Pattern of uptake of americium-241 by the rat skeleton and its subsequent redistribution and retention: implications for human dosimetry and toxicology.

The distribution and retention of intravenously injected 241Am 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 241Am intake by man. They showed that in the rat approximately one third of the intravenously injected 241Am was deposited in the skeleton where it appeared to be retained with a long biological half-time. The studies also showed: 1 241Am 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, 2 Bone accretion results in the burial of surface deposits of 241Am, 3 Bone resorption causes the removal of 241Am from surfaces, 4 Resorbed 241Am is retained by phagocytic cells (probably macrophages) in the bone marrow, 5 The transfer of 241Am from the phagocytic cells in the marrow to adjacent bone surfaces seems to occur, (local recycling). 6 The possibility that some of the 241Am removed from the bone surfaces enters the blood and is redeposited in bone, (systemic recycling) cannot be dismissed. These results show that 241Am deposition and redistribution in bone shares many characteristics with other 'bone surface-seeking radionuclides' typified by 239Pu. Consequently, it is suggested that a similar model to that used to calculate annual limits of intake for 239Pu in man would be suitable for the calculation of corresponding values for the 241Am isotopes.