Induction and Subsequent Repair of DNA Damage by Fast Neutrons in Cultured Mammalian Cells

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Abstract
The induction and repair of DNA damage were studied by a DNA unwinding method in mouse L5178Y cells exposed to fast neutrons. DNA lesions induced by fast neutrons were classified into three types from their repair profiles: fast-reparable breaks (T 1/2 = 3-5 min), slow-reparable breaks (T 1/2 = 70 min), and nonreparable breaks. The repairs rates of both fast-reparable and slow-reparable breaks were almost the same as those of corresponding damage induced by low-LET radiation. Neutrons induced a smaller amount of fast-reparable damage, an almost equal amount of slow-reparable damage, and a larger amount of nonreparable damage than those induced by equal doses of .gamma. rays or X rays. RBEs for fast- and slow-reparable damage were 0.3 and 0.9, respectively. The RBE for nonreparable damage owas dose dependent and was 1.4 at the level of 100 breaks/1012 Ca DNA. Among the three types of lesions, only the nonreparable damage levels correlated with the linear-quadratic shape of the survival curves and with the enhanced killing effectiveness of neutrons (RBE = 1.7 at D0).