�erent ways of making

the dose di�erence visible. In this project the compared contour �gure and the gamma �gured

are mostly used.

Another analysis is also performed in DoseLab, comparing two calculated dose distributions.

The two distributions used are the trial with 5 segments and the trial with 55 segments. The

trial with 5 segments is entered where the calculated dose distribution is to be entered and

the trial with 55 segments is entered where the measured distribution is to be entered. The

maximum dose for the trial with 55 segments is chosen. The comparison between the calcu-

lated dose distributions is performed for 01plan and 02sym.

In DoseLab the reference dose distribution is chosen to be the calculated dose distribution,

while this normally gives smoother contour lines [13].

The analyses performed in DoseLab are done with the user chosen dose di�erence and DTA.

The �rst analysis was performed with three di�erent sets of dose criteria (1mm/1%, 3mm/3%,

5mm/5%) to investigate which criteria to use. It is recommended to use the criterias as action

limits and not pass/fail test [2]. The criterias that is found to be the most suitable are used

both for the γ index and the NAT index calculations.

3.4 Process behavior charts template

The SPC tools used in this project is the PBC. An excel PBC template has been created to

simplify the process of creating the PBCs. In the template it has been chosen just to draw the

average and the PBLs and not any σ lines. To draw a PBC with this template the output of

the measurments are inserted into the Prostate or the H&N sheet, if the number of measure-

ments exceeds 20 the PBLs and the grand average will be calculated and drawn in the charts.

Both an X̄-chart and an R-chart will be drawn. The template can easily be changed to suit

the needs of di�erent departments. The number of �elds has in this project been chosen as

�ve for the measurement in both phantom parts.

28

Two di�erent methods of drawing the X̄-chart is tested, one using the total dose of all �ve

beams (n = 1) and one using the measurements for each beam (n = 5). The two di�erent

X̄-chart is then compared to investigate if one of the methods generates a stable process. The

method that creates the best PBC will be used in this template.

In this project the per cent di�erences of the measured dose and the calculated dose (see

Equation 20) are plotted in the PBCs. The measurements in the di�erent parts of the I'mRT

is separated and plotted in di�erent PBCs. The NAT indexes from the �lm measurements

will also be plotted in PBCs to investigate if it is possible.

4 Results

4.1 Ion chamber

The result of the ion chamber measurements is shown in Figure 18 to 21. The result is shown

both for all �elds in diagrams and for each beam in histograms. The calculated doses are

lower than the measured dose for 01plan and 02plan, see Figure 18 and 19, and for 01sym and

02sym it is the opposite way, see Figure 20 and 21. The tendency can be seen both for the

measurement of all �elds and for the measurements of the individual �elds.

In Figure 19(a) the dose for the trial with the smallest number of segments is lower than all the

other trials. The same thing can be seen in Figure 21(a) for the trial with the smallest number

of segments, but the di�erence is not as big as in Figure 19(a). The measurements of all �elds

for the prostate plans (01sym and 02sym) are good, meaning the per cent di�erence between

the measured and calculated dose are not bigger than 5%, for all measurements but one, the

01sym with 15 segments (see Figure 20(a)). The measurements of all �elds for the head and

neck plans (01plan and 02plan) are all within 5% of the calculated dose. The measurement of

the individual �elds shows a bigger di�erence between the measured dose and the calculated

dose. The head and neck measurements of the individual �elds are all within 10%, see Figure

18(b) and 19(b). The prostate measurements of the individual �elds are all within 15% except

for one �eld, see Figure 20(b) and 21(b).

4.1.1 Process behavior charts

PBCs drawn for the ion chamber measurements in the I'mRT phantom can be seen in Figure

22 to 27. The ion chamber measurements in this report were not enough in number to be able

to do a real PBC, so the limits has been calculated using all the measurements.

