Hyperthermia enhancement of radiation response and inhibition of recovery from radiation damage in human glioma cells

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Abstract
Three human glioma cell lines were tested for the effectiveness of hyperthermia and thermal radiosensitization. Thermal sensitization was evaluated from the perspective of increased radiosensitivity as well as inhibition of recovery from radiation damage. The three glioma cell lines tested showed large shoulders on the radiation survival curve and a large capacity for recovery of potentially lethal radiation damage. Hyperthermia caused radiosensitization in all three cell lines, which was primarily characterized by the reduction of the survival curve shoulder with moderate decreases in the survival curve slope. The radiosensitization was dependent on the time and temperature of the hyperthermia treatment. At 45°C for 60 min the shoulder of the radiation survival curve could be completely eliminated and the degree of enhanced cell killing at the 2 Gy level ranged from factors of 10 to 20 under the various conditions. When hyperthermia was given to cells which were irradiated and then plated immediately, or delayed for 8 h before plating to allow recovery, hyperthermia was found to cause radiosensitization under both conditions. In addition, when the hyperthermia dose was increased the difference between the immediate plating and the delayed plating survival curve decreased and for 45°C for 60 min this difference was completely eliminated, concomitantly with the elimination of the survival curve shoulder. These data indicate that hyperthermia may play a role in radiosensitization for the treatment of human glioma.