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.

Search for a genetic link for mammary cancer in a beagle colony

BACKGROUND

A possible genetic link for malignant mammary tumor (MMT) was investigated.

MATERIALS AND METHODS

Records of an internally-irradiated beagle colony followed from the early 1950's until about 1995 were searched and analyzed by standard statistical procedures.

RESULTS

Only a single ancestor yielded a "p" value (Fisher's Exact Test) for an overrepresentation among descendants with MMT at < 0.025 (one-sided test), and the number of comparisons for the 169 ancestors where the relative fraction of animals with MMT was greater than that for non MMT dogs (61) suggests that this could have occurred by chance alone. Results of other statistical tests were not remarkable.

CONCLUSION

No genetic link for MMT in this colony could be established with the available data. These findings may or may not be relevant to humans.

Survival and bone tumor hazard from internal deposition of 226Ra in beagles

The survival of 132 young adult control beagles and 117 beagles receiving graded injections of 226Ra ranging from 0.27 kBq kg-1 to 384.2 kBq kg-1 body mass was analyzed. The hazards of natural deaths, all deaths in injection groups, and deaths by bone tumors were assumed to follow a Weibull distribution with a common shape factor of 6.3. Only the scale factors of the Weibull distributions depend on the injection level. There were no significant sex differences. The relative risk with respect to controls for all causes of death increases up to 6,925 for 384.2 kBq kg-1. The dependence of the scale factors for all deaths and death by bone tumors on injected activity was fitted to an empirical regression model, which also contains a term representing radiation-caused deaths other than bone tumors. The risk of bone tumors increases nearly as the square of the injected activity, whereas the risk of the other radiation caused deaths increases approximately in proportion to the injected activity. By means of the regression model, it is possible to predict median survival times for all deaths, bone tumor and non-bone tumor deaths for an arbitrary intake level. Also, simple expressions for the fraction of animals with bone tumors and other radiation-caused deaths can be derived. The empirical model of bone tumor induction, which was based on results from a single injection design, can be generalized to an arbitrary systemic intake schema. This is achieved by using the average dose and dose rate to the skeleton as indices of detriment. Applying the generalized model, it was confirmed that no significant differences in survival can be expected for two groups of beagles receiving multiple injections, if compared to the corresponding single injection groups of about the same total activity. The general model also predicts that even extensive protraction of the intake would increase the survival times only to a very limited extent.

Biokinetic model of radium in humans and beagles

A compartmental model of the distribution of radium in humans and young adult beagle dogs (approximately 500-550 d) is presented. The model consists of one soft tissue compartment and seven skeletal compartments for humans, and five skeletal compartments for beagles. The number of transfer parameters to be estimated was reduced by using remodeling rates of bone and imposing several constraints deduced from known features of bone physiology, radium metabolism, and autoradiographic analyses. The model predictions are in good agreement with measured retentions in plasma, whole body, skeleton, and soft tissues of both species. Moreover, for beagles even the retention in individual bones can be predicted quite well if the relevant morphometric parameters are known. While some of the estimated transfer parameters are similar in both species, others differ by an order of magnitude or more. Wherever possible, a comparison of model parameters with those of previous models is given. The new model not only is instrumental for calculating local doses in the skeleton but also can be used for characterizing the microdistribution of radium in this organ.

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.

Biokinetic and dosimetric model of plutonium in the dog

A biokinetic model of the systemic distribution and dosimetry of 239Pu in the beagle dog is presented. To achieve maximum consistency with experimental data, known histomorphometric parameters and results of autoradiographic studies were adopted directly. The remaining parameters were determined from retention and excretion measurements by optimization procedures. The beagle model attempts to parallel the human model as much as possible, but only one liver compartment and one compartment representing other soft tissues were needed to describe the data adequately. The salient features and differences of the biokinetic behavior of 239Pu beagles and humans are compared. Generally the organ retention of the beagle in relation to the lifetime is longer than in humans. This is particularly pronounced in the skeleton. Trabecular deposits of plutonium are gradually shifted to cortical sites. For the dosimetric model some additional features disregarded in the human model were employed. These relate to bone volume labels, a gradation of concentrations in marrow, the energy-dependence of absorbed fractions, and the self-absorption in marrow. The model predicts that the contribution of surface deposits to the endosteal dose still exceeds the contributions from bone volume and marrow labels. The average endosteal dose is about eight times and the marrow dose about two times larger than the average skeletal dose. The model provides the basis for the analysis of survival and relative risks.

