Cellular radiation effects and hyperthermia. Influence of exposure temperature on survival of diploid yeast irradiated under oxygenated and hypoxic conditions

Stirred suspensions of wild-type stationary-phase diploid yeast cells have been exposed to X-rays under oxygenated and hypoxic conditions at temperatures between 230C and 500C, and the survival curves measured. The yeast cells were relatively resistant to this heat treatment, since there was no loss in colony-forming ability either in air or nitrogen for treatment times of up to 60 min at 450C although cell killing was observed after 45 min at 500C. The results of X-irradiation showed that increasing the exposure temperature to supranormal but still subcritical levels had only minor effects on the radiosensitivity. A significant difference was detected between hyperthermia during exposure and either pre- or post-treatment, suggesting the involvement of at least two mechanisms. There was only a small change in o.e.r. with temperature, demonstrating that the basic underlying processes, which are assumed to be similar in different systems, are not very temperature sensitive. These results do not suggest that the replacement of high LET-irradiation by heat treatment can be justified for hypoxic cells in radiotherapy. (U.K.)