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Kakakhel, Muhammad Basim, Baveas, Emmanual, Fielding, Andrew,Kairn, Tanya, Kenny, J, & Trapp, Jamie(2011)Validation and automation of the DYNJAWS component module of theBEAMnrc Monte Carlo code.Australasian Physical and Engineering Sciences in Medicine, 34(1), pp.83-90.

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1

Validation and automation of the DYNJAWS component module of the

BEAMnrc Monte Carlo code

M B Kakakhel1, 2

, E S Baveas3, A L Fielding

1, T Kairn

4, J Kenny

4, J V Trapp

1

1 Faculty of Science and Technology, Queensland University of Technology, GPO Box

2434,

Brisbane, Qld 4001, Australia

2 DPAM, PIEAS, PO Nilore, Islamabad, Pakistan

3 Radiation Oncology Mater Centre, Queensland Health, Australia

4 Premion, The Wesley Medical Centre, Suite 1, 40 Chasely St, Auchenflower, Qld

4066, Australia

Corresponding Author

J V Trapp

Faculty of Science and Technology

Portfolio MIPS-Physics

Queensland University of Technology

GPO Box 2434, Brisbane, QLD, 4001, Australia

Phone: +61(0)731381386, Fax +61(0)731389079

E-mail: j.trapp@qut.edu.au

Running Title:

Validation and automation of DYNJAWS

mailto:j.trapp@qut.edu.au

2

Abstract

The purpose of this work is to validate and automate the use of DYNJAWS; a new component module

(CM) in the BEAMnrc Monte Carlo (MC) user code. The DYNJAWS CM simulates dynamic wedges

and can be used in three modes; dynamic, step-and-shoot and static. The step-and-shoot and dynamic

modes require an additional input file defining the positions of the jaw that constitutes the dynamic

wedge, at regular intervals during its motion. A method for automating the generation of the input file

is presented which will allow for the more efficient use of the DYNJAWS CM. Wedged profiles have

been measured and simulated for 6 and 10 MV photons at three field sizes (5 cm x 5 cm , 10 cm x10

cm and 20 cm x 20 cm), four wedge angles (15, 30, 45 and 60 degrees), at dmax and at 10 cm depth.

Results of this study show agreement between the measured and the MC profiles to within 3% of

absolute dose or 3 mm distance to agreement for all wedge angles at both energies and depths. The

gamma analysis suggests that dynamic mode is more accurate than the step-and-shoot mode. The

DYNJAWS CM is an important addition to the BEAMnrc code and will enable the MC verification of

patient treatments involving dynamic wedges.

Key words Enhanced Dynamic Wedges (EDWs), Monte Carlo, DYNJAWS, Automation, BEAMnrc

3

Introduction

Physical wedge filters have been widely used for beam shaping in external beam radiotherapy [1, 2].

Despite their utility, physical wedges are associated with issues such as beam hardening, increased

scatter, dose discrepancies due to occasional misalignment, the potential hazard to patients and staff

due to the weight of the wedge, and the time inefficiencies of their physical removal and insertion [3].

With the introduction of computer control, it has become possible to deliver wedged fields by moving

the collimator jaws within a linear accelerator during the irradiation, allowing for more freedom in

terms of the possible wedge angles and the available field sizes. Such a computer controlled wedge is

normally termed a virtual or dynamic wedge [4-6].

Several authors have simulated dynamic wedges in the BEAMnrc Monte Carlo (MC) code [7]. Kapur

et al simulated the dynamic wedges by modifying the BEAM code and incorporating the Monitor Unit

(MU) settings for different wedge angles [8], and Verhaegen et al proposed a discretization approach

for simulating dynamic wedges on a Siemens machine [9]. Subsequently, the Position Probability

Sampling (PPS) and Static-Component Simulation (SCS) methods were developed to further improve

the accuracy of the MC dynamic wedge models [10]. In the PPS method the jaw position is treated as

a random variable, i.e. the jaw position is sampled from a cumulative probability distribution function

when each particle is initiated to start its transport in the simulation. By contrast, in the SCS method

individual static fields are simulated and then the results are integrated. Shih et al have also simulated

dynamic wedges by effectively integrating static fields using the ‘restart option’ in the BEAM code

[11, 12].

