Genetic injury in hybrid male mice exposed to low doses of 60Co gamma-rays or fission neutrons. III. Frequencies of abnormal sperm and reciprocal translocations measured during and following long-term weekly exposures.

Male B6CF1 mice were exposed to either fission neutrons or 60Co gamma-rays at once-weekly doses approaching occupational levels for periods up to 60 weeks. Both during and after the irradiation sequence, the mice were screened to determine the incidence of abnormal epididymal sperm and of reciprocal chromosome translocations in metaphase spermatocytes. Abnormal sperm frequencies equilibrated with dose/week by 10 weeks and also showed an additive nonlinear seasonal increment. The relative biological effectiveness (RBE) for these damages is 12 +/- 1. After exposures ended, sperm frequencies in gamma-irradiated mice quickly returned to near-normal levels. Neutron-irradiated males showed a significantly elevated level of abnormalities for approximately 30 weeks--a paradoxical finding--as no clear evidence of cumulative injury was seen during exposure. When assayed at 10 and 25 weeks of exposure but not later, translocation frequencies demonstrated an increment, significant in the neutron series, attributed to irradiated spermatocytes. Dose-response analysis with cumulative dose up to the 60-week maximum gave an RBE of 45 +/- 10. Post exposure, the incidence of translocations subsided slightly, but the RBE remained above 30.