Radiation-induced chromosome damage in human peripheral blood lymphocytes in vitro II. RBE and dose-rate studies with 60Co γ- and X-rays

Abstract Human peripheral blood lymphocytes were exposed, in vitro , to acute doses (2-min or 10-min exposures) of 250 kV X-rays and 60 Co γ-rays, or to chronic doses (24-, 48- or 72-h exposures) of γ-rays. Chromosome aberrations were scored at the first mitosis after stimulation with phytohemagglutinin. Yields of dicentric aberrations after X-irradiation agreed with those reported by Evans who obtained a dose exponent ( n ) of 1.17 ± 0.04 on a power law model. Our dose exponent for acute γ-irradiation was 1.24 ± 0.06 and the Relative Biological Effectiveness of 60 Co γ-rays compared with 250 kV X-rays was found to be 0.82 ± 0.03. The acute γ-ray data were analysed on a quadratic model on the assumption that the majority of dicentrics had been induced by a single ionizing track (the linear component) but that some were induced by two independent tracks (the quadratic component). On “classical” theory only the 2-track aberrations should be reduced by reducing the dose-rate (by increasing the exposure time) and the dose-response curve should become progressively more linear. In contrast we found that the dose-response curve for 24-h γ-exposures had a greater curvature ( n = 1.52 ± 0.10) than the acute response and the dicentric yields had fallen below the level of the I-track component of the acute curve. It was shown that the reduction in yield for chronic exposures was not the result of a depletion of oxygen during the 24-h irradiation. It is suggested that the analysis of dose-response curves for exchange aberrations in terms of a dose-rate dependent 2-track process and a dose-rate independent 1-track process might not be justified. However, further speculation on the present results must await resolution of the discrepancies between different laboratories in the shapes of dose-response curves for radiation-induced chromosome aberrations induced in peripheral blood lymphocytes in vitro