Normobaric oxygen as a sensitizer of hypoxic tumor cells.

A series of experiments have been performed to determine the relative radiosensitivities of a mouse mammary tumor (CA NT) irradiated in 100% normobaric oxygen or in air, using clinically relevant dose-fractionation schedules. The results demonstrate that normobaric oxygen is a potent radiosensitizer, its effect being more marked with smaller size fractions. Enhancement ratios (ER) of 1.23 to 1.45 were obtained. By contrast, no significant benefit was seen with single doses (ER = 1.05 +/- 0.14). The degree of radiosensitization with oxygen, in these fractionated regimens, is greater than that reported for the chemical radiosensitizer, misonidazole. It therefore seems appropriate to re-examine the clinical potential of normobaric oxygen and to consider how the conditions of normobaric radiotherapy achieved in the laboratory can be translated to man.