Dose Responses for Adaption to Low Doses of 60Co γ Rays and 3H β Particles in Normal Human Fibroblasts

Broome, E. J., Brown, D. L. and Mitchel, R. E. J. Dose Responses for Adaption to Low Doses of 60Co γ Rays and 3H β Particles in Normal Human Fibroblasts. Radiat. Res. 158, 181–186 (2002). The dose response for adaption to radiation at low doses was compared in normal human fibroblasts (AG1522) exposed to either 60Co γ rays or 3H β particles. Cells were grown in culture to confluence and exposed at either 37°C or 0°C to 3H β-particle or 60Co γ-ray adapting doses ranging from 0.1 mGy to 500 mGy. These cells, and unexposed control cells, were allowed to adapt during a fixed 3-h, 37°C incubation prior to a 4-Gy challenge dose of 60Co γ rays. Adaption was assessed by measuring micronucleus frequency in cytokinesis-blocked, binucleate cells. No adaption was detected in cells exposed to 60Co γ radiation at 37°C after a dose of 0.1 mGy given at a low dose rate or to 500 mGy given at a high dose rate. However, low-dose-rate exposure (1–3 mGy/min) to any dose between 1 and 500 mGy from either radiation, delivered at either temperature, caused cells to adapt and reduced the micronucleus frequency that resulted from the subsequent 4-Gy exposure. Within this dose range, the magnitude of the reduction was the same, regardless of the dose or radiation type. These results demonstrate that doses as low as (on average) about one track per cell (1 mGy) produce the same maximum adaptive response as do doses that deposit many tracks per cell, and that the two radiations were not different in this regard. Exposure at a temperature where metabolic processes, including DNA repair, were inactive (0°C) did not alter the result, indicating that the adaptive response is not sensitive to changes in the accumulation of DNA damage within this range. The results also show that the RBE for low doses of tritium β-particle radiation is 1, using adaption as the end point.