Monte Carlo simulation of out-of-field dose distribution in carbon-ion radiotherapy with passive beam

The National Institute of Radiological Sciences (NIRS) has achieved remarkable results in clinical trials and clinical practices for cancer treatment with carbon-ion radiotherapy at the Heavy-Ion Medical Accelerator in Chiba (HIMAC). These successful results allow the patient to live for a longer time and allow younger patients to receive this radiotherapy. While, undesired exposure which patients received during the treatment are continuously investigated. Based on our previous studies, the total secondary exposure doses in out-of-field volume per a treatment in carbon-ion radiotherapy with passive beam were comparable to or less than those in 3D-CRT and IMRT. In particular, as the position became closer to the field edge, the total dose equivalents in carbon-ion and proton radiotherapies were obviously less than those in 3D-CRT and IMRT. Now, our concern has shifted to the investigation on the assessment of the secondary cancer risk of patients receiving carbon-ion radiotherapy. No publications on the risk assessment of patients receiving carbon-ion radiotherapy are available at present, and several investigations on the risk assessment in proton radiotherapy have published. In the publications, the risk was estimated based on various risk models and dose distributions from the treatment planning system, measured data and calculated data by Monte Carlo simulation. Our other concern is the epidemiological study of patients receiving carbon-ion radiotherapy. In the dose estimation on the epidemiological studies, Monte Carlo simulation plays an important role because the treatment planning system can provide dose distribution only in/near the irradiation field and the measured data are limited. Moreover, since particles which influence the out-of-field dose in carbon-ion radiotherapy, are not only stray primary carbon ion but also secondary neutrons and various secondary charged particles, the distribution of the factor related to the risk such as LET and quality factor is needed as well as absorbed dose. Monte Carlo simulation is more suitable for estimating these distributions.This study is the first step in the risk assessment and epidemiological study in carbon-ion radiotherapy with passive beam at NIRS. In this presentation, we show 1) validation of the our calculational model with the Monte Carlo code, PHITS to simulate the dose and quality factor in water phantom by comparing with our experimental data, 2) distributions of dose and quality factor in phantom and 3) identification of partial contribution of each radiation type to the total dose.The 13th International Congress of the International Radiation Protection Associatio