SU‐E‐T‐540: MCNPX Simulation of Proton Dose Distributions in a Water Phantom

Abstract

Purpose: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.Methods: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 × 40 × 50 cm3 water phantom and the scoring volume was 20 × 20 × 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 × 40 × 10 cm3 water phantom and two scoring voxel sizes of 0.1 × 0.1 × 0.05 cm3 and 0.01 × 0.01 × 0.05 cm3 were used for the depth dose distribution, and 0.01 × 0.01 × 0.05 cm3 for the lateral profile distribution simulations.Results: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.Conclusion:Multiple Coulomb scattering is critical when there is in proton disequilibrium.