Thermal damage threshold of brain tissue--histological study of heated normal monkey brains

The thermal damage threshold of normal brain tissue was evaluated from immediate and delayed histological changes caused by hyperthermia treatment of normal monkey (Macaca fuscata) brains. A 2450 MHz microwave antenna and an antenna cooling system devised by our group were used for interstitial hyperthermia treatment. The antenna within the cooling system was inserted through a small craniectomy under general anesthesia. The temperature at a reference point, 4 mm radially away from the surface of the cooling system, was maintained at 42, 43, 44, 45, or 46 degrees C for 60 minutes. Eighteen animals were treated and sacrificed immediately after the treatment, while nine animals were treated and sacrificed 7 days after the treatment. The histological changes were studied microscopically on sections stained with HE or Kluver-Barrera's method. The non-survival experiment demonstrated that areas heated at 44 degrees C or below showed no obvious irreversible changes. The survival experiment showed areas heated at 44 degrees C or above developed coagulative necrosis. These histological findings indicate that thermal damage occurs in normal brain tissue after heating at 44 degrees C or above for 60 minutes, suggesting that the safety limit for brain hyperthermia is 43 degrees C for 60 minutes.