Survival of mammalian cells exposed to X rays at ultra-high dose-rates

X irradiation delivered in single, nanosecond-length pulses, so that the instantaneous dose-rate exceeded 7 × 1010 rads/second, was less effective in abolishing the subsequent reproductive capacity of HeLa and Chinese hamster cells in vitro than was irradiation delivered at lower dose-rates. It was felt that this was not due to the production of transient intracellular hypoxia during the radiation pulse in these mammalian cells, and an alternative hypothesis is proposed to explain these results. Possible applications of ultra-high dose-rate irradiation in radiotherapy are discussed.