J.L. Redpath, D. Liang, T.H. Taylor, C. James and E. Elmore
U.C. Irvine, Irvine CA 92697
The shape of the dose-response curve in the low dose region (<10 cGy) for the induction of cancer by radiation is a topic of both considerable importance and controversy. Current radiation risk estimation is based on a linear extrapolation of cancer incidences seen after high doses (>100 cGy). There is epidemiological evidence which is suggestive that such a linear extrapolation may be an overestimate. There are both biophysical and biological arguments that can be made to support the idea that linear extrapolation could well result in an overestimation of risk for low LET radiation exposure. One biological argument involves the real phenomenon of an adaptive response to low doses of radiation. We have previously observed an adaptive response (p<0.05) against spontaneous neoplastic transformation induced by 1 cGy of Cs-137 gamma radiation using the human skin fibroblast x HeLa hybrid cell neoplastic transformation assay (Radiat. Res. 149: 517-520, 1998). We now report an extension of that study to doses of 0.1 0.5, 5.0 and 10.0 cGy, as well as a repeat of the 1 cGy data. The results indicate that such an adaptive response is seen at each of these doses. Taken individually the data are not significant at the p<0.05 level, however taken as a group they are. Data will also be presented for the higher doses of 30 and 50 cGy so that a complete dose-response curve encompassing both high and low dose regions can be presented. Linear extrapolation from the higher doses through the zero dose point indicates a statistically significant departure from linearity at doses up to 10 cGy. Taken together with similar findings in epidemiological studies, provides further support to the notion that risks at low doses may indeed be lower than currently estimated. Supported by DOE Grant DE-FG07-99ER62876.