Molecular structure and biological and pharmacological properties of 3-hydroxy-2-methyl-1-(beta-D-ribofuranosyl or pyranosyl)-4-pyridinone: potential iron overload drugs for oral administration

Replacing alkyl groups by sugar moieties at N-1 position of 3-hydroxy-2-methyl-4-pyridinone did not affect the geometry of the iron chelating sites but increased the hydrophilic nature. The formation of a polymer cluster through the intermolecular hydrogen bonds was also revealed by X-ray crystal structure analysis for the first time in all known 3-hydroxy-4-pyridinone crystal structures. Iron removal from ferritin by the title compounds was more efficient than with DFO.

Microdistribution of 239Pu in the beagle skeleton

The microdistribution of 239Pu was analyzed in the humerus, lumbar vertebra, and proximal ulna of young adult beagles using neutron induced autoradiography. The animals were sacrificed serially in groups of three at 4, 8, 16, 32, and 64 wk after a single injection of 3.5 kBq kg(-1) body weight. The kinetic behavior of surface concentrations was modeled using a simple concept of deposition and clearance in skeletal regions. Bones with high turnover showed a larger initial uptake and a faster clearance than bones with low turnover rates. Using a regression procedure, the surface deposition and clearance of plutonium was calculated as a function of the turnover rate. With time after injection the initial nonuniformity of trabecular surface labels tends to become more uniform. The trabecular:cortical affinity ratio is about 10. Trabecular activity is gradually translocated to cortical sites. The affinity ratio of forming to resting surfaces is about three. In some bones a continuous increase of marrow stars was observed, whereas in other bones no clear-cut tendency could be seen. The highest level of marrow labeling occurred in the lumbar vertebra and the humerus shaft.

Bone tumor location in dogs given skeletal irradiation by 239Pu or 226Ra

Statistical analyses have indicated that there was a significant difference between dogs injected with bone volume-seeking 226Ra as compared to those given bone surface-seeking 239Pu with respect to location within the skeleton of 334 radiation-induced primary bone malignancies. Corresponding differences also were evident when dogs given bone volume-seeking 90Sr or bone surface-seeking 241Am, 228Th, (249,252)Cf, or 224Ra (which decays mostly on bone surfaces because of its short, 3.6 d half time) were included along with the 226Ra or 239Pu, respectively (562 total tumors). Further analysis suggested that higher values of percent red marrow (M) and bone turnover rate (R) are correlated with increased probability of tumor appearance at a particular location within the skeleton for the surface seekers. Proportionately higher values of M and R are associated with skeletal sites containing mostly trabecular bone as compared to those with mostly compact (cortical) bone. Coefficients of determination (r2) for the relationship between percent of total tumors vs the combination of percent red marrow and turnover rate (= MR) was about 0.7 for the surface seekers but only about 0.1 for the volume seekers. This indicates that the neoplastic effects of surface seekers, but not volume seekers, are associated with the presence of trabecular bone at the various sites of radionuclide deposition within the skeleton.

