Age dependence of the radiosensitivity of glial progenitors for in vivo fission-neutron and X irradiation

O-2A progenitor cells are the stem cells of the myelin-forming oligodendroc ytes in the central nervous system. In the epithermal reactor beams used fo r boron neutron capture therapy (BNCT) for treatment of brain tumors, fissi on neutrons are a contaminating component. To estimate the radiosensitivity of the O-2A progenitors for fission neutrons, an in vivo in vitro clonogen ic assay was used. Radiosensitivity of progenitors obtained from the spinal cord of 1- or 5-day-old rats or the optic nerve of 2- or 12-week-old rats for 1 MeV fission neutrons was compared to that for 300 kVp X rays. Dose-su rvival curves were fitted according to the linear-quadratic model. The resu lting beta component was very small to negligible. Progenitor cells obtaine d from rats of different ages show differences in radiosensitivity, charact erized by different or values. RBE values for fission neutrons were 3.5 for 1-day-old spinal cord, 3.2 for 5-day-old spinal cord, 3.0 for 2-week-old o ptic nerve, and 4.3 for 12-week-old optic nerve. These high RBE values indi cate the importance of minimizing the fast-neutron component in the epither mal neutron beams used for BNCT, (C) 2000 by Radiation Research Society.