A new component module (CM) called DYNJAWS has been added to the BEAMnrc code, which is

specifically designed to simulate dynamic wedges [13]. The approach taken in this CM for simulating

the dynamic wedges is similar to that of Verhaegen et al [10] but with many useful improvements (as

described in the Discussion section). One key advantage of using the DYNJAWS CM, rather than

developing or using in-house code for simulating dynamic wedges, is that DYNJAWS is now

distributed as a standard part of the BEAMnrc code, which is freely available, compatible with a wide

range of platforms and is well documented and maintained. Because DYNJAWS is a new addition to

4

the BEAMnrc code, this work aims to validate this CM and provide a method for automating the

generation of the DYNJAWS input file.

Background

DYNJAWS is based on the JAWS CM available in earlier releases of the BEAMnrc distribution [14].

In the DYNJAWS CM both dynamic (mode 1) and step-and-shoot (mode 2) modelling of the dynamic

wedge is possible. Moreover this CM functions as a JAWS CM in static mode (mode 0) where the

jaws are stationary during the simulation, representing standard orthogonal jaws.

Changes in the lateral position (opening) and longitudinal position (height, or distance from the

source) of the jaws can be varied during a simulation using the CM in mode 1 or mode 2, through the

use of a specific, detailed input file. This file is required in addition to the main input file used by the

BEAMnrc simulation. (Hereafter the term ‘input file’ is used to refer to this specific DYNJAWS file

and should be not be confused with the main BEAMnrc ‘egsinp’ input file). In mode 0, where the

jaws are stationary, no additional input file is required.

Input file for DYNJAWS

The input file for DYNJAWS first specifies the number of sets of jaw settings or subfields which will

be used in the simulation. The probability of selecting each subfield as well as the resulting jaw

positions (for both X and Y jaws) and the Z-distance (from source to the front and back of the jaws)

are also specified. The jaw position specification includes the front positive (FP), back positive (BP),

front negative (FN) and back negative (BN) coordinates (for both X and Y jaws) as shown in Figure

1. For example, in the case of the Y jaw, the front positive coordinate is abbreviated as YFP. The Z

distance includes Z-min and Z-max for each pair of jaws, where Z-min is the distance from the source

to the front of the jaw and Z-max represents the distance to the back of the jaw.

Figure 1.

In DYNJAWS the probability of selecting a particular subfield is represented by the index variable

which must be specified for all subfields [13]. In the step-and-shoot method, a random number, m1, is

generated from the interval [0, 1] at the beginning of each incident history. The segment, i is used if

5

1( 1) ( )index i m index i (1)

Segment 1i is used if

1 (1)m index (2)

To simulate the jaw motion in the dynamic mode, a similar comparison is carried out with a random

number (using equation 1 and 2); however the jaw settings are selected after interpolation between

segment i and 1i . By including this extra interpolation between the discrete subfields that define

each jaw motion, the dynamic mode (mode 1) provides a more realistic representation of the physical

sweeping motion of the jaw than the step-and-shoot mode (mode 2).

Methods and Materials

Validation

Experimental Measurements

Profiles produced using the Varian enhanced dynamic wedge (EDW) were measured for 6 and 10

MV photon beams using the MatriXX device with OmniPro I’mRT software (IBA dosimetry,

Schwarzenbruck, Germany) for three field sizes (5 cm x 5 cm, 10 cm x10 cm, and 20 cm x 20 cm)

and four wedge angles (15, 30, 45 and 60 degrees), at dmax and at 10 cm depth. The MatriXX ion

chamber array consists of 1020 vented parallel plate ionization chambers (0.5 cm height, 0.45 cm

diameter and 0.08 cc sensitive volume) arranged in a 32 x 32 matrix. Taking account of the inherent

build up of the MatriXX, plastic water was placed on the top of the array to achieve the depths of

interest. The SSD of 100 cm was set to the top of plastic water. The dose delivery was carried out on a

Varian Clinac iX machine (with up to 2 Gy delivered for each profile) with Y1-IN wedge orientation.

The ion chamber array was corrected for the background signal and the relative ion chamber

sensitivity. The dynalog files stored on the linear accelerator were recovered and used for the MC

simulations (for more details about the dynalog file refer to the Varian EDW documentation [15]).

