Abstract
A fundamental study was made on the possibility of applying hyperthermia using interstitial induction heating for deep seated brain tumors. This method uses a piece of thermosensitive magnetic metal or thermoseed, implanted in the tumor by direct surgery or stereotactic maneuver. Induction heating is performed by an extracranially placed magnetic coil which produces a high frequency magnetic field by means of a generator. The thermoseed must generate enough heat by loss of eddy current or hysteresis. It is also desirable to have the proper degree of Curie temperature to automatically regulate the temperature of the seed itself. Three kinds of implanting material [ferrite, stainless steel, and nickel-palladium (Ni-Pd) alloy] were developed and tested for the relationship between heat production and Curie temperature. Ni-Pd alloy was thought to be the best so far for this purpose, producing enough heat and a low Curie temperature, between 40 and 60 centigrade degrees. In vivo experiments were performed on a malignant brain tumor (T9) which was inoculated intracutaneously in the backs of CDF rats. When the tumors measured as large as 10 × 10 mm, the animals were subjected to local hyperthermia by induction heating. Three stainless steel needles (0.8 × 15 mm) were implanted in the tumor and a magnetic field was produced by a generator with a maximum power of 100 W and a field frequency of 250 kHz. The heating was continued for 60 minutes by controlling the magnetic field so that the temperature of the periphery of the tumor was maintained at 42°C. During the heating, the temperatures of the center of the tumor, a point 1 cm away from the tumor, and the skin over the tumor were measured, and were found to be steady at 45°C, 39°C and 38°C, respectively. Four tumors were treated by this method. Three tumors completely disappeared by the end of four weeks and another was markedly reduced in size compared to the control. In conclusion, magnetic induction heating using a ferromagnetic implant with low Curie temperature will be a useful method of interstitial hyperthermia for deep seated or unresectable brain tumors.
