Tumor eradication in the rabbit by radiofrequency heating.

Abstract

Summary A radiofrequency (RF) machine operating at the lower end of the short-wave medical diathermy range (13.56 MHz, 22-m wavelength) has been designed for heating tumors. The apparatus employs the electrostatic (condenser) field technique, whereby the tissue becomes part of the output circuit and is heated between two paddle electrodes applied to opposite sides of the tumor. Incorporation of tuned compensating coils in the paddle handles, a connector lead between the paddles, and a d.c. inverse feedback loop between the RF output and the crystal oscillator are special features of the circuitry developed to produce stable and readily controlled tumor heating. Thermistor and thermocouple needle-type sensors were examined for tissue temperature measurement in the low-power RF fields. At power levels up to 6 watts, temperature readings obtained with thermistors or copper-constantan thermocouples, in association with analog recording systems, were unaffected by the RF field. At 6- to 12-watt output, only the thermocouple-galvanometer system remained unaffected, the thermistors reading consistently 1.0–1.5° high. Digital meters with thermocouples proved to be less stable recording systems in these RF fields. With the machine, i.m. VX2 carcinomas (up to 22 ml in volume) in the hind limb of rabbits were heated at 47° for 30 min. A temperature differential of 2–3° was usually recorded between multiple sensors in large tumors. The temperature of skin and normal muscle within the RF field remained 3–4° below the minimal temperature in the tumor, and no marked elevation of body temperature occurred. Seven out of 10 VX2 carcinomas given a single treatment at 47° regressed completely, with cure of the host and without damage to normal tissues. At the time of heating, metastases were present in the regional and distant lymph nodes and in the lungs. The presence of tumor in the muscle did not significantly alter the electrical resistance of the leg, and the results suggest that the selective heating of the VX2 carcinoma in the RF field may be conditioned by poor blood flow through the tumor compared with normal adjacent tissue.