AbstractThe KHT sarcoma in C3H mice was exposed to a total dose of 3000 cGy x-irradiation delivered in three equal fractions at intervals of either 48, 72, or 96 hr. The dose rate was high, 212 cGy/min or low, 19 cGy/min. The effect of hyperthermia, 42.5°C for 30 minutes, was determined by inducing the hyperthermia using either water or ultrasound. For radiation alone at high dose rate, tumor control was maximum at a fraction interval of 96 hr. At low dose rate, local tumor control was not influenced by fractionation interval and was not different from the maximum tumor control achieved at high dose rate. Hyperthermia alone was ineffective for tumor control. When high dose rate irradiation and hyperthermia were combined, thermal enhancement was highest at fractionation intervals of less than 96 hr where the effects or irradiation alone were least. At low dose rate, the thermal enhancement was independent of fractionation interval. Only for irradiation at high dose rate was the ultrasound-induced hyperthennia more effective at potentiating the radiation than water bath-induced hyperthermia. A possible explanation for the interaction of the dose rate effect with fractionation and hyperthermia is proposed.