Kumar, P. R. V., Mohankumar, M. N., Hamza, V. Z. and Jeevanram, R. K. Dose-Rate Effect on the Induction of HPRT Mutants in Human G₀ Lymphocytes Exposed In Vitro to Gamma Radiation. Radiat. Res. 165, 43–50 (2006).

The influence of dose rate on expression time, cell survival and mutant frequency at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) locus was evaluated in human G₀ peripheral blood lymphocytes exposed in vitro to γ rays at low (0.0014 Gy/min) and high (0.85 Gy/min) dose rates. A cloning assay performed on different days of postirradiation incubation indicated an 8-day maximum expression period for the induction of HPRT mutants at both high and low dose rates. Cell survival increased markedly with decreasing dose rate, yielding D₀ values of 3.04 Gy and 1.3 Gy at low and high dose rates, respectively. The D₀ of 3.04 Gy obtained at low dose rate could be attributed to the repair of sublethal DNA damage taking place during prolonged exposure to low-LET radiation. Regression analysis of the mutant frequency yielded slopes of 12.35 × 10⁻⁶ and 3.66 × 10⁻⁶ mutants per gray at high and low dose rate, respectively. A dose and dose-rate effectiveness factor of 3.4 indicated a marked dose-rate effect on the induced HPRT mutant frequency. The results indicate that information obtained from in vitro measurements of dose-rate effects in human G₀ lymphocytes may be a useful parameter for risk estimation in radiation protection.