The Role of Dose Rate in the Induction of Micronuclei in Deep-Lung Fibroblasts In Vivo after Exposure to Cobalt-60 Gamma Rays

To evaluate the influence of low-dose-rate exposures on biological damage, it is necessary to have cells that can be maintained in the same stage of the cell cycle for long periods. Normal rat lung fibroblasts represent a stable cell type with a slow turnover rate in vivo. These cells can be stimulated to divide by placing them in tissue culture. Therefore, a constant cell population can be exposed over a protracted time and stimulated to divide, and the cytogenetic damage can be evaluated at the first cell division after exposure. By placing rats at different distances from a 60 Co source, they were exposed to graded doses of γ rays-0.0, 3.9, 7.4 and 11.3 Gy-protracted over either 4 or 67 h. Fibroblasts were isolated from the lung and cultured for 24 h; after cytochalasin B was added, the cells were cultured for an additional 69 to 72 h. The percentage binucleated cells in fibroblasts of animals exposed for 4 or 67 h was 47.1 ± 4.3 and 62.1 ± 3.9. There was no influence of dose on the percentage binucleated cells, but the fraction of cells that divided at 67 h was significantly higher (P < 0.05) than observed at 4 h. Cells were scored for micronuclei on coded slides. The dose-response data from animals exposed for 4 and 67 h were fitted to the following linear dose-response relationships, where D = dose: micronuclei/binucleated $\text{cell}=0.02\pm 0.03+2.38\pm 0.44\times 10^{-2}\ D$, and micronuclei/binucleated $\text{cell}=0.01\pm 0.06+1.01\pm 0.10\times 10^{-2}\ D$, respectively. The r2 values for the two curves were 0.67 and 0.91, indicating the goodness of fit for the data for the 4- and 67-h treatments. The slopes were different from zero and each other at the P < 0.05 level of significance. The effectiveness of the 60 Co exposure decreased as the dose rate decreased. At dose rates below 0.17 Gy/h, the effectiveness remained constant over the range of doses and dose rates used. Comparing the slope of the dose response for the lowest exposure rate to that from information published previously, the dose-rate effectiveness factor was 6.14 ± 0.65 for the induction of micronuclei in deep-lung fibroblasts. This paper describes how these data, derived at low dose rates, were used to estimate that the relative biological effectiveness (${\rm RBE}_{{\rm M}}$) of 60 Co and radon was 65.2 ± 8.4 when the exposures were delivered over similar lengths of time. These studies emphasize the importance of using the proper dose rate for low-LET radiation in estimation