The thermal damage threshold of brain tissue was estimated from the immediate and delayed histological changes caused by 2450 MHz microwave antenna and an antenna cooling system of a device used for interstitial hyperthermia treatment. An antenna within a cooling system was inserted through the small cranietomy under general anesthesia. The temperature at a reference point, 4 mm radially away from the surface of the cooling system, was maintained at 42 °C, 43 °C, 44 °C, 45 °C or 46 °C for 60 minutes. In a non-survival experiment, 18 animals were used and sacrificed immediately after the treatment. In a survival experiment, 9 animals were used and sacrificed seven days after the treatment. The histological changes were evaluated by microscopic examination with hematoxylin and eosin, Klüver-Barrera, or Bodian stainings. In the non-survival experiment, areas heated below 44 °C showed no obvious irreversible change. In the survival experiment, areas heated at 44 °C or above showed coagulative necrosis. Those histological findings indicate that the thermal damage occurs in normal brain tissue after heating at 44 °C or above for 60 minutes, and suggest that the safety limit for brain hyperthermia is 43 °C for 60 minutes.
interstitial microwave irradiation
malignant brain tumors
antenna cooling system