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doi:10.1016/j.ni

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Nuclear Instruments and Methods in Physics Research A 562 (2006) 1005–1008

www.elsevier.com/locate/nima

Monte carlo modelling of a clinical proton beam-line for thetreatment of ocular tumours

Colin Bakera,�, David Shipleyb, Hugo Palmansb, Andrzej Kacperekc

aRadiotherapy Division, School of Health Sciences, University of Liverpool, UKbQuality of Life Division, National Physical Laboratory, Teddington, Middlesex, UK

cDouglas Cyclotron, Clatterbridge Centre for Oncology, Wirral, UK

Available online 3 March 2006

Abstract

Three Monte Carlo codes have been compared in their prediction of beam characteristics for a clinical proton facility used for the

treatment of ocular tumours. MCNPX 2.5d, GEANT4 and McPTRAN.RZ were used to model the transport of a nominal 62.5MeV

proton beam through scattering foils, range shifter and modulator wheels of the Clatterbridge proton beam-line.

Depth and radial dose profiles measured using a silicon PIN photodiode, PTW Markus ion chamber and film were used to verify

Monte Carlo predictions. Three beam configurations were investigated: Full energy (Bragg peak), full modulation and a modulated and

range-shifted beam typical of those used clinically.

All three codes were able to model the range of beam configurations studied. For GEANT4 and McPTRAN.RZ simulations,

matching to the full energy Bragg peak required the use of an energy spread of the incident proton beam that was significantly larger than

the experimentally derived estimate.

r 2006 Elsevier B.V. All rights reserved.

PACS: 02.70.Lq; 14.20.Dh; 24.10.Lx; 29.27.�a; 41.75.�i; 41.85.�p; 87.53.�j

Keywords: Monte carlo; Protons; Radiotherapy

1. Introduction

The use of Monte Carlo (MC) simulation to model thegeneration and subsequent dose distributions of photonand electron beams relevant to radiotherapy is wellestablished. Simulation of therapeutic proton beams isfollowing a similar pattern [1–5].

A number of established MC codes are available forproton beam simulations. These codes differ in either orboth radiation transport algorithms and representation ofproblem geometry, potentially leading to significantlydifferent predictions of energy deposition.

This paper compares predicted beam characteristics for anominally 62.5MeV proton beam used in the treatment ofocular tumours at the Clatterbridge Centre for Oncology[6]. A schematic representation of this beam-line is shown

e front matter r 2006 Elsevier B.V. All rights reserved.

ma.2006.02.082

ing author. Tel.: +44 151 794 5754; fax: +44 151 794 5751.

ess: colin.baker@liverpool.ac.uk (C. Baker).

in Fig. 1. MCNPX 2.5d [7], GEANT4 (6.2.p02 with LowEnergy EM Physics package G4EMLOW 2.3) [8–10] andMcPTRAN.RZ [3,11] MC codes were compared in theirprediction of beam characteristics against each other andagainst measured depth and radial dose profiles for selectedbeam conditions.

2. Methods

2.1. Measurements

Measurements in PMMA (polymethyl-methacrylate) ofthe full energy Bragg peak (BP) were made using a PTW-Markus ionization chamber and PIN diode (SiemensBPW34). BP height relative to surface dose agreed towithin 3.5% between each detector, the higher value beingobserved with the diode, in agreement with Herault andIborra [5]. Readings were uncorrected for the variation ofstopping power ratio with depth, which is expected to

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1 2 3 4 5 6, 7 8 9 10 11 12

VacuumAir

Fig. 1. Schematic representation of the proton beam-line geometry:

collimators (1, 8, 11), scattering foils (2, 4), central stopper (3), range

shifter and modulator wheel (6, 7), monitor chambers (9), nozzle (10) and

phantom (12). Protons exiting the cyclotron are incident from the left.

C. Baker et al. / Nuclear Instruments and Methods in Physics Research A 562 (2006) 1005–10081006

introduce a correction of the order of 2% for an air-filledionization chamber [12]. The average of these two profileswas then used as the definitive BP for comparison with MCpredictions. Radial dose profiles at depths in PMMA weremeasured using diodes and radiographic film (Kodak X-Omat V).

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

2.2 2.4 2.6 2.8Depth in pmma [cm]

Rel

ativ

e do

se

Fig. 2. Comparison of MC simulated BP with measurement. Relative

dose is normalized to unity at 0.3 cm depth. Measurement (solid line),

MCNPX (o), GEANT4 (x) and McPTRAN (&).

2.2. MC simulations and source parameters

MCNPX v2.5d was installed in parallel, under MPI, on aSun Grid Engine consisting of 16 UltraSPARC III900MHz processors. Typically 20 million proton historieswere performed, resulting in estimated statistical uncer-tainties typically less than 2% in energy deposition tallies.Wall-clock time for these runs was approximately 8 h.

GEANT4 and McPTRAN.RZ were run on a PC clusterof 1–2GHz Pentium/Xeon processors under MandrakeLinux. Typical timings were 48 h for 25 million histories ona single 2GHz Xeon processor.

