Toxicity of ²⁴¹Am in male C57BL mice: Relative risk versus ²²⁶Ra

A life-span study on male C57BL mice after injection of various doses of ²⁴¹Am 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 Bq ²⁴¹Am per g. The mice died early from nonneoplastic diseases at the higher dose levels (190, 373, and 1197 Bq ²⁴¹Am/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 after ²⁴¹Am injection and death with bone tumor were compared to data collected previously for ²²⁶Ra-injected mice of the same C57BL strain. This enabled direct comparison in the same strain of the effects of the bone-surface seeker ²⁴¹Am to the effects of the bone-volume seeker ²²⁶Ra. The proportional hazards model was applied and the rate of death with bone tumor was 12.9 ± 5.2 times higher after ²⁴¹Am injection than after ²²⁶Ra 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 after ²⁴¹Am injection was 20.4 ± 3.6 times higher than after ²²⁶Ra injection if regressed on average dose to the skeleton.