Combined Effects of X Irradiation and Hyperthermia on CHO Cells for Various Temperatures and Orders of Application

Abstract

Survival of CHO [Chinese hamster ovary] cells to hyperthermic treatment combined with radiation indicates that heat given immediately before or immediately after irradiation radiosensitizes S[synthetic]-phase cells more than G1 [1st phase] cells, thus resulting in similar absolute levels of survival for each phase. No difference in effect was observed for different temperatures (42.0 to 45.5.degree. C) applied before irradiation in G1 or S when times of heating were adjusted to obtain the same survival (0.5-0.6) from heat alone. When heat was administered after irradiation and time between treatments was increased, repair during G1 of radiation damage which interacted with subsequent heat damage occurred over a 2-h period. Survival increased from a synergistic level to an independent level with kinetics similar to those seen for repair between split X-ray doses. For this experiment, heat treatments were administered at 42.5 or 45.5.degree. C with times of heating adjusted to obtain the same survival (0.15) from heat alone. When cells were treated similarly in S phase using 42.5 or 45.5.degree. C (survival from heat alone was 0.2), recovery from a synergistic level of survival was similar to that observed in G1; however, survival did not reach an independent level by 120 min between treatments. When relatively sublethal heat doses at 42.5 or 45.5.degree. C were applied before, during or after irradiation, maximum reduction in survival of asynchronous cells occurred when heat was present during and immediately following irradiation, presumably due to heat increasing the fixation of radiation damage. A 6-fold difference in survival was observed with about a 5-min change in timing of radiation with respect to heating. This sensitivity of survival to changes in protocol may have considerable implications in the combined use of hyperthermia and radiation for cancer therapy.