Inhalation carcinogenesis of high-fired 241AmO2 in rats

Wistar rats were given a single inhalation exposure to high-fired 241AmO2 particles and examined over their life span. A total of 310 rats were used: 259 exposed to 241Am for life-span study, 30 exposed to 241Am for early metabolism study, and 21 unexposed life-span controls. The activity median aerodynamic diameter of the aerosols was 0.75-1.39 microns. About 55% of alveolarly deposited 241Am was cleared from the lung with a half-life of 0.5 days, 37% with a half-life of 7 days, and 8% with a half-life of 580 days. Group mean lung doses ranged from less than 5.7 rad up to 1500 rad. Significant early mortality due to radiation pneumonitis was seen only in the highest exposure group. The percentage of rats with lung tumors was 0% for controls (21 rats), 1% at lifetime lung doses less than 10 rad (139 rats), 7% at 10-50 rad (86 rats), 0% at 50-100 rad (9 rats), 60% at 100-500 rad (10 rats), and 7% at 500 rad (15 rats). Only one liver and one bone tumor were found in all exposed rats, both at lifetime tissue doses less than 10 rad. The fate and carcinogenicity of inhaled 241AmO2 in the lung of rats were similar to what has previously been described for inhaled 244CmO2.