Effect of hyperthermia on the radiation response of Chinese hamster small intestine

The Chinese hamster small intestine was surgically exteriorised from the ligament of Trietz to the ileocaecal junction and heated for 8 min at 44.3 degrees C (a non-lethal dose) at various intervals before or after whole-body 60Co irradiation. Hyperthermia exposure immediately after irradiation reduced the LD50/7 from the control of 1,230 cGy to 798 cGy (thermal enhancement ratio, TER, of 1.5). At an interval between irradiation and hyperthermia of 2 h the LD50/7 was 935 cGy, and it remained at that value for intervals of 2-24 h. The increase in the LD50/7 by 2 hours after irradiation probably represents the repair of radiation damage that can interact with hyperthermia, and after two hours the two modalities interact independently. When the small intestine was exposed to 44.3 degrees C hyperthermia immediately prior to irradiation, the LD50/7 was reduced to 658 cGy (TER = 1.8). As hyperthermia and radiation treatments were separated the LD50/7 returned to the control value by 24 h and was unchanged over 24-72 h. This indicates that by 24 h, recovery from hyperthermia damage that could interact with radiation damage was complete. For the sequence hyperthermia----radiation, 33% of the hyperthermia damage was repaired by 1 h; whereas for radiation----hyperthermia, 85% of the radiation damage was repaired by 1 h. The sequence-dependent interaction of hyperthermia and radiation damage in normal tissues is complex, but the kinetics of interaction for the sequence radiation----hyperthermia seem to be predictable for several normal tissues in different species.