The effect of very low radiation doses on the human bladder carcinoma cell line RT112.

Human RT112 cells were exposed to single doses of X-rays (0.05-5 Gy), and cell survival was measured using a Dynamic Microscopic Imaging Processing Scanner (DMIPS) with which individual cells can be located in tissue-culture flasks, their positions recorded, and after an appropriate incubation time the recorded positions revisited to allow the accurate scoring of survivors. The response over the X-ray dose range 1-5 Gy showed a good fit to a Linear-Quadratic (LQ) model. For X-ray doses < 1 Gy, an increased effect of X-rays was observed with cell survival below the prediction from the LQ model extrapolated from higher doses. Several arguments suggest that this phenomenon could reflect an induced radioresistance so that in this cell line, low single doses of X-rays are more effective per Gray than higher doses in reducing cell survival because only at higher doses, above a threshold, is there sufficient damage to trigger radioprotective mechanisms.