Probability calculation

The probability of selecting a subfield (the index variable in DYNJAWS) was calculated from the

Segmented Treatment Tables (STTs) supplied by the vendor for each wedge angle and energy [15].

The STT specifies the jaw position versus the dose delivery information at different points in an EDW

6

delivery. The first step was to select the relevant STT based on the photon energy and the wedge

angle. The selected STT was then truncated according to the delivered EDW field size. The delivered

jaw positions were read from the dynalog files. Jaw positions from the dynalog files were compared to

the jaw positions in the STT. If they were matching then the corresponding dose value entry in the

STT was used as the required probability for selecting that jaw position or subfield. However, if the

dynalog derived jaw position did not match the STT jaw position entry, then interpolation was carried

out to determine the probability value from the STT. This procedure was repeated for all subfields.

After calculating the probability for all subfields, the probability values were renormalized with the

probability for the last subfield set to unity (i.e. dividing all the subfield probability values by the

probability of the last subfield) so that at the last subfield 100% the photon histories for a given

wedged field were delivered.

The probability values and the jaw positions were then written to the input file, with each jaw position

corrected for the trigonometric difference between the jaw position projected to the isocentre (listed in

the dynalog files) and the physical distance between the jaw and the central axis (required by the

BEAMnrc input file).

Automation of the input file generation

A script, named AUTODJAWS was written in Matlab (version 7.8.0.342, The MathWorks, Natick,

MA, USA) to automate and generate the input file. The GUI of the AUTODJAWS software is shown

in Figure 2. When using AUTODJAWS, the user selects the relevant dynalog file, with the ‘Select

Dynalog file’ button, and then selects the wedge angle, jaw orientation, beam energy and collimator

angle. For a given field size the coordinates in the dynalog file are identical for all wedge angles,

however the number of MU delivered at each coordinate position differ according to the wedge angle.

Therefore for a given field size, the user only needs one dynalog file and can select the appropriate

wedge angle to generate the required input files for any wedge angle. Similarly, for 6 and 10 MV the

jaw coordinates are the same for a particular field size; however the proportion of the total number of

MUs that are delivered at a particular position is different. The user may also specify the position of

the X-jaws, which by default will be parked at 10 cm on each side of the central axis. Finally by

selecting the ‘Generate Input File’ button the required file is created (this includes the probability

calculation described in the previous section).

7

Figure 2.

MC simulations

All MC simulations were carried out on a SGI Altix XE Cluster, with 200 cores. Simulations were

performed to generate the wedge dose profiles at photon beam energies of 6 MV and 10 MV for three

different field sizes (5 cm x 5 cm, 10 cm x10 cm and 20 cm x 20 cm) at two depths (dmax and 10 cm)

for four wedge angles (15, 30, 45, and 60 degrees). The optimized electron beam FWHM and incident

electron energy combination for the models were found to be 1 mm, 5.875 MeV and 1 mm, 9.8 MeV

for the linear accelerator operating at 6 and 10 MV respectively during the MC commissioning

process. A phase-space file, containing the positions, trajectories, energies and charges of all particles

exiting the linear accelerator was scored (starting with 22 million initial electron histories) at a

distance of 55 cm from the photon source. This scored phase-space file was used as an input in the

phantom dose calculations in the second stage.

The simulated phantom dimensions were 56 cm x 32 cm x 6 cm (length x width x height). The

MatriXX array was modelled in 3 layers, with reference to the manufacturer’s specifications. The first

(build-up) layer has a physical thickness of 3.3 cm, and an unknown composition, but is described in

the manufacturer’s documentation as having a 0.33 cm water-equivalent thickness. This layer was

therefore modelled as a 3.3 cm thickness of water with a density of 0.1g/cm^3. The second (active)

layer consists of an array of 0.5 cm thick ionisation chambers embedded in a water-equivalent

medium. This layer was modelled as a 0.5 cm thickness of water, according to the common procedure

of modelling ion chambers as water equivalent in MC simulations [16-18]. The final (backscatter)

layer of the MatriXX consists of a 2.2 cm thickness of polystyrene (98%) and titanium dioxide (2%),

with an unspecified structure, at a density of 1.045 g/cm^3. Given that a material composed of 98%

polystyrene and 2% titanium dioxide has an electron density 0.998 times the electron density of

standard polystyrene (evaluated using the mass fractions, molecular masses and atomic numbers of

the component elements [19]), this backscatter layer was modelled, for simplicity, as a 2.2 cm

thickness of polystyrene with a density of 1.045 g/cm^3. The additional solid water placed on the

