Analysis of the physical interactions of therapeutic proton beams in water with the use of Geant4 Monte Carlo calculations.

The processes that occur when protons traverse a medium are investigated theoretically for a full therapeutic range of energies [20 MeV, 220 MeV]. The investigation is undertaken using the Geant4 toolkit for water medium. The beam is simulated only inside the phantom, effects of beamline are included in the overall beam properties as lateral width and momentum bandwidth. Every energy deposition is catalogued according to the particle and the process that caused it. The catalogued depositions are analysed statistically. There are only few important processes such as proton ionisation and nuclear scattering (elastic/inelastic) that constitute the main features of the energy distribution. At the same time processes concerning electrons are used very often without obvious effect to the result. Such processes can be therefore approximated in the simulation codes in order to improve the performance of the code. Neutron depositions are most important before the Bragg peak, still they are by an order of magnitude smaller than those of protons. In the region behind the Bragg peak only a small number of neutrons is created in the simulation and their energy contribution through secondary protons is by orders smaller than the effect of proton-produced secondary protons within the Bragg peak. Hence, the effects of neutrons created in the calculation volume can be neglected.