Kinetics of Interaction of Hyperthermia and Ionizing Radiation in Tribolium confusum

The interaction of hyperthermia and ionizing radiation was investigated using the flour beetle, Tribolium confusum. A series of temperature-tolerance curves from 43.5 to 46.0°C in 0.5°C increments were determined. Three nonlethal hyperthermia schemes, i.e., 45.0°C for 2 hr, 43.0°C for 2 hr, and 43.0°C for 12 hr, were applied either immediately before or immediately after irradiation. The sensitizing effect of hyperthermia was indicated by the shifting of the regression line of survival in probits on dose to the left of that of the control. The sensitizing effect as measured by decreased LD₅₀ did not reveal any definite trend related to the order of application of the two modalities in immediate sequence. The importance of the order of application of the two modalities was immediately apparent if various time intervals intervened between a fixed radiation dose and a fixed hyperthermia treatment. The effect of hyperthermia given 60 min or longer after irradiation of male beetles was not detectable, as the survival was indistinguishable from that of the irradiated controls. This disappearance of interaction within 60 min seemed to be independent of radiation dose and of the temperature of hyperthermic treatment. In contrast, hyperthermia before irradiation exerted an effect lasting at least 5 hr. The results are in keeping with the hypothesis that hyperthermia affects the repair capability of the beetles.