The X̄-chart is drawn both for n = 1 and n = 5 and the result can be seen comparing

Figure 22 and 23 and Figure 24 and 25. For the measurements in the head and neck part

of the phantom it can be seen that the process is in State II for both methods, see Figure

29

(a)

(b)

Figure 18: The result of the ion chamber measurements in the head and neck part of theI'mRT phantom for 01plan. Part (a) showing the measurement of the whole treatment planand part (b) showing a histogram for the per cent di�erence for measurements of each beamof the treatment plan.

30

(a)

(b)

Figure 19: The result of the ion chamber measurements in the head and neck part of theI'mRT phantom for 02plan. Part (a) showing the measurement of the whole treatment planand part (b) showing a histogram for the per cent di�erence for measurements of each beamof the treatment plan.

31

(a)

(b)

Figure 20: The result of the ion chamber measurements in the prostate part of the I'mRTphantom for 01sym. Part (a) showing the measurement of the whole treatment plan and part(b) showing a histogram for the per cent di�erence for measurements of each beam of thetreatment plan.

32

(a)

(b)

Figure 21: The result of the ion chamber measurements in the prostate part of the I'mRTphantom for 02sym. Part (a) showing the measurement of the whole treatment plan and part(b) showing a histogram for the per cent di�erence for measurements of each beam of thetreatment plan.

33

Figure 22: A PBC X̄-chart for the ion chamber measurements in the I'mRT phantom for01plan and 02 plan, each with six di�erent trials where only the maximum number of segmentshas been varied. The PBC is made using the measurment of each beam (n = 5). The outputplotted is the per cent di�erence between the measured dose and the calculated dose.

Figure 23: A PBC X̄-chart for the ion chamber measurements in the I'mRT phantom for01plan and 02 plan, each with six di�erent trials where only the maximum number of segmentshas been varied. The PBC is made using the total measurment of the treatment plan (n = 1).The output plotted is the per cent di�erence between the measured dose and the calculateddose.

34

Figure 24: A PBC X̄-chart for the ion chamber measurements in the I'mRT phantom for01sym and 02sym, each with six di�erent trials where only the maximum number of segmentshas been varied. The output plotted is the per cent di�erence between the measured dose andthe calculated dose.

Figure 25: A PBC X̄-chart for the ion chamber measurements in the I'mRT phantom for01sym and 02sym, each with six di�erent trials where only the maximum number of segmentshas been varied. The PBC is made using the total measurment of the treatment plan (n = 1).The output plotted is the per cent di�erence between the measured dose and the calculateddose.

35

Figure 26: A PBC R-chart for the ion chamber measurements in the I'mRT phantom for01plan and 02plan, each with six di�erent trials where only the maximum number of segmentshas been varied.

Figure 27: A PBC R-chart for the ion chamber measurements in the I'mRT phantom for01sym and 02sym, each with six di�erent trials where only the maximum number of segmentshas been varied.

36

22 and 23. The data points are inside the PBLs, but he PBLs are not inside the speci�ca-

tion limits. The measurements in the prostate part of the phantom are a bit more dispersed

than the measurments in the head and neck part of the phantom. The PBLs are outside

the speci�cation limits, see Figure 24 and 25. Figure 24 is showing a process in State IV,

as can be seen while one data point is outside the PBLs. Figure 25 is showing a process in

state II, as can be seen while the data points are all within the PBLs. The PBC template is

constructed to only draw the X̄-chart for the measurements of the individual �elds (n = 5),

this choice was made based on the fact that if the process is stable, the speci�cations will be

met if the PBLs are inside the speci�cation limits, which is easiest reached by using subgroups.

4.2 Film

4.2.1 Criteria

Three di�erent gamma images, Figure 28, and three di�erent NAT images, Figure 29, are

compared to decide which criteria is to be used. The NAT indexes and the number of pixels

with NAT index equal to 0 is also displayed, Table 7 and Figure 30.

Table 7: The normalised agreement test (NAT) index and the per cent of pixels having a NATindex value of 0 for the �ve di�erent dose criteria and DTA that are tested.

