Specification of Carbon Ion Dose at the National Institute of Radiological Sciences (NIRS).

The clinical dose distributions of therapeutic carbon beams, currently used at NIRS HIMAC, are based on in-vitro Human Salivary Gland (HSG) cell survival response and clinical experience from fast neutron radiotherapy. Moderate radiosensitivity of HSG cells is expected to be a typical response of tumours to carbon beams. At first, the biological dose distribution is designed so as to cause a flat biological effect on HSG cells in the spread-out Bragg peak (SOBP) region. Then, the entire biological dose distribution is evenly raised in order to attain a RBE (relative biological effectiveness) = 3.0 at a depth where dose-averaged LET (linear energy transfer) is 80 keV/mum. At that point, biological experiments have shown that carbon ions can be expected to have a biological effect identical to fast neutrons, which showed a clinical RBE of 3.0 for fast neutron radiotherapy at NIRS. The resulting clinical dose distribution in this approximation is not dependent on dose level, tumour type or fractionation scheme and thus reduces the unknown parameters in the analysis of the clinical results. The width SOBP and the clinical / physical dose at the center of SOBP specify the dose distribution. The clinical results analysed in terms of TCP were found to show good agreement with the expected RBE value at higher TCP levels. The TCP analysis method was applied for the prospective dose estimation of hypofractionation.