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Verification of the dose distributions with GEANT4 simulation for proton therapy

The GEANT4 based simulation of an irradiation system for the proton therapy has been developed for the verification of dose distributions. The simulation represents a treatment room with the 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, so that the dose distribution is achieved in three dimensionally. The simulation was carried out for the proton beams at the isocentric gantry nozzle for the therapeutic energy of 150, 190, and 230 MeV, respectively. The simulated dose distributions are compared with measurements, where dose distributions are obtained using a water phantom at an isocenter, which simulate practical situations of the beam irradiation to the patients. The validation of the simulation was performed for the proton ranges in important materials at beam line and lateral uniformity of the irradiation field at an isocenter, respectively. Then, the dose distribution in simulation based on GEANT4 were verified with measurements for Bragg peak and spread out Bragg peak, respectively. The result of verification shows the depth-dose distributions in simulation are in good agreement with measurements.