MatriXX surface to achieve the desired depths was also modelled in the phantom design. The voxel

size used was 7.62 mm along the wedge direction, which is similar to the inherent resolution of the

8

MatriXX array. For the dose calculations in this simulated phantom, 6 billion histories were run

producing calculated doses with a statistical uncertainty below 1%.

Back scatter correction

An important consideration for modelling an EDW is the effect of backscatter into the monitor

chamber from the upper Y-jaws; this usually results in shorter delivery time for small field sizes.

Various authors have quantified the backscatter into the monitor chamber [10, 20-23]. In this study

the empirical correction suggested by Ahmed et al has been incorporated into the STTs to account for

the back scatter [24]. Equation 3 and 4 has been used for 6 and 10 MV photons respectively.

4( ) 1.03 7.03 10BCF y y (3)

4( ) 1.02 4.9 10

BCF y y (4)

Whereas FBC represents the back scatter correction function, and y represents the Y-jaw position. This

correction is reflected in the probability values calculated from the backscatter corrected STTs.

Results

Input file Generation

Input files were generated using the AUTODJAWS script. These input files specified the details for

all the subfields including the probability of selection of a subfield, X and Y jaw coordinates and the

respective Z distance to the front and back of the jaws from the target plane. The accuracy of these

input files was confirmed by independent manual calculation of the probabilities and the jaw

positions.

Validation

Wedged Profiles

Comparison of wedged profiles for the 6 and 10 MV beams at dmax and 10 cm depth is shown in

Figure 3 for 20 cm x 20 cm and 10 cm x 10 cm field sizes. For a particular field size, energy and

depth combination, all wedge angles simulated are plotted on the same graph. It can be seen in Figure

3 that there is an excellent agreement between the measured and simulated data for 6 and10 MV

photons at both depths (within 3% or 3 mm for all data points). Similar agreement was observed for 5

9

cm x 5 cm (results not shown here). All the simulations were carried out using dynamic mode of the

DYNJAWS (mode 1).

Figure 3.

Open field and off axis profiles

In Figure 4(a-c) measured and simulated wedged profiles for 6 MV photons have been compared at

dmax using the static mode. Figure 4(d) compares measured and simulated profiles for a corner field

(using a 60 degree dynamic wedge and a 10 cm x 10 cm off-axis field). For all data points the results

agreed within 3% or 3 mm criteria. Similar results were found for 10 MV (not shown).

Figure 4.

Dynamic versus step-and-shoot

Figure 5 illustrates the comparison of dynamic and step-and-shoot modes for 60 and 15 degree

simulated wedged profiles with the measurements for two field sizes (20 cm x 20 cm and 10 cm x 10

cm) for 6 MV photons at dmax. A gamma comparison (at 3% or 3 mm) of the dynamic and step-and

shoot modes was carried out and the mean gamma values are reported in Table 1.The results suggest

that the dynamic mode is more accurate, especially for smaller wedge angles at large field sizes. For

example, the dynamic mode was substantially more accurate than the step-and-shoot mode when

simulating a 15 degree dynamic wedge with a 20 cm x 20 cm field.

Figure 5.

Table 1.

Discussion

The results confirm the suitability of the new DYNJAWS CM for modelling the dynamic wedges. In

addition to being a dedicated CM for modelling dynamic wedges, DYNJAWS eliminates the need for

the intermediate phase space files that were required in some previously reported approaches [25]. In

the SCS method a number of phase space files were created at different jaw positions and then

summed [11]. When using DYNJAWS, a single phase space file is created whether simulating in

10

dynamic or step-and-shoot modes. The dynamic mode of the new CM is truly dynamic because it

carries out an extra interpolation for selecting the jaw positions after a particular subfield is selected

for the delivery. The step-and-shoot mode in the DYNJAWS CM is based on Verhaegen and Liu’s

PPS approach [10], while the dynamic mode is a new addition. The open field profiles in Figure 4

demonstrate that the DYNJAWS CM can be used as a standard JAWS CM when applied in the static

mode. Figure 4(d) also provides a useful example of the use of the DYNJAWS CM to simulate a non-

symmetric wedged field.

