Verification of the dose distributions with GEANT4 simulation for proton therapy

A GEANT4-based simulation of an irradiation system for proton therapy has been developed for the verification of dose distributions. The simulation represents the treatment room and beam irradiation system at the Hyogo Ion Beam Medical Center (HIBMC). The beam irradiation system consists of a lateral beam-spreading system and a range modulating system, which enable a three-dimensional dose distribution to be achieved. The simulation was carried out for proton beams at the isocentric gantry nozzle for therapeutic energies of 150, 190, and 230 MeV. The simulated distributions were compared to measured dose distributions obtained by using a water phantom at the isocenter. This arrangement simulates the practical aspects of the beam irradiation of a patient. The validation of the simulation was performed in proton range for important materials in the beam line, and in the lateral uniformity of the radiation field at the isocenter. Dose distributions based on GEANT4 were verified with measurements of the Bragg peak and the spread-out Bragg peak. This verification shows that the simulations are in good agreement with measurements.