Application of Monte-Carlo treatment planning system 'JCDS-FX with PHITS' to proton radiotherapy

In proton radiotherapy, proton irradiated in the body generates secondary neutron and photon. And incidental doses caused by the secondary radiations are given to around target region in addition to therapeutic dose. However in current treatment planning of the proton radiotherapy, conventional dose estimation algorithm like pencil beam method can't estimate influence of the secondary radiations. Thus, to improve accuracy of the dose estimation of the radiotherapy, a new multi-modal Monte-Carlo treatment planning system is being developed. The new system has employed Monte-Carlo transport code PHITS to dose calculation engine of the system. The aim of this study is to investigate the influence of the secondary radiations in the proton radiotherapy by numerical simulation with Monte-Carlo calculation. To investigate the secondary radiations and incidental doses, a proton irradiation with water phantom was simulated, secondary radiations and doses were determined using PHITS. The estimation results demonstrated that maximum incidental dose was approximately 3 mGy per 1 Gy proton dose. And a lot of external neutrons generated at upstream devices are irradiated to surface of the phantom. However at deep region, influences of the external neutrons were small, neutrons generated in the body accounted for a large amount of the total neutrons at the region. The results demonstrated that it is impossible to release completely the incidental dose, even if shielding of the external neutron and application of the active scanning method are achieved. (author)