Interpretation of Cytogenetic Damage Induced in the Germ Line of Male Mice Exposed for over 1 Year to 239 Pu Alpha Particles, Fission Neutrons, or 60 Co Gamma Rays

The relative biological effectiveness (RBE) of 239 Pu α particles, fission neutrons (0.85 MeV), and 60 Co γ rays has been evaluated for the induction of reciprocal chromosome translocations in spermatogonia and of chromosome/chromatid fragments and chromatid rearrangements in the primary spermatocyte of adult male ${\rm B}6{\rm CF}_{1}$ mice. Age concurrency was maintained for both internal and external radiations which were delivered at about 1 rad/week for 239 Pu (single intravenous dose of 10 μCi/kg), 0.67, 1.67, and 2.67 rad/week for neutrons, and 6.95, 17.4, and 32 rad/week for γ rays for at least 60 weeks. In terms of frequency of translocations, the response to the alpha emitter was nonlinear (concave downward) with little dose-response predictability; to cumulative neutron exposures the response was linear, without evidence of a dose-rate effect; and to γ radiation to responses were linear, and a significant dose-rate effect was seen. RBE estimates are variable. For translocations, the n/γ ratio is between 10 and 24, depending upon weekly dose level, and the ratio is 1 or less for the α particle relative to the neutron. For fragments, the n/γ ratio is 18 to 22, depending upon age factors, and α/n is 1.5. For chromatid rearrangements, n/γ is 7 and α/n is essentially indeterminate, but much below one. The overall response to the α emitter is interpreted to be a complex function of (a) microdosimetric heterogeneity, (b) a nearly invariant deposition pattern in the gonad, (c) the high sensitivity of differentiating spermatogonia to cell killing, and (d) the capacity of stem cells in relatively radiation-free areas to progressively assume the major spermatogenic role.