DTA/Dose criteria NAT index NAT index= 0 (%)

1mm/1% 61.8 75.12mm/2% 17.6 86.93mm/3% 7.0 92.94mm/4% 3.4 96.65mm/5% 2.0 98.6

The choice of the criteria is made so that as much information as possible can be seen in the

�gures. The criterias should also be clinical signi�cant. Budgell et al. 2004 [2] concluded that

the criteria of 3mm/3% should be used.

4.2.2 Dose di�erence comparision

The dose di�erences are visualised in combined contour images, see Figure 31 and 32, where

the solid lines are the calculated dose contours and the dashed lines are the measured dose

contours. The dose di�erences can also be seen in the gamma images Figure 33 and 34. A

third way of looking at the dose di�erence is to look on the number of pixels failing di�erent

criterias, see Table 8.

In Table 8 the head and neck parameters are almost similar in value. The per cent passing

the gamma index has the highest value for the measurement performed with the least number

of segments. The gamma �gure of the same measurement (Figure 33(a)) is the one with the

37

(a) (b)

(c)

Figure 28: The comparison of how the gamma image depends on the dose criteria and theDTA for 02sym and the trial with a maximum number of segments of 5. The di�erent criteriaused are (a) 1mm/1%, (b) 3mm/3% and (c) 5mm/5%.

38

(a) (b)

(c)

Figure 29: The comparison of how the normalised agreement test (NAT) image depends onthe dose criteria and the DTA for 01plan and the trial with a maximum number of segmentsof 5. The di�erent criteria used are (a) 1mm/1%, (b) 3mm/3% and (c) 5mm/5%.

39

Figure 30: The normalised agreement test (NAT) indexes for di�erent set of dose criteria andDTA, (1) 1mm/1%, (2) 2mm/2%, (3) 3mm/3%, (4) 4mm/4% and (5) 5mm/5%.

Table 8: Per cent di�erence of di�erent analysis parameters derived from DoseLab4 for 01planand 02sym.

Treatment site/ Per cent Per cent passing Per cent passing both Per centNumber of segments passing DTA dose di�erence DTA and dose di�erence passing gamma

H&N/5 50.5 10.7 56.9 45.2H&N/15 47.9 13.5 50.2 43.2H&N/25 49.1 13.3 51.7 41.0H&N/35 45.7 8.5 48.0 43.6H&N/45 52.4 5.7 53.0 40.7H&N/55 50.1 8.2 50.7 40.9

prostate/5 36.8 40.3 56.1 71.4prostate/15 45.9 50.9 71.3 91.6prostate/25 51.4 57.4 77.5 93.3prostate/35 42.5 45.5 66.8 85.2prostate/45 50.4 60.4 76.5 93.2prostate/55 57.5 56.9 76.5 88.4

40

(a) (b)

(c) (d)

(e) (f)

Figure 31: Combined contours images for the �lm measurements for 01plan in the head andneck part of the I'mRT phantom taken from DoseLab4. Solid lines are the calculated dosedistribution and the dashed lines are the measured dose distribution.

41

(a) (b)

(c) (d)

(e) (f)

Figure 32: Combined contours images for the �lm measurements for 02sym in the prostate partof the I'mRT phantom taken from DoseLab4. Solid lines are the calculated dose distributionand the dashed lines are the measured dose distribution.

42

(a) (b)

(c) (d)

(e) (f)

Figure 33: Gamma images taken from DoseLab for the analysis of the measurements of 01plandone in the head and neck part of the I'mRT phantom.

43

(a) (b)

(c) (d)

(e) (f)

Figure 34: Gamma images taken from DoseLab for the analysis of the measurements of 02symdone in the prostate part of the I'mRT phantom.

44

least yellow/red pixels as well. which means the same thing. By comparing the di�erent parts

of Figure 33 the pixels failing gamma (yellow/red pixels) seems to move into the target as the

number of segments are increased.

The prostate measurements instead shows a bigger tendency with all the parameters for the

measurement with the lowest number of segments having the smallest value. By comparing

Figure 34(a) to the rest of the images in Figure 34 it also has the biggest number of yellow/red

pixels.

