The Response of Chinese Hamster Ovary Cells to Fast-Neutron Radiotherapy Beams: II. Sublethal and Potentially Lethal Damage Recovery Capabilities

The recovery capabilities of mammalian cells in culture were investigated after irradiation with fast neutrons. Two different neutron beams (mean neutron energy 7 and 21 MeV) produced at the Texas A & M Variable Energy Cyclotron were used in these studies. The ability of fast-neutron-irradiated cells to recover from sublethal damage was tested in fractionated dose experiments. No recovery could be detected after irradiation with total doses of fast neutrons up to 300 rad. After a total dose of 500 rad, a recovery factor of about 1.5 was observed, which was considerably less than that observed when the cells were irradiated with 60 Co γ rays to a comparable survival level. The recovery capabilities of cells were not significantly different for the two neutron beams. The ability of fast-neutron-irradiated cells to recover from potentially lethal damage was tested by irradiating cells in the plateau phase of growth and plating the cells either immediately after the irradiation or 8 hr later. No difference in survival was observed when neutron-irradiated plateau-phase cells were plated at these two times, whereas 60 Co γ-irradiated cells showed a factor of 2 to 3 increase in survival when plated 8 hr after irradiation.