Accumulative effect of two low doses of irradiation in inducing an adaptive response in human lymphocytes.

A chromosomal adaptive response to Co-60 gamma-rays in human lymphocytes was observed over a range of 1-20 cGy pre-exposure doses, with 1 cGy giving optimal reduction in chromatid breaks. A 0.5 cGy dose, which in itself did not induce an adaptive response, did so when given twice within the same cell cycle, and the magnitude of the accumulative effect was strongest when there was an interval of 6 h between the two adaptive doses and between the second adaptive dose and a challenge dose. Reductions equivalent in effect to a single 1 cGy dose were seen when a 0.5 cGy dose was given twice. Delivering two 1 cGy doses had no greater effect than did a single 1 cGy dose.