The influence of dose and dose rate on the oncogenic effectiveness of 1-MeV neutrons, 5-MeV neutrons, 250-kVp x-rays, and60 Co gamma-rays was investigated in male and female RF mice exposed to various doses of whole-body radiation at dose rates of 10-6 to$10^{2}\ \text{rads}/{\rm min}$, beginning at 10 weeks of age. Incidence of myeloid leukemia and of thymic lymphoma increased with dose, also varying with dose rate of gamma-rays but not of neutrons. Neutrons were equally or slightly less leukemogenic than x- or gamma-rays at high dose rates but several times as leukemogenic as gamma-rays at low dose rates. Incidence of ovarian tumors also varied as a function of dose and dose rate in gamma-irradiated mice, but relatively few such tumors were induced by neutrons. The cumulative incidence of nonthymic lymphomas and lung tumors was increased by gamma-irradiation at low dose rates but was decreased at high dose rates, the decrease being greater and less dependent on intensity with neutrons than with x- and gamma-rays. Other solid neoplasms were also observed but in insufficient numbers to define quantitative dose-effect relations.

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