A study was initiated to investigate the relationship between the concentration of ²³⁹Pu on trabecular bone surfaces and the rate of trabecular bone formation at specific skeletal sites of the beagle. The contribution of the above parameters to the incidence of ²³⁹Pu induced osteosarcomas was also evaluated. Young adult beagles were administered single intravenous injections of ~0.016 uCi/kg of monomeric ²³⁹Pu-citrate. Beagles injected with citrate only served as controls. All animals were administered tetracycline derivatives intravenously in order to label actively forming bone surfaces. Two ²³⁹Pu-injected beagles and a control were sacrificed at each of seven different time periods from 7 to 365 days post-injection. Neutron-induced autoradio graphs were produced from thick bone sections subjected to a thermal neutron fluence of 10¹⁶ n/cm². The ²³⁹Pu concentration on trabecular bone surfaces was determined from microscopic counts of fission fragment tracks. The rate of trabecular bone formation was calculated from values for the surface/volume ratio of trabecular bone, the fraction of trabecular surface undergoing active bone formation, and the bone apposition rate.
The lumbar vertebra, pelvis, and proximal humerus, each of which exhibits a high incidence of ²³⁹Pu-induced osteosarcoma, were found to have a high initial concentration of 239pu on their trabecular surfaces (~7-8 pCi/cm²) and a relatively high rate of trabecular bone formation. The ²³⁹Pu concentration at these sites decreased to 22-3 pCi/cm² at the end of the first year. On the other hand, the skeletal sites with a low tumor incidence, the proximal ulna and distal humerus, had a low initial concentration of ²³⁹Pu on their trabecular surfaces (~1-2 pCi/cm²) and a significantly lower rate of trabecular bone formation. The ²³⁹Pu concentration at these sites remained nearly constant throughout the experimental period. These data suggest that the microdistribution and retention of ²³⁹Pu on the surfaces of trabecular bone are intimately related to the rate of trabecular bone turnover. Furthermore, it appears that a high initial deposition of ²³⁹Pu on trabecular surfaces and a high rate of trabecular bone turnover may contribute to the genesis of ²³⁹Pu-induced osteosarcomas.
This study revealed that there are significant intraskeletal variations in the rate of trabecular bone formation, the rate of trabecular bone apposition, the initial deposition of ²³⁹Pu on trabecular surfaces, and the incidence of ²³⁹Pu-induced osteosarcoma. These variations appear to be related to the distribution of red and yellow marrow within the beagle skeleton, and may be the result of differences in the degree of vascularity of the two types of bone marrow.