Kinetics of the Skeletal Retention of <tex-math>${}^{239}{\rm Pu}({\rm IV})$</tex-math>

A kinetic analysis of the observed skeletal retention of <tex-math>${}^{239}{\rm Pu}({\rm IV})$</tex-math> in the beagle is presented. Since skeletal retention is directly related to bone-remodeling processes, the analysis applies only to beagles that are injected in young adulthood at dose levels <latex>$<0.095\ \mu {\rm Ci}\ {}^{239}{\rm Pu}\ ({\rm IV})/{\rm kg}$</latex>. At these dose levels the effect of irradiation on bone remodeling and, hence, skeletal retention, is less than the variation between animals. The kinetics are derived from these macro- and micro-observations: (1) The initial deposition and the rate of decrease in retention are both higher in bones that have a relatively greater amount of trabecular bone. One ulna, which has a relatively greater amount of cortical bone, was analyzed from each of 40 beagles at dose levels from 0.00064 to 0.095 μCi/kg and at times from 35 to 4549 days after injection. Retention of <tex-math>${}^{239}{\rm Pu}({\rm IV})$</tex-math> in the ulna was approximately constant over 12.5 yr, which is in contrast with the retention in the humerus and in the third lumbar vertebra. In these bones retention decreased during the first several years and then approached approximately constant values. Also, comparison of the initial concentrations shows that that of the ulna was only ≃25% that of the other two bones. (2) W. S. S. Jee has shown that <tex-math>${}^{239}{\rm Pu}({\rm IV})$</tex-math> deposits initially on osseous surfaces. Then through osteoblastic activity surface deposits are buried and²³⁹ Pu appears diffusely in new bone. Through osteoclastic activity surface deposits are resorbed and²³⁹ Pu appears in osteoclasts. Later²³⁹ Pu appears in macrophages, which in the final step disappear from bone. The macro- and micro-events are consistent in time. The kinetic model that is derived could lead to the identification of a more meaningful dose rate parameter and also could provide a basis for estimating bone remodeling rates in the young adult beagle.