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

Abstract

HeLa cells cultured in vitro were exposed to Co60 gamma-rays delivered as acute exposures (103 rads/min.) or protracted exposure (30 rads/hr). The radiation was given under conditions of normal aeration and under threshold hypoxia, defined to be the presence of about 340 parts/million of O2 in the gas phase. The survival curves for acute exposure under normal aeration and under thrshold hypoxia both had extrapolation numbers (N) of 1.5, and had 37% doseslopes (Do) of 180 and 435 rads respectively; the O2 enhancement ratio is about 2.4. For protracted exposures, the survival curve under aerated conditions exhibited an N of 1.0 and a Do of 260 rads; under threshold hypoxia the N was 1.5 and the Do was 405 rads. At this low dose-rate, O2 did not appear to be a simple dose-modifying agent; the O2 enhancement ratio calculated from the ratio of the limiting slopes was about 1.56.