The Influence of Dose and Dose Rate on the Incidence of Neoplastic Disease in RFM Mice after Neutron Irradiation

The influence of dose and dose rate on tumorigenicity after neutron irradiation was investigated in female RFM mice exposed to various doses of neutrons at dose rates of 5 and 25 rad/min or 1 rad/day. Comparison of neutron irradiation with acute gamma irradiation showed that tumors were induced more effectively with neutrons than with gamma rays, particularly at low doses. Linear relationships were obtained at both dose rates for thymic lymphoma induction after neutron irradiation, while a dose-squared relationship was obtained for gamma rays. At low doses little dose-rate dependence was seen with neutrons, while at the highest doses the low dose rate was more effective. An increased incidence of lung adenomas was seen after neutron irradiation with doses as low as 20 rad, while the incidence after gamma-ray irradiation decreased with increased dose except at approximately 300 rad. In spite of the apparent sensitivity, the neutron dose-response curve for lung tumors was not linear. Neutron irradiation at low dose rates appeared to induce thymic lymphomas and possibly lung adenomas more efficiently than irradiation at high dose rates, but was less effective than high dose rates in inducing ovarian and pituitary tumors. Both the importance of a more complete understanding of dose and dose-rate relationships (particularly in the low dose range) and the need for examination of the basis of such relationships are discussed.