There are insufficient data at present to inform cancer risk after radiation exposure, during fetal and childhood periods, especially for heavy ions and neutrons. Using animal models, we studied the age-at-exposure effect of radiation on cancer induction and lifespan shortening, in order to propose age-weighting factors and relative biological effectiveness (RBE) of heavy ions for radiation protection of fetuses and children. Fifty female and male B6C3F1 mice per group were exposed to gamma rays (137Cs), carbon ions (13 keV/µm) generated from Heavy Ion Accerelator in Chiba (HIMAC), or neutrons (average energy, 1-2 MeV) from Neutron Exposure Accelerator System for Biological Effect Experiments (NASBEE), at various ages from fetal to mature adult periods. Mouse ages at the time of irradiation included 3 days post-conception (dpc), 13 dpc, 17 dpc, and 1 week, 3 weeks, 7 weeks and 15 weeks after birth. The doses ranged between 0.2 and 4 Gy for gamma rays, 0.2 and 2 Gy for carbon ions, and 0.05 and 1 Gy for neutrons. Mice were observed until moribund and their lifespan and tumor burden were analyzed.Female mice appeared to be more susceptible to radiation-induced lifespan shortening than male mice. The effect of gamma-rays on lifespan shortening was more pronounced when irradiated at 1 week rather than 7 weeks of age. Surprisingly, irradiation with gamma rays at the late fetal stage had little influence on lifespan shortening compared to infant and adult exposures. Carbon ions and neutrons were more potent in reducing lifespan than gamma rays when 1 week-old mice and fetuses were exposed. These results on lifespan shortening suggest the highest RBE value were obtained for neutrons. Currently, pathological examination is undertaken to clarify the cause of life shortening.59ｔｈ Annual Meeting of the Radiation Research Societ
