Synergism between hyperthermia, ultrasound and gamma irradiation

Hyperthermia is now established as a successful therapy for some tumours. It has been found that, for some tumours, the combination of heat and ionizing radiation is more effective than either type of treatment on its own. Ultrasound is commonly used to produce hyperthermia. We have therefore studied the effect of combined heat, ultrasound and 60Co gamma irradiation treatments on suspension cultures of V79 Chinese hamster lung fibroblasts. Cells were exposed to 43 degrees C hyperthermia with or without simultaneous continuous wave ultrasound (2.6 MHz, 2.3 W cm-2) either before or after 60Co gamma exposure (428 c Gy). It was found that when the two types of exposure followed one immediately after the other, then the ultrasound exposure increased the amount of cell killing produced by the combination of heat and ionizing radiation, whether the heat or the ionizing radiation came first. If an interval for repair of 72 minutes was left between the heat and ionizing radiation, then this enhancement was lost. The increase in cell killing produced by the ultrasound is shown to be caused by a mechanism that is nonthermal in origin.