Abstract
Based on the determination of protons fluence at the phantom's surface, a 3D dose distribution is calculated inside a water phantom using a fast method. The dose contribution of secondary particles, originating from inelastic nuclear interactions, is also taken into account. This is achieved by assuming that 60% of the energy transferred to secondary particles is locally absorbed. Secondary radiation delivers approximately 16.8% of the total dose in the plateau region of the Bragg curve for monoenergetic protons of energy 190 MeV. The physical dose beyond the Bragg peak is obtained for a proton beam of 190 MeV using a Geant4 simulation. It is found that the dose beyond the Bragg peak is <0.02% of the maximum dose and is mainly delivered by protons produced via reactions of the secondary neutrons. The relative dose profile is also calculated by simulation of the proposed beam line in Geant4 code. The dose profile produced by our method agrees, within 2%, with the results predicted by the Fermi Eyges distribution function and the results of the Geant4 simulation. It is expected that the fast numerical approach proposed herein may be utilised in 3D deterministic treatment planning programs, to model proton propagation in order to analyse the effect of modifying the beam line.
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