DOSE-RATE EFFECT FOR DNA-DAMAGE INDUCED BY IONIZING-RADIATION IN HUMAN TUMOR-CELLS

The effect of dose rate on clonogenic cell survival and DNA double-str and breaks (DSBs) has been examined in a human bladder carcinoma cell line, RT112, treated with ionizing radiation. Cell survival changed ma rkedly over the range of dose rates used (0.01-1.28 Gy/min) with the c urves becoming shallower and straighter as the dose rate was lowered. Similarly, the number of DSBs measured by pulsed-field gel electrophor esis (PFGE) immediately after irradiation varied with dose rate. Fewer DSBs were detectable after low-dose-rate irradiation. However, when a 4-h repair period was allowed after irradiation, cells treated at all dose rates exhibited approximately the same amount of damage. The fin al level of unrejoined DSBs, as detected by PFGE, therefore does not c orrelate with cell survival at different dose rates.