HYPERSENSITIVITY OF A HUMAN TUMOR-CELL LINE TO VERY-LOW RADIATION-DOSES

Survival of HT29 cells was measured after irradiation with single dose s of X-rays (0.05-5 Gy) and neutrons (0.025-1-5 Gy), using a Dynamic M icroscopic Imaging Processing Scanner (DMIPS) with which individual ce lls can be accurately located in tissue culture flasks, their position s recorded, and after an appropriate incubation time the recorded posi tions revisited to allow the scoring of survivors. The response over t he X-ray dose range 2-5 Gy showed a good fit to a Linear-Quadratic (LQ ) model. For X-ray doses below 1 Gy, an increased X-ray effectiveness was observed with cell survival bclow the high-dose LQ prediction. The value of -dose/log(e)(SF) for each experimental data point, plotted a gainst dose, demonstrated clearly how X-rays are maximally effective a t doses approaching zero, becoming less effective as the dose increase s and with minimal effectiveness at about 0-6Gy then becoming more eff ective again as the dose increases above 1.5 Gy. This phenomenon was n ot seen with neutrons. Neutron RBE was calculated for each X-ray data point by taking each X-ray survival value and comparing it with the co mmon LQ fit to all the neutron data. Over the X-ray dose range 0.05-0. 2Gy, the RBE is close to 1 indicating that these very low doses of X-r ays are of similar effectiveness to neutrons in killing cells. The inc rease in RBE with increasing dose over the range 0.05-1 Gy, and the sl ight decrease in RBE above 1 Gy, reflect primarily the changes in X-ra y sensitivity over the whole dose range of 0.05-5 Gy. Several argument s suggest that this phenomenon could reflect an induced radioresistanc e so that in this system low single doses of X-rays are more effective per Gy than higher doses in reducing cell survival because only at hi gher doses, above a threshold, is there sufficient damage to trigger r adioprotective mechanisms.