Presentation Conceptual design of reference radiation field simulating secondary neutrons during carbon-ion radiotherapy

米内, 俊祐

The risk of radiation-induced secondary cancer is of great concern to long-term cancer survivors who received radiotherapy and people involved in radiotherapy, because normal tissues around the treatment target are potentially exposed to undesired doses. Estimations of the risk for conventional radiotherapy with high-energy x-ray and electrons have been reported based on epidemiologic studies including dose estimations. On the other hand, for ion-beam radiotherapy (IBRT) such as proton and carbon-ion radiotherapies, the risk estimation derived from epidemiologic studies is difficult yet because it is more modern radiotherapy modality. In addition, since many radiation species such as secondary charged particles and neutrons are produced by primary beam through interactions with irradiation devices and patient body during IBRT compared to conventional radiotherapy, dose-response relationships depending on radiation quality has to be investigated. In particular, secondary neutrons are a high-LET radiation which leads to high relative biological effectiveness (RBE) and provide whole-body exposure, though the neutron dose in IBRT is low according to published dosimetric studies. So it is very important to clarify the dose-response relationship for neutron exposures following IBRT. However, the epidemiologic approach is not realistic yet as mentioned before, biological experiments with cells and animals have still a key role. The goal of this study is a development of reference neutron field for simulating secondary neutrons during carbon ion radiotherapy (CIRT) with the broad beam method. As is well known, there is the dependence on neutron energy in RBE, so it is ideal to perform experiments in the real treatment room. But, such experiments should be limited considering constraints of time and biological hazard. It is, therefore, required for the reference field to simulate well the neutron energy spectrum in the treatment room. Here, conceptual design of the reference neutron field using Monte Carlo code, PHITS is shown.
World Congress on Medical Physics and Biomedical Engineering

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