We have recently demonstrated that postirradiation hypoxia during colony formation in vitro enhanced the radiation sensitivity of murine tumor cells irradiated and maintained at 0.1% O2. The effect of postirradiation hypoxia was expressed by a significant reduction in the oxygen enhancement ratio. We now demonstrate that this enhancement of radiation sensitivity by postirradiation hypoxia is also observed in a human tumor cell line. The effect was observed at$[{\rm O}_{2}]\leq 0.1\%$, but was not present at$[{\rm O}_{2}]\geq 0.5\%$. Time-course experiments suggested that this enhancement of cell killing by X rays required prolonged exposure to hypoxic conditions.

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