Thermal radiosensitization of human tumour cell lines with different sensitivities to 41.1oC

While much work on radiosensitization by hyperthermia in the 43oC and higher temperature range has been done, relatively little work has been done at temperatures in the 41-42oC range. In this moderate hyperthermia range there are dramatic differences in the resistance of mammalian cells to hyperthermia. Therefore, thermal radiosensitization was measured in two human colon adenocarcinoma cell lines, one that expresses chronic thermotolerance and proliferates at 41.1oC, NSY 42129 (NSY) cells and one that is slowly killed at 41.1oC, HCT15 cells. Heat-resistant NSY cells were found to be more radioresistant than heatsensitive HCT15 cells. Hyperthermia at 41.1oC enhanced the radiation sensitivity in NSY cells, but no significant induction of heat-induced radiosensitization was observed in HCT15 cells. The radiation sensitivity induced by 41.1oC in NSY cells appeared to be related to both intrinsic heat-induced radiosensitization (HIR) and cell-cycle redistribution at 41.1oC. Incidentally, cells incubated at 41.1oC for between 8-16h displayed an identical radiosensitivity to those heated for 24h. This result implies that modest hyperthermia for 2h or more can have a radiosensitizing effect in heat-resistant cells.