Data shown in Table 1 suggest that the use of the dynamic mode simulations is especially beneficial

when wedge angles are small, field sizes are large or the region under examination is located away

from the central axis. In these circumstances, the step-and-shoot mode’s lack of interpolation between

the jaw positions at each subfield defined in the dynalog file leads to noticeable inaccuracy in the

positioning of the simulated jaw. We have observed that the dynamic mode appears to require slightly

more CPU time than the step-and-shoot mode, possibly due to the additional interpolation of the jaw

values that is carried out in this mode.

The AUTODJAWS script accurately generates the required input files which simplifies the use of the

DYNJAWS CM. This script is available by contacting the authors.

Conclusion

These results validate the capability of the DYNJAWS CM for simulating the dynamic wedges. The

use of this CM can be made further efficient by automating the process of the input file generation.

The DYNJAWS CM is an important addition to the BEAMnrc code and will allow the simulation and

Monte Carlo validation of complex patient treatments involving the use of dynamic wedges.

Acknowledgments

The authors are thankful to: Blake Walters at NRCC for his useful suggestions and for sharing the

code of DYNJAWS before its final release; Bruce Faddegon for his useful discussions and ideas on

modelling the dynamic wedges; and Frank Verhaegen for his work on simulation of the dynamic

wedges. Scott Crowe at QUT helped with the 6 MV BEAMnrc model. Computational resources and

services used in this work were provided by the High Performance Computing (HPC) and Research

Support Unit, QUT, Brisbane, Australia.

11

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Target Plane Central Axis

Z-max

Z-min

Front Negative (FN) Front Positive (FN)

Back Negative (BN) Back Positive (BN)

Figure 1. Coordinate specification in DYNAJWS CM.

14

Figure 2. AUTODJAWS GUI – the user simply selects the required parameters to generate the DYNJAWS

input file.

15

a) 6 MV-20x20 cm- dmax

Off Axis Distance (cm)

-15 -10 -5 0 5 10 15

Rela

tive D

ose (

%)

0

50

100

150

200

250

Measured Profiles

60 Deg Simulated

45 Deg Simulated

30 Deg Simulated

15 Deg Simulated

b) 6 MV-20x20 cm-10 cm depth

Off Axis Distance (cm)

-15 -10 -5 0 5 10 15

Rela

tive D

ose (

%)

0

50

100

150

200

250

Measured Profiles

60 Deg Simulated

45 Deg Simulated

30 Deg Simulated

15 Deg Simulated

c) 6 MV-10x10 cm- dmax

Off Axis Distance (cm)

-8 -6 -4 -2 0 2 4 6 8

Re

lati

ve

Do

se

(%

)

0

50

100

150

200

Measured Profiles

60 Deg Simulated

45 Deg Simulated

30 Deg Simulated

15 Deg Simulated

d) 6 MV-10x10 cm- 10 cm depth

Off Axis Distance (cm)

-8 -6 -4 -2 0 2 4 6 8

Rela

tive D

ose

(%

)

0

50

100

150

200

Measured Profiles

60 Deg Simulated

45 Deg Simulated

30 Deg Simulated

15 Deg Simulated

e) 6 MV-10x10 cm- dmax

Off Axis Distance (cm)

-8 -6 -4 -2 0 2 4 6 8

Re

lati

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Do

se

(%

)

0

50

100

150

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Measured Profiles

60 Deg Simulated

45 Deg Simulated

30 Deg Simulated

15 Deg Simulated

f) 6 MV-10x10 cm- 10 cm depth

Off Axis Distance (cm)

-8 -6 -4 -2 0 2 4 6 8

Rela

tive D

ose

(%

)

