Gamma rays: further evidence for lack of a threshold dose for lethality to human cells

In experiments designed to measure human cell survival sith +/- 2 percent accuracy it was found that low doses (21 to 87 rad) of gamma-rays inactivated the colony-forming ability of cultured human cells with a probability of 0.00226 +/- 0.00012 per rad. There appears to be no threshold for the lethality of radiation to human cells in vitro.