A life-span study on male C57BL mice after injection of various doses of241 Am was conducted. The effects on life span were evaluated and the incidence of tumors was determined by procedures that take competing risks into account. Bone tumors were induced in the mice by injections of 22 and 58 Bq241 Am per g. The mice died early from nonneoplastic diseases at the higher dose levels (190, 373, and 1197 Bq${}^{241}{\rm Am}/{\rm g}$. Additionally, spontaneously occurring tumors such as liver carcinomas, lymphosarcomas, and lymphoreticulosarcomas occurred at an enhanced rate with increasing dose level. The data for survival time after241 Am injection and death with bone tumor were compared to data collected previously for${}^{226}{\rm Ra}\text{-injected}$ mice of the same C57BL strain. This enabled direct comparison in the same strain of the effects of the bone-surface seeker241 Am to the effects of the bone-volume seeker226 Ra. The proportional hazards model was applied and the rate of death with bone tumor was 12.9 ± 5.2 times higher after241 Am injection than after226 Ra injection if the regression covariate was the average dose to the skeleton. The relative risk was 3.5 ± 1.7 if regressed on the injected radioactivity. The mortality rate after241 Am injection was 20.4 ± 3.6 times higher than after226 Ra injection if regressed on average dose to the skeleton.

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