Inhibition of repair of radiation-induced strand breaks by hyperthermia, and its relationship to cell survival after hyperthermia alone.

Ehrlich ascites cells growing in vitro have been used to investigate the influence of hyperthermia on the formation and repair of DNA strand breaks after X-irradiation. Different heat pretreatments were given immediately prior to a dose of 6 Gy of X-rays. When a temperature of 42 degrees C was used, up to 4 hours of pretreatment had only a slight inhibitory effect on the repair of DNA strand breaks induced by radiation. At a temperature of 43 degrees C progressively more inhibition was observed with longer treatment times. This was also the case for temperatures of 44 degrees C and 45 degrees C. When the treatment times at 43-45 degrees C were longer, strand breaks in DNA were induced by the hyperthermic treatment alone. Under these conditions almost no repair was found of strand breaks induced by a subsequent radiation dose. The data obtained strongly suggest a correlation between the effect of the hyperthermic treatment alone on cell survival and the kinetics of repair of strand breaks in DNA as induced by radiation.