4.2.3 Normalised agreement test

The NAT indexes are shown in Table 9, as for the ion chamber measurements the �lm mea-

surements also show the opposite tendencies when the di�erent plans are compared. The head

and neck measurements has the lowest NAT index for the exposure with the least number of

segments and the prostate measurements has the highest NAT index for the exposure with the

least number of segments. The head and neck NAT indexes are all higher than the prostate

NAT indexes.

Table 9: Normalised agreement test (NAT) indexes and per cent of pixels with a NAT valueof 0 derived from DoseLab4 for 01plan and 02sym.

Treatment site/ Per cent withNumber of segments NAT index NAT value= 0

H&N/5 13.4 82.9H&N/15 33.9 75.4H&N/25 34.1 76.6H&N/35 39.7 75.3H&N/45 37.4 73.9H&N/55 35.3 73.3

prostate/5 7.0 92.9prostate/15 1.3 95.4prostate/25 2.9 94.0prostate/35 4.3 90.3prostate/45 1.0 96.5prostate/55 2.6 94.0

4.2.4 Process behavior charts

It is possible to draw a PBCs for the NAT indexes. The data can be shown over time and

PBLs can be calculated. The speci�cation limits are not set in this project.

4.3 Number of segments

The solid lines in Figure 31 and 32 are the calculated dose distributions. Comparisons by

eye between the 180 cGy contour for 01plan (Figure 31) shows a big resemblance, but the

45

Figure 35: A PBC X̄-chart for 01plan using the NAT-indexes derived from DoseLab4.

Figure 36: A PBC X̄-chart for 02sym using the NAT-indexes derived from DoseLab4.

46

trial with 5 segments are wider and have a rounder shape than the other. The same kind of

comparison of the 02sym (Figure 32) shows bigger discrepancies, but just for the two trials

with the least number of trials.

The comparison in DoseLab shows that what was seen by eye is true, see Figure 37 and

38. The resemblance for 01plan is big, as can also be seen in Table 11. The parameters for

the 02sym comparison are a bit worser, especially for the NAT index.

Figure 37: Combined contour image for the comparison of the calculated dose distributions ofthe trial with 5 segments (solid lines) and the trial with 55 segments (dashed lines) for 01planderived from DoseLab4.

Table 10: Parameters derived from DoseLab4 for the comparison between calculated dosedistributions of the trial with 5 segments and the trial with 55 segments. The 5 segment trialwas used as calculated dose distribution and the 55 segment trial was used as measured dosedistribution.

Parameter 01plan 02sym

NAT index 5.8 103.3NAT index= 0 93.6% 57.2%Pass γ index 77% 42.6%

Pass dose criteria 27.3% 17.1%Pass DTA 46.4% 26.4%

47

Figure 38: Combined contour image for the comparison of the calculated dose distributions ofthe trial with 5 segments (solid lines) and the trial with 55 segments (dashed lines) for 01planderived from DoseLab4.

48

5 Discussion

The �ducial points, the marks the needles make in the �lm in the prostate part of the phantom,

will create incorrect analysis, but for a very small point. It can easily be seen in the NAT

images in Figure 29 and also in some of the gamma images for the prostate measurements,

see Figure 34.

5.1 Treatment plans

The treatment plans produced in this project are random geometries. The �rst plan that was

produced, 01sym, has to small target. The target was big enough to place an ion chamber in

it and get good dose agreement, but it was hard to �nd a point with even dose distribution.

The second plan done to the prostate part of the I'mRT phantom (02sym) was done with

bigger ROIs, both for the target and for the sensitive ROIs. The �rst plan done on the head

and neck part of the phantom (01plan) was done with the same thinking as for 02sym. The

02plan was done to try to be a bit more challenging to the dose optimisation, with a ROI

which is c-shaped around one end of the target.

The trials with �ve segments are the same as 3D CRT. With one segment per beam there are

no possibility to modulate the intensity of the �elds, but the shape of each �eld can be made

to �t the target and try to avoid the sensitive areas with the IMRT inverse planning algorithm

used by Pinnacle.

