High-resolution measurements of the survival of asynchronous Chinese hamster V79-379A cells in vitro after single doses of X rays (0.01-10.0 Gy) and neutrons (0.02-3.0 Gy) were made using a computerized microscope for locating and identifying cells (Palcic and Jaggi, Int. J. Radiat. Biol. 50, 345-352, 1986). The X-ray response from 1 to 10 Gy showed a good fit to a linear-quadratic (LQ) dose-survival model, but with X-ray doses below 0.6 Gy, an increased X-ray effectiveness was observed, with cell survival below the prediction made from the data above 1 Gy using the LQ model. The effect per unit dose <tex-math>$(-{\rm log}_{e}{\rm SF}/\text{dose})$</tex-math> increased by a factor of ∼2, from <tex-math>$0.19\ {\rm Gy}^{-1}$</tex-math> at a dose of 1 Gy to <tex-math>$0.37\ {\rm Gy}^{-1}$</tex-math> at a dose of 0.1 Gy. This phenomenon was not seen with neutrons, and cell survival decreased exponentially over the whole neutron dose range studied. Further data suggest that this phenomenon is unlikely to be due to a subpopulation of X-ray-sensitive cells determined either genetically or phenotypically by distribution of the population within the cell cycle. The existence of low-dose sensitivity also appeared to be independent of dose rate in the range <tex-math>$0.016-1.7\ {\rm Gy}\ {\rm min}^{-1}$</tex-math>. A possible explanation of these results is that the phenomenon reflects "induced repair" or a stress response: low doses in vitro (or low doses per fraction in vivo) are more effective per gray than higher doses because only at the higher doses is there sufficient damage to trigger repair systems or other radioprotective mechanisms.

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