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MCNPX Simulation of Proton Dose Distributions in a Water Phantom

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S Chen

C Lee1,2,3 , Y Lee4 , S Chen1*, B Chiang1 , C Tung1,3 , T Chao1,3 , (1) Department of Medical Imaging and Radiological Sciences, College of Medicine, Chang Gung University, Kwei-Shan 333, Taiwan, (2) Department of Radiation Oncology, Chang Gung Memorial Hospital, Kwei-Shan 333, Taiwan, (3) Institute for Radiological Research, Chang Gung University/Chang Gung Memorial Hospital, Kwei-Shan Tao-Yuan 333, Taiwan, (4) Department of Radiation Oncology, Ministry of Health and Welfare Nantou Hospital, Nan-Tou 540, Taiwan


SU-E-T-540 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

In this study, fluence and energy deposition of proton and proton by-products and dose distributions were simulated. Lateral dose distributions were also been discussed to understand the difference between Monte Carlo simulations and pencil beam algorithm.

MCNPX codes were used to build a water phantom by using “repeated structures” technique and the doses and fluences in each cell was recorded by mesh tally. This study includes, proton equilibrium and proton disequilibrium case. For the proton equilibrium case, the doses difference between proton and proton by-products were studied. A 160 MeV proton pencil beam was perpendicularly incident into a 40 Χ 40 Χ 50 cm³ water phantom and the scoring volume was 20 Χ 20 Χ 0.2 cm³. Energy deposition and fluence were calculated from MCNPX with (1) proton only; and (2) proton and secondary particles. For the proton disequilibrium case, the dose distribution variation using different multiple Coulomb scattering were studied. A 70 MeV proton pencil beam was perpendicularly incident into a 40 Χ 40 Χ 10 cm³ water phantom and two scoring voxel sizes of 0.1 Χ 0.1 Χ 0.05 cm³ and 0.01 Χ 0.01 Χ 0.05 cm³ were used for the depth dose distribution, and 0.01 Χ 0.01 Χ 0.05 cm³ for the lateral profile distribution simulations.

In the water phantom, proton fluence and dose in depths beyond the Bragg peak were slightly perturbed by the choice of the simulated particle types. The dose from secondary particles was about three orders smaller, but its simulation consumed significant computing time. The depth dose distributions and lateral dose distributions of 70 MeV proton pencil beam obtained from MCNPX, GEANT4, and the pencil beam algorithm showed the significant deviations, probably caused by multiple Coulomb scattering.

Multiple Coulomb scattering is critical when there is in proton disequilibrium.

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