Pattern of Uptake of Americium-241 by the Rat Skeleton and its Subsequent Redistribution and Retention: Implications for Human Dosimetry and Toxicology

Abstract

<jats:p> The distribution and retention of intravenously injected <jats:sup>241</jats:sup>Am in the skeleton of the female rat has been investigated using autoradiographic and radiochemical techniques. The studies were designed to assess the dosimetric and toxicologic implications of an <jats:sup>241</jats:sup>Am intake by man. They showed that in the rat approximately one third of the intravenously injected <jats:sup>241</jats:sup>Am was deposited in the skeleton where it appeared to be retained with a long biological half-time. The studies also showed: </jats:p><jats:p> 1<jats:sup>241</jats:sup>Am is initially deposited onto all types of bone surface including endosteal surfaces, periosteal surfaces and those of the vascular canals within cortical bone, but seems to be preferentially deposited onto those that are resorbing, </jats:p><jats:p> 2 Bone accretion results in the burial of surface deposits of <jats:sup>241</jats:sup>Am, </jats:p><jats:p> 3 Bone resorption causes the removal of 241 Am from surfaces, </jats:p><jats:p> 4 Resorbed <jats:sup> 241</jats:sup>Am is retained by phagocytic cells (probably macrophages) in the bone marrow, </jats:p><jats:p> 5 The transfer of <jats:sup> 241</jats:sup>Am from the phagocytic cells in the marrow to adjacent bone surfaces seems to occur, (local recycling). </jats:p><jats:p> 6 The possibility that some of the <jats:sup>241</jats:sup>Am removed from the bone surfaces enters the blood and is redeposited in bone, (systemic recycling) cannot be dismissed </jats:p><jats:p> These results show that <jats:sup> 241</jats:sup>Am deposition and redistribution in bone shares many characteristics with other 'bone surface-seeking radionuclides' typified by <jats:sup>239</jats:sup>Pu. Consequently, it is suggested that a similar model to that used to calculate annual limits of intake for <jats:sup>239</jats:sup>Pu in man would be suitable for the calculation of corresponding values for the <jats:sup>241</jats:sup>Am isotopes. </jats:p>

Autoradiographic Studies of the Distribution of Radium-226 in Rat Bone: Their Implications for Human Radiation Dosimetry and Toxicity

“…Discussion ' Pattern of uptake The results of the present experiments show that most of the 226 Ra deposited in the rat skeleton is present as a volume deposit and is fairly evenly distributed throughout the bone matrix. This pattern of uptake is very different to that described for other a-emitting bone-seeking radionuclides such as 239pu (Green et al, 1978;Jee, 1972;Priest & Giannola, (1980),241Am (Polig, 1978;Priest et al, 1982b) and 233U (Stevens et al, 1980;Priest et al, 1982a) which deposit almost exclusively on bone surfaces. The attachment of 226 Ra to the bone matrix must in part be reversible as in excess of 20 % of the bone radium is lost to excretion within the first four days after its injection.…”

ICRP Publication 141: Occupational Intakes of Radionuclides: Part 4

“…Results of early autoradiographic studies on rodents (Hamilton, 1948) indicated that the sites of deposition on bone surfaces are similar for americium, curium, and actinium, but that the surface distribution of these elements differed from that of plutonium. Later studies involving refined techniques and various animal species yielded relatively detailed descriptions of the distribution of some actinide elements, particularly plutonium and americium, on bone surfaces (Herring et al., 1962; Lloyd et al., 1972a; Durbin, 1973; Priest et al., 1983). Plutonium deposits mainly on endosteal surfaces, especially the surfaces of the trabeculae of spongy bone near the sinusoidal circulation of active marrow (Durbin, 2011).…”

“…Long-term studies on dogs (Lloyd et al., 1970; Mewhinney et al., 1982) indicate that a large portion of the initial liver burden gradually transfers to the skeleton. (950) Hamilton (1948) described the sites of bone deposition of americium and curium in rodents as indistinguishable from those of the trivalent elements cerium, promethium, and actinium, but different from sites of deposition of the tetravalent elements plutonium, thorium, and zirconium. Later studies involving a variety of animal species indicate that americium deposits on all types of bone surfaces, including resorbing and forming surfaces (Herring, 1962; Lloyd et al., 1972a; Durbin, 1973; Priest et al., 1983). Deposition on bone surfaces is more uniform than that of plutonium, although there are gradations in the intensity of the americium label.…”

“…Americium deposits to a greater extent than plutonium on cortical vascular channels (Hamilton, 1948; Herring et al., 1962). (951) Priest et al. (1983) studied the systemic behaviour of 241 Am in rats over the first month after administration, with emphasis on its behaviour in bone.…”

Microdistribution of237Np in the Skeleton of Female Rats

“…Days after injection 1982, Crawley et al 1976, Priest et al . 1983) with the lowest levels in the tail and paws and highest levels in the ribs and the pelvis .…”