Radiosensitivity and Thermosensitization of Thermotolerant Chinese Hamster Cells and RIF-1 Tumors

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Abstract
CHO [Chinese hamster ovary] cells subline HA-1 were made thermotolerant by a priming heat treatment (43.degree. C, 30 min). Later 4, 16, or 24 h they were either irradiated or heated (43.degree. C, 30 min) and irradiated. Thermotolerance had no effect on the radiation sensitivity of the cells as measured by the D0 value of the clonogenic survival curve. However, the N value of the curve (width of shoulder) showed a significant increase at 24 h, indicating an increased capacity to accumulate sublethal damage. This indicates that the fractionation schedule 43.degree. C, 30 min + 37.degree. C, 24 h + 43.degree. C, 30 min + X-ray required .apprx. 100 rad more radiation than 43.degree. C, 30 min + X-ray to reduce survival to the same level. The same priming treatment was given to rat sarcoma RIF-1 tumors growing in C3H mice. Later, 24 h, when the tumors were either irradiated or heated (43.degree. C, 30 min) and irradiated, thermotolerance had no effect on the radiosensitivity of the cells as measured by in vitro assay. However, thermal radiosensitization was not apparent 24 h after the priming treatment.