Repair of Sublethal Damage in Mammalian Cells Irradiated at Ultrahigh Dose Rates

The lethal response of asynchronous Chinese hamster ovary (CHO) cells exposed to single and split doses of radiation at conventional or ultrahigh dose rates has been examined to determine whether repair of sublethal damage occurs in cells irradiated at ultrahigh dose rates. The high-intensity irradiations were performed with electrons delivered in single 3-nsec pulses from a 600-kV field emission source under medium-removed, thin-layer conditions. Conventional dose-rate experiments were done under identical thin-layer conditions with 50-kVp X-rays, or under full-medium conditions with 280-kVp X rays. Oxygenated cells were irradiated and maintained at 22-24°C between exposures. Survival did not increase as the time between two doses of pulsed electrons increased from 0 to 4 min, indicating no evidence of fast repair. However, increased survival was observed when 30 to 90 min was allowed to elapse between the split doses. The half-time for maximum repair was ≃30 min irrespective of the exposure conditions and radiation modality used. Observed repair ratios increased from ≃2 to 4 as the single-dose surviving fraction decreased from $10^{-2}\ \text{to}\ 5\times 10^{-4}$. Over this survival range the repair ratios, measured at the same value of surviving fraction, were independent of dose rate. The observed repair ratios imply that the shoulder regions of the nonfractionated X-ray and pulsed-electron survival curves were not completely restored between the split doses. However, the fraction of the shoulder restored between split doses of radiation was dose-rate-independent. It is concluded that sublethal damage can be repaired in oxygenated CHO cells irradiated at dose rates of the order of 1011 rad/sec.