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An interstitial microwave antenna array hyperthermia (IMAAH) system was evaluated in a series of acute experiments for its ability to generate localized hyperthermic fields in the normal adult and developing dog brain. Using a single microwave antenna, the maximum diameter of heating greater than or equal to 42Â°C was 1.3 cm. Using four microwave antennas, temperatures greater than or equal to 42Â°C were obtained over a 2.5 cm cross-sectional diameter. Normal gray matter cannot be heated above 42.2Â°C for 60 min without causing acute damage to cortical neurons. Edema formation can occur at temperatures as low as 42Â°C in both gray and white matter. Disruption of myelin tracts in white matter is apparent at temperatures of 43.0-43.5Â°C. The IMAAH system generates significantly higher and more variable temperature distributions in virally induced experimental brain tumors than in the developing dog brain at the same power levels.