Tumor induction in BALB/c female mice after fission neutron or γ irradiation

This study was designed to examine the dose-response relationships for tumor induction after neutron irradiation in female BALB/c mice, with emphasis on the response in the dose range 0 to 50 rad. Tumors induced after radiation exposure included ovarian tumors, lung adenocarcinomas, and mammary adenocarcinomas. For comparison the dose responses for induction of these tumors after ¹³⁷Cs γ irradiation were also examined. As previously described for the female RFM mouse, the data for ovarian tumor induction after neutron and γ irradiation were consistent with a threshold model. For lung and mammary tumors the dose-response curve after neutron irradiation appeared to 'bend over' in the dose range 10 to 20 rad. The factors responsible for this bend-over and their relative contributions to the overall form of the dose-response relationship are not presently known. However, these data strongly indicate that extrapolation from data above 50 rad could result in a significant underestimate of risks. Further, it is clear that current models of neutron carcinogenesis are inadequate, since such a bend-over is not predicted at these low dose levels.