LET dependency of relative biological effectiveness in normal human dermal fibroblasts irradiated with carbon ion beam

Mizohata, Kensuke*; Yoshida, Yukari*; Matsumura, Akihiko*; Isono, Mayu*; Yako, Tomoko*; Ando, Koichi*; Funayama, Tomoo; Ono, Tatsuya*; Nakano, Takashi*; Kanai, Tatsuaki*

Clinical RBE (cRBE), which was used to decide a clinical dose of carbon ion radiotherapy, is calculated by multiply experimental RBE (eRBE) by scaling factor. A value of eRBE was measured by colony formation assay of HSG cells in past, and adopted to the therapy of all patients and organ. However, there are question whether it was proper to use conventional eRBE in a different organization, different cell type. Therefore, we irradiated X-rays or a carbon line to NHDF cell and performed a colony assay to determine RBE of different cell type. The value of RBE was calculated from the cell survival rate fit in LQ model. As a result, the value of RBE was increased in LET-dependent manner, and the RBE obtained from the NHDF cell exhibited higher value than that of HSG cells. Thus, we concluded that it might be necessary to change scaling factor when evaluate different organ.