Radiosensitivity and the oxygen effect for mammalian cells culturedin vitroin stationary phase

HeLa cells grown in medium 199 supplemented with 15 per cent human serum show increased radiosensitivity when irradiated in unfed confluent culture (“stationary phase”), compared with their response when irradiated during exponential growth (“log. phase”). In addition, a biphasic dose-response curve for stationary phase cells irradiated, under hypoxic conditions indicates that the oxygen enhancement ratio (OER) is reduced from circa 3·0 to circa 1·8 for a significant proportion of the cells. Thus, up to doses (if 500–600 rads, greater depopulation occurred in hypoxic stationary phase cells than in aerobic, exponentially growing cells. However, the same cells grown in human serum-supplemented medium F10 showed increased radiosensitivity in confluent culture but an unaltered OER. By contrast, Chinese hamster cells grown in confluent culture with or without medium changes showed no significant changes in the slope or shoulder of their dose-survival curves compared with those irradiated during exponential growth, and no alteration in OER. If the stationary phase 199-grown HeLa cells in their folic acid-deficient, thymidine-deficient, asparagine-deficient medium are a suitable model for the hypoxic and metabolically starved tumour cell at a distance from a capillary blood supply, their unexpected radiosensitivity and reduced response to hypoxia could explain why fractionated-dose radiotherapy does cure many human tumours.