0

50

100

150

200

Measured Profiles

60 Deg Simulated

45 Deg Simulated

30 Deg Simulated

15 Deg Simulated

Figure 3. Comparison of measured and simulated wedged profiles for 20 cm x 20 cm and 10 cm x 10 cm field

size at 100 cm SSD for four wedge angles (from top to bottom 60, 45, 30 and 15 degrees) at dmax( 1.5 cm for 6

MV and 2 cm for 10 MV) and 10 cm depth. Both the simulated and measured doses are normalized to 100% at

the centre of the field. a) 20 cm x 20 cm- 6 MV profiles at dmax b) 20 cm x 20 cm- 6 MV profile at 10 cm depth

c) 20 cm x 20 cm -10 MV profile at dmax d) 20 cm x 20 cm - 10 MV profile at 10 cm depth. e) 10 cm x 10 cm -

6 MV profiles at dmax f) 10 cm x 10 cm - 6 MV profile at 10 cm depth g) 10 cm x 10 cm - 10 MV profile at

dmax and h) 10 cm x 10 cm - 10 MV profile at 10 cm depth. The error in Monte Carlo dose calculation was less

than 1 %. The 10 x 10 cm measured profiles were smoothed using a spline interpolation.

16

b) 6 MV-10x10 cm- Open field

Off Axis Distance (cm)

-10 -5 0 5 10

Rela

tive D

ose (

%)

0

20

40

60

80

100

120

Measured

Simulated

c) 6 MV-5x5 cm- Open field

Off Axis Distance (cm)

-8 -6 -4 -2 0 2 4 6 8

Re

lati

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Do

se

(%

)

0

20

40

60

80

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Measured

Simulated

a) 6 MV-20x20 cm- Open field

Off Axis Distance (cm)

-15 -10 -5 0 5 10 15

Re

lati

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se

(%

)

0

20

40

60

80

100

120

Measured

Simulated

d) 6 MV-60 degree-corner field.

Off Axis Distance (cm)

-2 0 2 4 6 8 10 12 14

Rela

tive D

ose

(%

)

0

20

40

60

80

100

120

140

160

Measured

Simulated

Figure 4. Comparison of measured and simulated open field and wedged profiles at dmax. Both the simulated

and measured doses are normalized to 100% at the centre of the field. a) 20 cm x 20 cm open field for 6 MV b)

10 cm x 10 cm open field for 6 MV c) 5 cm x 5 cm open field for 6 MV d) 6 MV Corner field (60 degree) 10 cm

x 10 cm profile. The error in Monte Carlo dose calculation was less than 1 %.

17

a) 6x-20x20 cm-60 Deg

Off Axis Distance (cm)

-15 -10 -5 0 5 10 15

Rela

tive D

ose (

%)

0

50

100

150

200

250Measured

Dynamic

Step and Shoot

b) 6x-20x20 cm-15 Deg

Off Axis Distance (cm)

-15 -10 -5 0 5 10 15

Re

lati

ve

Do

se

(%

)

0

20

40

60

80

100

120

140 Measured

Dynamic

Step & Shoot

c) 6x-10x10 cm-60 Deg

Off Axis Distance (cm)

-8 -6 -4 -2 0 2 4 6 8

Re

lati

ve

Do

se

(%

)

0

20

40

60

80

100

120

140

160

Measured

Dynamic

Step & Shoot

d) 6x-10x10 cm-15 Deg

Off Axis Distance (cm)

-8 -6 -4 -2 0 2 4 6 8

Rela

tive D

ose (

%)

0

20

40

60

80

100

120

Measured

Dynamic

Step & Shoot

Figure 5. Comparison of dynamic and step-and-shoot mode for 6 MV at dmax. Both the simulated and

measured doses are normalized to 100% at the centre of the field. a) 20 cm x 20 cm-6 MV- 60 degree b) 10 cm x

10 cm-6 MV- 60 degree c) 10 cm x 10 cm-6 MV- 60 degree d) 5 cm x 5 cm-6 MV- 60 degree

18

Table 1. Comparison of gamma values for dynamic and step-and-shoot modes for 6 MV photon

beams.

Mean Gamma Value (3% or 3 mm)

Dynamic Step-and-shoot

Field size (cm2) 60

o 15

o 60

o 15

o

10 x 10 0.17 0.17 0.22 0.19

20 x 20 0.40 0.17 0.46 0.30