It is well known that cells of human tumor cell lines display a wide range of sensitivity to radiation, at least a part of which can be attributed to different capacities to process and repair radiation damage correctly. We have examined the response to very low-dose radiation of cells of five human tumor cell lines that display varying sensitivity to radiation, using an improved assay for measurement of radiation survival. This assay improves on the precision of conventional techniques by accurately determining the numbers of cells at risk, and has allowed us to measure radiation survival to doses as low as 0.05 Gy. Because of the statistical limitations in measuring radiation survival at very low doses, extensive averaging of data was used to determine the survival response accurately. Our results show that the four most resistant cell lines exhibit a region of initial low-dose hypersensitivity. This hypersensitivity is followed by an increase in radioresistance over the dose range 0.3 to 0.7 Gy, beyond which the response is typical of that seen in most survival curves. Mathematical modeling of the responses suggests that this phenomenon is not due to a small subpopulation of sensitive cells (e.g. mitotic), but rather is a reflection of the induction of resistance in the whole cell population, or at least a significant proportion of the whole cell population. These results suggest that a dose-dependent alteration in the processing of DNA damage over the initial low-dose region of cell survival may contribute to radioresistance in some cell lines.

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