The Effect of Radiation on the Survival in Vitro of Rat Thymocytes of Different Size

exposures (10 to 1500 R; 60Co). The survival following all doses of radiation decreased as the cell size increased. The exponential dose-survival curves for each of the five size classifications consisted of two components; the percentage of survival decreased with doses up to about 300 R but became flat and independent of the radiation exposure at higher doses. The level of the flat portion varied inversely with cell diameter, decreasing from about 68% survival for the smallest (4.9 microns) to 12% for the largest (10.4 microns) cells. These data suggest that, once thymocytes are lethally injured by radiation, the rate of degeneration is governed by the rate of physical and chemical processes within the cell; the speed of these processes increases as the cell size increases. The slopes of the dose-dependent component of the dose-survival curves were found to be similar for cells from 4.9 to 7.9 microns in diameter, but were two- and fourfold greater for larger cells, indicating an increased radiosensitivity among the large cells of the isolated population.