The number of segments does not change the dose distribution that much for the two trials

measured with �lm. Geometries with irrigular shapes are expected to show bigger discrepan-

cies for trials with di�erent numbers of segments.

5.2 γ index and NAT index

The per cent of the pixels failing the γ index for the head and neck measurements are all

over 50%, which is more than twice the allowed failed per cent of pixels that Childress et al.

[12] uses. The NAT indexes are though all under their tolerance limit and the limits are used

together as complements.

The NAT index and the γ index can show di�erent result, this because of the di�erent methods

of calculation. The NAT index ignores cold areas outside the PTV contrary to the γ index

which takes these into acount as well. Cold areas are areas where the measured dose is lower

than the calculated dose. Outside the PTV these should not be as clinically signi�cant as

when the cold spots occur within the high dose region [13].

The comparison in DoseLab gives both the per cent of the pixels that pass the γ index and the

NAT index. The calculation of these to parameters will not take any extra time and to note

49

them in a list or put them into the PBC will not take much extra time. A data base of these

parameters could be useful in the future and perhaps facilitate the IMRT QA. With a few

of the parameters collected, the size of the parameters can be investigated and preliminary

department speci�c speci�cation limits can be set. It is still recommended to do the �nal

decision based on dose comparison images.

5.3 Dose distribution

The investigation of the accuracy of the calculated dose distribution has in this project been

chosen to be visualised using combined contour images and γ images. The combined contour

images is a good tool for the physicist to quickly see the agreement between the calculated

dose distribution and the measured dose distribution. The γ image was chosen to be used

because it shows all the discrepancies and not as the NAT images excludes the cold regions

outside the PTV. Investigations of the IMRT inverse planning algorithm should include all

discrepancies, so the calculations in all di�erent dose ranges can be investigated.

5.4 Process behavior charts

To draw a PBC for clinical measurements using a set subgroup size can be hard, because the

number of �elds can vary between di�erent patients. Subgroup size of 1 makes it harder to

produce a stable process, while the PBC constants are bigger. An idea to get around this

problem can be to measure all �elds more than one time. The subgroup size has to be held

constant as well as the method to construct a PBC.

The ion chamber measurements in this project were too few to create real PBCs using 15−20

historical data from a predictable process. With more measurements the process can be judged

more fair. The speci�cation limits are chosen to be ±5%, which makes it easier to reach a

higher state than using speci�cation limits of ±3%. The choice of the speci�cation should not

be made to make it easier to produce a process in State I, but to represent the departments

requirements on the treatment plans. For this project agreement with the treatment plan to

within ±5% is accepted.

The PBCs drawn for the NAT indexes (Figure 35 and 36) are, as for the ion chamber mea-

surments, limited by too few data points. The PBCs does not include any speci�cation limits

while there are no good guidances on this. The speci�cation lines have to be decided, and for

that the NAT index has to be further investigated for the QA method that is going to be used.

The PBLX̄lowfor the 02sym is negative and, because the NAT index can not be negative, it

was set to 0.

PBCs for �lm measurements could be constructed using other parameters, such as the number

of pixels failing/passing the γ index. The same problem will arise when the speci�cation limits

are to be set and when the PBLX̄upis calculated, if it exceeds 100% or fall below 0%. There

50

are no speci�cated speci�cation limits for the number of pixels that are allowed to fail or have

to pass the γ index, as for the NAT index. The PBC usage for �lm measurement will need

further investigations to be able to draw speci�cation limits. At this point the PBC can be

used to observe the �lm dosimetry process in the IMRT QA.

The measurments in the two di�erent parts of the I'mRT phantom are drawn in separate

PBCs, which also is used by Pawlicki et al. 2008 [7]. Real IMRT plans di�er much between

prostate plans and head and neck plans and it is therefore good to draw separate PBCs. All

data points in a PBC must come from the same methods and a part of the method is the

phantom choice, therefore the data in this project also are drawn in separate PBCs.

