| Abstract: | Cultured murine mammary carcinoma cells M8013 could be made thermotolerant by a priming heat treatment, 30 min at 43 degrees C, applied 5 h prior to subsequent heat treatment. The sensitivity of non-tolerant and thermotolerant cells to either radiation or heat combined with radiation was investigated. Analysis of survival curves with respect to D0 and N showed that thermotolerance had no influence on the radiation sensitivity of the cells. Thermal enhancement of radiation effects (in combined heat/irradiation treatments) was however reduced as a result of thermotolerance. When thermal enhancement ratios were (D0) plotted as a function of the cell killing effects of heat treatment alone, thermotolerance did not seem to have any influence. This latter observation suggests that thermotolerance modifies the effectiveness of the heat treatment for heat-induced cell lethality and radiosensitization equally. Comparison of our in vitro results with several in vivo data on normal tissues suggest that the reduction in 'effective' treatment temperature which has been observed in the in vivo studies as a result of thermotolerance may be explained by equal modification of the effects of heat by thermotolerance both for its direct effects and the radiosensitization. |
