Dose responses for chromosome aberrations produced in noncycling primary human fibroblasts by alpha particles, and by gamma rays delivered at sublimiting low dose rates

As the total dose of X or γ rays is delivered at lower and lower rates, the yield of chromosome aberrations progressively diminishes. Simultaneously, the shape of the dose response changes from one exhibiting pronounced upward curvature at high dose rates to one approaching linearity at low dose rates. Although the maximum sparing effect caused by lowering the dose rate can be predicted from classical cytogenetic theory, it has yet to be verified experimentally. Here, noncycling normal human fibroblasts were exposed to graded doses of ¹³⁷Cs γ rays at chronic dose rates of 6.3 and 2.8 cGy h^-1, dose rates that we reasoned should be lower than those required to achieve maximal sparing. This was indeed shown to be the case, after it was determined that the two chronic dose rates produced identical linear dose responses of 0.05 total aberrations per cell Gy^-1. Consistent with cytogenetic theory, this value was statistically indistinguishable from the linear coefficient derived from a fit to aberration frequencies produced by high-dose-rate exposure. Exposure to ²³⁸Pu α particles also produced a linear dose response for total aberrations, whose slope - with respect to ¹³⁷CS γ rays as a reference radiation - implied a maximum RBE of 35 ± 2.