Threshold Hypoxia: Its Effect on the Survival of Mammalian Cells Irradiated at high and low dose-rates

HeLa cells cultured in vitro were exposed to 60Co γ rays delivered as acute exposures (103 rads/minute) or protracted exposures (30 rads/hour). The radiation was given under conditions of normal aeration and under threshold hypoxia, defined to be the presence of about 340 parts per million of oxygen in the gas phase. The survival curves for acute exposures under normal aeration and under threshold hypoxia both had extrapolation numbers (N) of 1·5, and had 37 per cent dose-slopes (D0) of 180 and 435 rads respectively; the oxygen enhancement ratio is thus about 2·4. For protracted exposures, the survival curve under aerated conditions exhibited an N of 1·0 and a D0 of 260 rads; under threshold hypoxia the N was 1·5 and the D0 was 405 rads. At this low dose-rate, oxygen did not appear to be a simple dose-modifying agent; the oxygen enhancement ratio calculated from the ratio of the limiting slopes was about 1·56.