Soft tissue tumors induced by monomeric 239Pu

Ancestry of beagles in lifespan studies of radionuclide toxicity at the University of Utah

Analysis of the ancestry of the 1,262 lifespan beagles (LSB) entered into lifespan studies at the Radiobiology Division, University of Utah, beginning in 1952 and ending in 1980, indicated that about 97% of ancestor citations in the various pedigrees were of only 10 breeding animals (breeders) among breeders within the beagle colony. In turn, just 18 AKC-registered "champion" beagles from outside of this colony (founders) accounted for about 98% of all ancestor citations among founders for the LSB. We conclude from this study that the animals used in the lifespan radionuclide experiments can be considered to be somewhat genetically interrelated.

Does low dose internal radiation increase lifespan?

No significant differences in lifespan could be established between control dogs and dogs given 241Am, 228Th, 90Sr, 228Ra, 226Ra or monomeric 239Pu at low dosage levels that induced less than 10% skeletal malignancies (low dose animals) in the Utah beagle colony when all dogs surviving at least 1 y were included in the analysis and dogs given individual radionuclides were considered separately or together. Censoring or exclusion of dogs from these groups that were diagnosed with skeletal malignancies or that died in a gran mal epileptic seizure made no important difference to these results. Therefore, an enhanced lifespan of low dose dogs as compared with controls could not be established. It is concluded that low doses from internal (mainly skeletal) deposits of these radionuclides probably do not benefit the survival of individuals so exposed.

Does soft tissue cancer affect the development of radionuclide-induced skeletal malignancy?

Data in the archives of the beagle project at the University of Utah were used to determine whether there might have been an association between the occurrence of soft tissue cancer and bone tumor (skeletal malignancy) among the dogs injected with bone-seeking radionuclides. In addition to comparing the numbers of animals with and without bone tumors and with and without soft tissue cancers, data on bone sarcomas and soft tissue sarcomas were compared. Longevity (postinjection survival) of the dogs was investigated. It appeared that survival (time between radionuclide injection and death) was important in the eventual expression of malignancies. There could not be established a possible susceptibility to skeletal malignancy associated with the occurrence of soft tissue cancer (or soft tissue sarcoma). We concluded that, although the literature reports an increased susceptibility for subsequent malignancies for individuals with naturally-occurring or radiation-induced cancer, the data derived from this study do not confirm the concept that the appearance of soft tissue cancer is strongly associated with an increased risk of developing a radionuclide-induced skeletal malignancy.

Review of 239Pu and 226Ra effects in beagles

A long term biological study has been completed that was designed to assess the predicted effects in humans of internally deposited 239Pu by comparison with 226Ra in beagles. Herein we summarize for the first time results of several previous reports about the effects of these two radionuclides in our beagles in an attempt to elucidate what has been learned since the beginning of the study in the early 1950's. Perhaps the most important finding was that bone surface-seeking plutonium is more toxic at equal mean skeletal radiation doses (<3 Gy for 239Pu, <20 Gy for 226Ra) than bone volume-seeking radium for the induction of skeletal malignancy by about a factor of 16 for a single intravenous injection of monomeric 239Pu. In addition, ancillary studies have shown that when plutonium transfers continuously onto bone surfaces from a depot of particulate 239Pu in phagocytic cells, its relative toxicity per Gy average skeletal dose is enhanced by about a factor of 2. Juvenile animals or dogs injected as mature adults were only about half as sensitive for equal mean skeletal doses as dogs injected as young adults. Male and female dogs were about equally sensitive to radiation of the skeleton by either radionuclide. Findings about radiation-induced fractures are summarized as well as data on the induction of soft-tissue malignancies by 239Pu or 226Ra. Natural survival was not affected at the lower dosage levels of either 226Ra or 239Pu as compared with control dogs given no radioactivity, but the survival of animals at higher levels was reduced. No additional life-shortening effects beyond those attributable to occurrence of radiation-induced malignancies or other radiation-induced effects were suggested by analysis of data for low dosage levels.

Does longevity in beagles injected with bone-seeking radionuclides depend upon radiation dose in the absence of known radiation effects?

Regression analyses of longevity as a function of skeletal radiation dose among groups of beagles injected with 226Ra, 228Ra, 228Th, 241Am, 90Sr or monomeric 239Pu suggested that at low doses and dose-rates (those at which induced effects are low), age at death seems to be independent of dose when animals dying with specific radiation effects were excluded, although longevity does appear to be a function of dose when animals dying with established radiation effects and at all doses were included. We conclude tentatively that, for mammals receiving skeletal dose from bone-seeking radionuclides at low doses and low dose-rates, longevity may not be dependent upon skeletal radiation dose in the absence of radiation-induced malignancies or other radiation effects.

Fracture occurrence from radionuclides in the skeleton

Because skeletal fractures were an important finding among persons contaminated with 226Ra, experience with fractures among dogs in our colony was summarized to determine the projected significance for persons contaminated with bone-seeking radionuclides. Comparison by Fisher's Exact Test of lifetime fracture occurrence in the skeletons of beagles injected as young adults suggested that for animals given 226Ra, 228Ra, 228Th, or 239Pu citrate, there was probably an excess over controls in fractures of the ribs, leg bones, spinous processes, and pelvis (os coxae) plus the mandible for dogs given 226Ra and the scapulae for dogs given 228Ra or 228Th. Regression analysis indicated that significantly elevated fracture occurrence was especially notable at the higher radiation doses, at about 50 Gy average skeletal dose for 239Pu, 140 Gy for 226Ra, about 40 Gy for 228Ra, and more than 15 Gy for 228Th. The average number of fractures per dog was significantly elevated over that noted in controls for the highest radiation doses of 239Pu and 226Ra and for the higher doses of 228Ra and 228Th. For those dogs given 90Sr citrate, there was virtually no important difference from control beagles not given radionuclides, even at group mean cumulative skeletal radiation doses up to 101 Gy. Because of a large proportion of dogs with fractures that died with bone malignancy (even at dosage levels lower than those exhibiting an excess average number of fractures per dog), we conclude that fracture would not be an important endpoint at lower levels of plutonium contamination in humans such as would be expected to occur from occupational or environmental exposure.