The aim of this project was to investigate the use of PBCs in IMRT dosimetry QA. The

ion chamber process was found to be suitable to visualise in PBCs. The data points can

easily be compared to the speci�cation limits and the process can be analysed as well. The

�lm dosimetry process can also be analysed in PBCs, but because there are no guidances

on speci�cation limits for the parameters that can be viewed in a PBC the analysis of each

�lm measurement can not be analysed in the PBC. It is also recommended to look on dose

distribution images to do the analysis.

51

Acknowledgement

I would like to thank my supervisor Dr Sean Geoghegan for all the guidance throughout the

project and for giving me the opportunity to do this project at the Royal Perth Hospital.

I would also like to thank all the other physicists and the sta� at department of Medical

Engineering and Physics for making me feel welcome.

Another thank goes to my travel and study friend Emma Hedin for all the discussions during

the project, which motivated me.

All measurements were performed at Perth Radiation Oncology Wembley.

52

References

[1] Huq M. S. et al. A method for evaluating quality assurance needs in radiation therapy.

International Journal of Radiation Oncology Biology Physics, 71:170�173, 2008.

[2] Budgell G. J. et al. Quantitative analysis of patient-speci�c dosimetric imrt veri�cation.

Physics in Medicine and Biology, 50:103�119, 2005.

[3] Kahn F. E. The physics of radiation therapy, third edition. Lippincott Williams &Wilkins,

2003.

[4] Williams P. C. Imrt: delivery techniques and quality assurance. The British Journal of

Radiology, 76:766�776, July 2003.

[5] Pawlicki T. et al. Variation and control of process behavior. International Journal of

Radiation Oncology Biology Physics, 71:210�214, May 2008.

[6] Tague N. R. The quality toolbox. www.asq.se, 2004. Visited 15/10/2008.

[7] Pawlicki T. et al. Process control analysis of imrt qa: implications for clinical trials.

Physics in Medicine and Biology, 53:5193�5205, August 2008.

[8] The michigan chemical process dynamics and controls open text book.

http://controls.engin.umich.edu/wiki/index.php/Main_Page, November 2007. Vis-

ited 09/10/2008.

[9] Andreo P. et al. Trs-398. Technical report, IAEA, 2004.

[10] Low D. A. et al. A technique for the quantitative evaluation of dose distributions. Medical

Physics, 25:656�661, 1998.

[11] Depuydt T. et al. A quantitative evaluation of imrt dose distributions: re�nement and

clinical assessment of the gamma evaluation. Radiotherapy and Oncology, 64:309�319,

2002.

[12] Childress N. L. et al. Retrospective analysis of 2d patient-speci�c imrt veri�cations.

Medical Physics, 32:838�850, 2005.

[13] Childress N. L et al. The design and testing of novel clinical parameters for dose compar-

ison. International Journal of Radiation Oncology Biology Physics, 56:1464�1479, 2003.

[14] International Specialty Products. Gafchromic ebt - self-developing �lm for radiotherapy

dosimetry. Technical report, 2004.

[15] Martisikova M. et al. Analysis of uncertainties in gafchromic ebt �lm dosimetry of photon

beams. Physics in Medicine and Biology, 53:7013�7027, 2008.

[16] Paelinck L. et al. Precautions and strategies in using a commercial �atbed scanner for

radiochromic �lm dosimetry. Physics in Medicine and Biology, 52:231�242, 2007.

53

[17] International Specialty Products. EBT User's Protocol.

54

APPENDIX

List of abbreviations

Table 11: List of the abbreviations used in the thesis.

Abbreviation

CSV Comma Separated ValuesCT Computed TomographyDMPO Direct Machine Parameter OptimisationDTA Distance-To-AgreementEBT External Beam TherapyFMEA Failure Modes and E�ects AnalysisIMRT Intensity Modulated Radiation TherapyMLC MultiLeaf CollimatorsMU Monitor UnitsNAT Normalised Agreement TestPBC Process Behavior ChartPBL Process Behavior LimitPTV Planned Target VolumePV Pixel ValueQA Quality AssuranceROI Region Of InterestSDD Source-to-Detector DistanceSPC Statistical Process ControlTPS Treatment Planning System

55