Studies on the effects of ultrasound on several solid tumours in experimental animals have indicated that tumour growth rates can be reduced. These data are generally consistent with a thermal mechanism of action. Application of combined ultrasound and X-irradiation have shown that with some experimental animal tumours the radiation dose required to locally control 50% of the tumours can be reduced by ultrasound. These results were also consistent with a thermal mechanism of action hypothesis. Fractionated X-irradiation was not enhanced as much as single dose. Pulsing the ultrasound with the same time-average intensity resulted in the same radiosensitivity enhancement. The combined effects of ultrasound and cancer chemotherapy drugs have been studied in mouse leukaemia. The treatment was applied in vitro with cells in suspension. Subsequent to treatment, the cells were inoculated into host mice and survival was monitored. Cytotoxic action of 5 of 10 drugs studied was enhanced by ultrasound. A thermal mechanism of action apparently was not involved. Cavitation in the suspension probably played a role in the cytotoxic enhancement. Experimental data are consistent with the concept that ultrasound causes rapidly reversible cell damage which, in the presence of cytotoxic drugs, is not so readily reversed and results in significant loss of lethal potential of the malignant cells to the host
