Radiation dose rates to the proximal humerus of growing beagles injected with 239Pu

The anatomical distribution of 239Pu in the proximal humerus was studied in four beagles injected at 90 days of age and sacrificed 7, 28, 56, and 128 days after injection, respectively. Initial deposition in each humerus was 3.1% of the injected Pu; 41% of that was retained with a net halftime of 248 days; for the remaining 59% no decrease was observed with time. Sixty-one percent of the initial deposit was concentrated in the proximal epiphysis and metaphysis. Average radiation dose rates to trabecular bone of the epiphysis, to those parts of the metaphysis that were formed before and/or after injection, and to the area corresponding to the primary and secondary spongiosa were determined by fission track autoradiography. Local dose rates within the range of 10 microns of the endosteal surface, to the marrow and to osseous tissue were calculated. Because of the rapid growth and modeling rate, surface deposits declined rapidly. In the metaphysis, bone formed after injection contained less than 1/2 the Pu concentration of bone which was present at the time of injection. At 100 days of age, bone mineralization as determined by the ash weight/wet weight ratio, and the fraction of bone volume occupied by mineralized tissue was 1/2 that seen in young adult beagles. The surface/volume ratio was twice as high as in young adult beagles. The possible effects of local radiation doses were correlated with areal densities of cells and types of cells at risk. Comparisons were made with corresponding sites from the humerus of beagles injected as young adults.