Kinetic model of the distribution of 239Pu in the beagle skeleton

A model is presented to analyze the retention of Pu in the major deposition organs of the beagle dog and predict the dynamic behavior of skeletal labels. The kinetic part describing the gross organ distribution was represented by a compartment model. A fit to empirical retention equations of liver and skeleton yielded skeletal clearance corresponding to turnover rates of 93.8% y-1 and 3.8% y-1 in trabecular and cortical bone, respectively. Initially about 9% of skeletal Pu is deposited in cortical bone. The blood flow changes over a period of 3000 d from an initial 0.15% of the injected dose per day to 0.05% d-1 at the end of this period. More than 100% of the injected Pu is recirculated back to the skeleton during this interval. The calculation of the label dynamics showed that nearly complete volumization of Pu was only possible assuming a very high affinity ratio of forming vs. resting surfaces and high bone turnover rates. There was a steady increase in the fractional activity of surface and secondary diffuse labels in cortical bone whereas in trabecular bone these labels showed maximum activity at about 2 y after injection. The Pu concentration on pre-existing trabecular bone surfaces that were not remodeled within 3000 d post injection increased by a factor of 3.6. The model may be applied to single bones or the skeleton as a whole. The flow of Pu in the blood can be derived in human cases where urinary excretion rates are available. With the blood flow known, the model enables one to simulate the dynamic behavior of skeletal labels even for very general conditions of human contamination, including inhalation.