An investigation of some factors determining the production of local ultrasound hyperthermia with the view to treating neoplasms
The possibility of using elevated temperatures (hyperthermia) induced by ultrasound in cancer therapy has been investigated. Computer models were used to find the optimum frequency for the focussed ultrasonic transducer and lenses. It was found to be around 1 MHz when deep-lying tumours are to be treated. The effect of blood flow, thermal conduction and ultrasonic absorption on the temperature distributions during hyperthermia treatment was simulated with a 3-dimensional computer model. The decreased blood perfusion in some malignant tissues appeared to produce higher temperatures in the tumour than in the surrounding tissue despite equal energy input. The temperature distributions produced by focussed or overlapping fields were measured both in phantoms and in pigs' muscle in vivo. The thermal conduction smoothed the temperature distributions in both experiments. However, it was possible to produce the maximum temperature at the depth of 50 mm in muscle in vivo. By using 7 unfocussed overlapping fields the temperature of a large tissue volume could be increased. The biological effects of ultrasound hyperthermia were studied in transplanted rat's Yoshida sarcoma. The elevated temperature 49 °C was induced by a therapy transducer with the operating frequency of 0.75 MHz. The cell killing effect appeared to be similar to that reported by other research groups using different methods for induction of hyperthermia. Thus the most significant reason for the tissue damage in this experimental situation appears to be the thermal effect of ultrasound.