The distribution and retention of 241Am was measured in two adult female baboons after single intravenous injections of 241Am citrate. Retention of 241Am in the whole body, skeleton and liver was determined by in vivo scintillation measurements of the 60 keV gamma-ray. In addition to “specific site” in viuo measurements, serial biopsy samples were obtained to determine the rate of clearance of 241Am from the liver. Clearance of 241Am from the blood was rapid. Less than 1 % of the injected dose remained at 48 hr and less than 0.05 % remained one month post injection. Greater than 90 % of the activity was associated with the plasma fraction. From analysis of daily excreta, 10 % of the injected dose was excreted by the end of the first week and 15 % by the end of the first month. Approximately 70 % was urinary excretion; however, the urine to fecal clearance ratio varied from 0.06 during the first week to greater than 20 at 3 months. Estimates of the effective half time of americium in the total body from excreta analysis and from in vivo measurements were 274 and 263 days, respectively. Clearance half times from the liver evaluated by in vivo and biopsy analysis were 150 and 154 days, respectively. In vivo measurements over the head showed no detectable clearance from the skull during the same period. The animals were sacrificed at one and three months post-injection. In order of decreasing total activity the major loci were the skeleton, liver, lungs and kidney, which accounted for about 85 % of the retained americium. At the end of one month the highest concentration of activity was found in the liver with concentrations decreasing in the order of bone, lung, aorta, kidney and spleen. Among various bones, the concentration of 241Am varied within a factor of 4, being highest in vertebrae. The concentration in the ends of long bones was 2–3 times higher than in the shafts.
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