Enhanced cytotoxicity in combination of low dose-rate irradiation with hyperthermia in vitro.

We have pursued an in vitro investigation using a rodent cell line to characterize the interaction of simultaneous low dose-rate irradiation (LDRI) and hyperthermia and to determine the role of LDRI in the development of thermotolerance in a fractionated hyperthermia schedule. Yoshida sarcoma cells growing in vitro were used in this study. Cell survivals were estimated by the Courteney soft agar clonogenic assay. A treatment device for LDRI treatment, which held eight 137Cs sources and agar plus containing the cells, gave an irradiation dose of 51.8 cGy/h to the cells. In the experiment of simultaneous LDRI and hyperthermia, the LDRI cytotoxicities were enhanced by hyperthermia over a non-lethal temperature range. High synergistic effects were observed in a lower temperature range (40, 41 and 41.5 degrees C) in contrast to higher temperature (42, 43 and 44 degrees C). In the experiment of alternate LDRI and hyperthermia, thermotolerance induced by initial heating was well developed during the LDR exposure but less expressed in comparison with the cells which had no LDRI exposure. The treatment of LDRI for 8 h may have affected the thermotolerance development in our experimental condition.