Genetic injury in hybrid male mice exposed to low doses of 60Co gamma-rays or fission neutrons. I. Response to single doses.

Young adult male B6CF1 mice were exposed to single whole body doses of fission neutrons or 60Co gamma rays. Postspermatogonial dominant lethal injury, incidence of reciprocal chromosome translocations induced in spermatogonia, incidence of abnormal epididymal sperm 4-6 weeks after exposure, and testis weight loss 3-6 weeks after exposure were all measured. Emphasis is on response to neutron doses between 1 and 40 rad, and gamma-ray doses between 22.5 and 145 rad, although more limited data from a 4-fold higher dose range were integrated into the analysis. Significant effects were seen at 1 and 2.5 rad of neutrons consistent with extrapolation from higher doses, with the exception of dominant lethal mutations, which occurred in significant excess of expectation. Dose-response functions were linear or linear-quadratic, depending upon end point, radiation quality, and dose range. For translocation frequencies, the D2 term was negative for neutron and positive for gamma-ray irradiations. RBE values varied with dose and end point. For testis weight loss and abnormal sperm over the full dose range, the RBEs were between 5 and 6. They were between 7 and 9 at lower doses (less than 10 rad) for translocations. RBEs for postimplantation and total dominant lethal rates were 5-6 above 10 rad and 10-14 below 10 rad. The RBEs for preimplant losses were between 15 and 25 above 10 rad and possibly higher below 10 rad, although the data are statistically "noisy". The tentative interpretations of unusual results at lowest doses involve variation in cell sensitivity, cell selection, probability of neutron traversal per cell, variance of magnitude of the energy deposition events, dose rate, and DNA repair.