Effects of different ionizing radiations on human cells in tissue culture; II. Biological experiments

The differences observed in the degree of damage caused by the same doses of various ionizing radiations are generally assumed to be due to differences in spatial distribution of ionizations. Though many experiments on the relative biological effectiveness of various radiations have been carried out, the mechanism of action responsible for the high effectiveness with which densily ionizing radiations damage the reproductive capacity of many types of cells is still imperfectly understood (1, 2). A few years ago, Puck et al. (3) showed the feasibility of plating single mammalian cells such that each of them develops into a colony (clone) of macroscopic size. Investigation of the effects of different radiations on this reproductive capacity is of interest, since Puck (4) showed such cells to be extremely sensitive to X-irradiation. Results reported in this paper show that after a dose of 210 rads of X-radiation or 45 rads of a-radiation 50 % of the cells have lost the ability to form clones. The experiments to be described pertain to irradiations of cells from human origin with a-, B-, and X-radiation. The investigations included effects of single and fractionated doses, combination of aand X-radiation, and variation of dose rates. The irradiation techniques and dosimetry have been described in a preceding paper (5). In all experiments the cells were plated on modified petri dishes with a bottom of Melinex, 0.85 mg/cm2 thick. Experiments on the modification of the RBE by anoxia and protecting agents are in progress.