Search

During the last decade there has been great interest in the use of hyperthermia in cancer therapy. The reasons why elevated temperatures, either alone or together with radiotherapy or chemotherapy, may prove useful have been the subjects of several recent reviews (Connor et al., 1977; Field and Bleehen, 1979).*Clinical applications of hyperthermia fall into two broad categories—whole-body and localized hyperthermia. During whole-body hyperthermia, which has been used on patients with advanced and widespread malignancies, core temperature is maintained at 41.8–42°C for a few hours. This appears to be the range of maximum tolerated temperatures; the organs which limit such treatment are the liver, which may be relatively sensitive to thermal damage, and the heart. Methods of inducing whole-body hyperthermia include the use of wax baths (Pettigrew et al., 1974), hot air cabinets (Pomp, 1978), water blankets (Barlogie et al., 1979) and space suits (Bull et al., 1979). Common features of these methods are the reduction of heat losses from the patient and the introduction of thermal energy into the body to enable the desired level of hyperthermia to be reached rapidly. The heating techniques and associated monitoring of temperature are relatively simple and no further comment on them will be made here.Local hyperthermia offers a wider range of therapeutic possibilities, especially if the temperature in a tumour can be maintained above that of surrounding normal tissue.

Although recent research and experience have shown that hyperthermia administered alone, or in combination with ionizing radiations or drugs, may be useful in the treatment of cancer, there are many questions to be answered if such therapy is to be optimized. Many studies with experimental animals have been made using water-bath heating because of its simplicity. However, recent reports (Hume et al., 1979a; Robinson et al., 1978) indicate that temperature distributions in tissues heated in this way may be markedly affected by blood flow. Heating methods in which energy may be deposited at depth in tissues may produce more uniform temperature distributions than can be obtained by immersing tissues in hot water. Since the effects of hyperthermia are very strongly dependent on temperature, the development of methods to heat tissues uniformly is an important requirement both experimentally and clinically. Expertise acquired during the development of microwave heating techniques should prove particularly valuable since this means of inducing hyperthermia has been chosen in a number of clinical trials currently underway.In the present report a microwave heating system which allows four volumes of tissue to be treated simultaneously is described. It is particularly useful in experiments involving rodents. The temperature in each field may be monitored and individually controlled.

The non-uniform effects of hyperthermia observed in mouse intestine when heating is achieved by immersion of an exteriorized loop of jejunum in a hot bath were quantified. Intestinal damage was assayed by crypt destruction and correlated with local temperature and distance from major mesenteric blood vessels.The rate of rise of temperature during heating was much greater in regions far from blood vessels than in those close to vessels. Equilibrium temperatures in regions close to vessels were as much as 2°C below liquid bath temperature. Such temperature differentials agreed well with those predicted from the heterogeneity observed in crypt destruction. Both histological and thermometric data suggest that major blood vessels may influence tissue temperature over a distance of approximately 2 mm and, within this region, temperature gradients of 1°C per mm may exist.These findings raise questions concerning the interpretation of previously reported data from water-bath heating of both tumours and normal tissues. They also emphasize the need for consideration of the influence of blood flow on heating patterns during hyperthermal treatment of patients.

Obtaining benefit from the use of hyperthermia in treating cancer is strongly dependent on the success of the heat treatment in raising the temperature adequately throughout a tumour volume (Dewhirst & Sim, 1984). Interstitial hyperthermia offers one of the best methods of uniform heat delivery (Doss & McCabe, 1976). The radiofrequency (RF) system described here is relatively simple and integrates well with interstitial brachytherapy treatment using afterloaded iridium-192 wires.Under general anaesthesia, hollow stainless steel needles (1.8 mm o.d.) are implanted into the tumour using a central trochar. The desired arrangement of needles is two parallel planes of equal length, evenly spaced, with 1 cm between planes and between needles. Perspex strips or plastic tubing with pre-cut holes are used as jigs to ensure appropriate geometric arrangement, since unacceptable overheating will occur wherever two adjacent needles converge.

In a recent technical note ter Haar and Dunn (1984) describe the assembly of a linear thermocouple array for in vivotemperature measurement in ultrasonically induced hyperthermia fields.