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

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 62.5 MeV 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