Toxicity and mutagenicity of low dose rates of ionizing radiation from tritiated water in human lymphoblastoid cells.

In order to study the toxic and mutagenic effects of low-dose-rate exposure to ionizing radiation, human lymphoblast cells were grown continuously in tritiated water (3H2O) for up to 8 days. Dose rates ranged from 0.0054 to 0.064 rad/min. Mutation to trifluorothymidine resistance (TK locus) and 6-thioguanine resistance (HGPRT locus) was measured; comparable results were observed at both loci. The mutant fraction as a function of total absorbed dose was independent of dose rate over the range studied. At the lower doses, the dose-response curve was linear, with no indication of a threshold. Overall, it appeared to be slightly biphasic with a diminished slope at higher total doses. These data are discussed in relation to earlier studies utilizing high-dose-rate X-irradiation and incorporated [3H]TdR; 3H2O and [3H]TdR were more mutagenic per rad than X-rays, but 3H2O was less cytotoxic than either X-rays or [3H]TdR.