Enhancement of radiation sensitivity by postradiation hypoxia.

We demonstrate that postradiation hypoxia during colony formation in vitro enhances radiation sensitivity when cells are irradiated in severely hypoxic states. The presence and magnitude of this phenomenon, if it occurs in vivo, raise questions about the importance of the oxygen effect in the radiation response of tumor cells in vivo and suggest that hypoxia may not be an important factor in the probability of tumor control following radiation therapy.