Combined electron radiation and hyperthermia. Repair of DNA strand breaks in NHIK 3025 cells irradiated and incubated at 37, 42.5, or 45 degrees C.

Induction of DNA strand breaks by a short electron pulse (18.5 Gy) and the subsequent strand-break rejoining were investigated at hyperthermia (42.5 and 45 degrees C) and at 37 degrees C during irradiation and repair. The cells were irradiated immediately after 2.5 min equilibration (i.e., from 37 to 42.5 or 45 degrees C) to investigate the effect of short-duration hyperthermia on radiation damage and subsequent repair. Due to a high radiation dose rate and a rapid lysis technique, the cells could be kept at the actual temperature during irradiation and repair, and the strand-break frequency could be measured only seconds after irradiation. At all temperatures, a constant or possible increase in the initial number of breaks was observed during the first 7 sec after the electron pulse. At 37 degrees C, strand-break rejoining was nearly complete within 1 hr. Hyperthermia at 42.5 degrees C had only minor influence on the net rate of strand-break rejoining. At 45 degrees C, 50% of the breaks remained after 1 hr. Subsequent incubation for 23 hr at 37 degrees C reduced by half the number of breaks remaining at 1 hr in irradiated samples. Unirradiated samples exposed to the same heat treatment showed a significant increase in the number of DNA strand breaks. Thus, heat treatment at 45 degrees C may lead to a combined effect of reduced rejoining capacity and formation of breaks after the electron pulse which in turn may be responsible for increased cell death when both modalities are employed.