Interstitial microwave hyperthermia in a canine brain model.

A dual frequency microwave system was constructed for interstitial heating of brain tissue. Single-junction dipole antennas were tested in a phantom model and in normal dog brain to determine how variations in physical factors affected temperature distributions. Non-survival studies were performed at both 915 and 2450 MHz to determine heating patterns that could be achieved within normal brain using this system. Chronic survival studies were performed using a single dipole antenna inserted laterally into one hemisphere of brain and driven at 2450 MHz. Temperatures of 43 or 44 degrees C for 30 min at a reference point 0.5 cm from the antenna junction were used to induce a thermal lesion of approximately 1 cm diameter in the right cerebral hemisphere of dogs. Neurologic and physical changes in dogs were monitored daily for up to 16 weeks after induction of cerebral lesions. The extent and development of thermal lesions was monitored with weekly computed tomographic (CT) examinations and, after death, at histopathologic examination. Results of the phantom studies showed that the longitudinal heating pattern was bell-shaped at both frequencies used and that there was some variation in heating length that depended on insertion depth. Acute studies in dog brain showed that 915 MHz antennas implanted less than 6.5 cm deep produced erratic heating patterns that usually included excessive heating of the surface of the brain. Conversely, 2 cm-long antennas driven at 2450 MHz gave reproducible temperature distributions both longitudinally along and radially away from the antenna. The steepest gradients--about 1 degree C/mm--occurred in the radial direction away from the antenna junction. A single 30 min heat treatment produced a large focal lesion that consisted of central coagulation necrosis surrounded by a sharply demarcated hypervascular zone. Edematous changes were minimal and were observed only during the first week after treatment. As assessed by serial CT scans, thermal lesions reached a maximum size by the first week after treatment and were essentially resolved by 16 weeks after treatment.