An unmodulated, mono-energetic source of 62.5MeVprotons was first simulated with each code. Gaussianbroadening of the incident proton beam energy was thenperformed to represent the nominal true spread for theClatterbridge cyclotron (0.1% standard deviation (SD)[13]) and to provide matching to measured BP height [1].

All three MC codes approached the simulation of themodulator wheel in different ways, due to differences intheir respective geometry capabilities. As MCNPX doesnot currently support entry of dynamic geometry, a protonphase-space file was scored in a plane at the position of thecentral step of the modulator wheel, in its absence. Theeffect of the physical modulator wheel was then approxi-mated by restarting this phase-space file at depths in aPMMA block corresponding to thicknesses of the physicalmodulator, with a weight corresponding to the appropriatevane angles, using the MCNPX SSR card. Whilst this doesnot represent an accurate physical model, it enablessimulations of any arbitrary modulator wheel (includingno wheel) to be performed simply by modifying theappropriate step weights.

In McPTRAN.RZ, the modulator wheel was representedas a slab with a dynamic thickness, randomly sampled foreach incident proton from a distribution of thicknesseswith probabilities proportional to the true vane angle.

In GEANT4 the modulator wheel was accuratelyrepresented as a moving (spinning) object by specifyingthe physical geometry and rotating the wheel in 11 stepsbetween 0.51 and 44.51. Energy deposition was scored foreach angle and then summed.

3. Results

All three codes were able to reproduce the full energy BPheight to within experimental and simulation uncertainties.For MCNPX this was achieved using the experimentally-derived upper limit on the energy spread of protonsemerging from the cyclotron (Scanditronix MC60PF) of0.1% SD. Inclusion of this magnitude of energy spreadmade no significant difference to BP height compared tothe use of a mono-energetic source. For GEANT4 andMcPTRAN, a significantly larger energy spread (0.8% SD)was required to reduce the predicted BP height to thatdetermined by measurement, resulting in broadening of theBP width. It should be noted that the nominal mean energyof 62.5MeV was derived to provide the observed range forMCNPX simulations. Fig. 2 shows that a slightly lowerenergy would have been appropriate to match GEANT4and McPTRAN (the distance between curves is of theorder of 0.2mm PMMA, or 0.2MeV). It should also benoted that GEANT4 offers a number of different hadronicmodels for proton interactions [9], only one of which hasbeen evaluated here [4]. The fitted energy spread is sensitive

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0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 2 3Depth in pmma [cm]

Rel

ativ

e do

se

Diode

MCNPX

GEANT4

MCPTRAN

1

Fig. 3. Comparison of depth-dose profiles for a fully modulated beam

with measurement, normalized to 0.3 cm depth.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.5 1 1.5 2 2.5Depth in pmma [cm]

Rel

ativ

e do

se

DiodeMCNPXGEANT4

Fig. 5. Comparison of depth-dose profiles for a range-shifted and

modulated beam, typical of clinical conditions, normalized to 0.3 cm

depth.

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2Radial distance [cm]

Rel

ativ

e do

se

Diode

MCNPX

GEANT4

McPTRAN

Fig. 4. Comparison of simulated and measured lateral dose profiles in

PMMA at 0.1mm depth, for a 3 cm diameter field.

C. Baker et al. / Nuclear Instruments and Methods in Physics Research A 562 (2006) 1005–1008 1007

to the exact height of the BP, which has been observed tovary by several percent over time.

Simulation results for a fully modulated beam arecompared with Markus and diode readings in Fig. 3.GEANT4 results are seen to predict a slope in the

modulated beam, possibly due to the excessive width ofthe fitted BP.Radial dose profiles near the phantom surface (0.1mm

depth) and at 50% of the proton range were scored with allMC codes. Fig. 4 shows the comparison for surfaceprofiles. Similarly good agreement for MCNPX andGEANT4 was obtained at 50% depth. Artefacts in theMcPTRAN results are expected to be due to inaccuratemodelling of proton scatter in the scattering foils.Comparisons of depth-dose profiles for a range-shifted

and modulated beam, more typical of those used clinically(0.9 cm range shift, 1.34 cm modulated width, eye tissueequivalent) are shown in Fig. 5. Artefacts in the MCNPXcurve, also present to a lesser degree in the fully modulatedbeam simulation require further investigation.

4. Conclusions

Measured depth and radial dose profiles in PMMA for anominal 62.5MeV clinical proton beam under a range ofbeam conditions, from full energy to full modulation havebeen compared with MC simulations using MCNPX 2.5d,GEANT4 and McPTRAN.RZ.Good agreement between simulation and measurement

was generally obtained for all codes. However, forGEANT4 and McPTRAN codes, a spread in the incidentproton beam energy significantly larger than the experi-mentally- derived estimate was required in order to reducepredicted BP height to that determined by measurement.

ARTICLE IN PRESSC. Baker et al. / Nuclear Instruments and Methods in Physics Research A 562 (2006) 1005–10081008

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