Comparison of internal emitter radiobiology in animals and humans

Investigations of radionuclide metabolism and effects in various mammalian species revealed important similarities between animals and humans and between some animal species. These include skeletal deposition of radium and radiostrontium in bone volume; deposition on bone surfaces of plutonium and other actinides; liver deposition of actinides; induction of skeletal or liver malignancies by these radionuclides; induction of tooth and jaw abnormalities; mammary cancer induction by radium in humans and in the beagle; depression of circulating cells in blood; and induction of bone fractures. There are also inter-species differences that may not have been noted if multiple species (including humans) had not been studied. Some of these are more rapid excretion of radium in humans compared with most other mammals; induction by radium of eye melanomas in animals but not humans; rapid loss of deposited plutonium from liver in many species of mice and rats but not in humans and dog; substantial sex-related differences in skeletal plutonium retention and bone sarcoma induction in mice but not in humans or dog; and induction of head sinus carcinomas by 226Ra in humans but not the beagle. Leukemia and other related neoplasms were not induced in radionuclide-injected lifespan dogs in excess of the occurrence in control animals. Much of our current understanding of skeletal biology and radionuclide behavior in mammals was derived from this and related projects. The primary goal of the Utah experiment of estimating toxicities of bone-seeking radionuclides relative to radium has been accomplished. For 226Ra = 1.0, comparative toxicities (ratios) of a single injection for bone tumor induction in beagles were about 16 +/- 5 for monomeric 239Pu (32 +/- 10 for chronic exposure), 6 +/- 0.8 for 241Am, 8.5 +/- 2.3 for 228Th, 6 +/- 3 for 249Cf, 4 +/- 2 for 252Cf, 6 +/- 2 for 224Ra (16 +/- 5 for 50 weekly injections), 2 +/- 0.5 for 228Ra, and between 0.01 +/- 0.01 and 1.0 +/- 0.5 for 90Sr, depending on the dose-rate, with the lowest dose-rates approaching a ratio of zero. Corresponding ratios in mice for 226Ra = 1.0 were 16 +/- 4 for monomeric 239Pu, 5.4 +/- 2.0 for 224Ra (16 for 50 weekly injections), 4.9 +/- 1.4 for 241Am, 5.0 +/- 1.4 for 249Cf, 2.6 +/- 0.8 for 252Cf, 4.4 +/- 1.8 for 243,244Cm and about 1.0 for 90Sr at high doses, decreasing to near zero for low doses.

Uranium skeletal dosimetry and distribution in young adult beagles: a guide for calculating uranium skeletal doses in humans

Uranium isotopes were given via single intravenous injection into 22 young adult beagle dogs of both sexes to determine the metabolism of this element. Animals were given either 232U, 233U, 238U, or a combination of 232 (+) 233U. Calculations to assign a value of skeletal dose for each dog were performed using published radioactive properties of each uranium isotope and the metabolic data (including measured retention and skeletal distribution) derived from this study during a period of up to 2 y after injection. We believe that the procedures illustrated in this communication can serve as a useful pattern for estimating skeletal radiation doses to humans contaminated with 232U, 233U, or 238U.

Soft tissue tumors induced by monomeric 239Pu

Individual records of soft tissue tumor occurrence (lifetime incidence) among 236 beagles injected with 239Pu citrate as young adults and 131 comparable control beagles given no radioactivity enabled us to analyze the possible effects on soft tissue tumor induction resulting from internal exposure to 239Pu. A significant trend was identified in the proportion of animals having malignant liver tumors with increasing radiation dose from 239Pu. There was also a significant difference in the relative numbers of both malignant liver tumors (3.2 expected, 22 observed) and benign liver tumors (18.1 expected, 66 observed). Malignant tumors of the mouth, pancreas, and skin were more frequent among controls than among the dogs given 239Pu as were all tumors (malignant plus benign) of the mouth, pancreas, testis, and vagina. For all other tumor sites or types, there was no significant difference for both malignant and all (malignant plus benign) tumors. Mammary tumor occurrence appeared not to be associated with 239Pu incorporation. We conclude that the only soft-tissue neoplasia induced by the intake of 239Pu directly into blood is probably a liver tumor.

Mammary tumor occurrence in beagles given 239Pu

Comparison of 120 young adult female beagles given 0.026 to 106 kBq 239Pu kg-1 by intravenous injection and 63 comparable female control beagles showed that there were no significant differences in the risk of mammary tumor appearance between the two groups. This was the case for benign tumors only, for malignant tumors only, and for both malignant and benign tumors considered together. For malignant tumors the observed number was 73 as compared with 69 expected; for benign tumors, there were 131 observed and 126 expected; for all tumors (separate analysis, not just the addition of malignant plus benign), there were 199 observed and 199 expected. Chi-square analysis indicated that the p values for all these comparisons were > 0.05. There were 45 controls (71.4%) with any tumor vs. 67 dogs (55.8%) given Pu (95% C. I. = 46.9% to 86.2%). No significant differences could be established (Kaplan-Meier analysis) between these two groups for survival age at diagnosis of the first mammary tumor, 11.75 +/- 0.30 y for dogs given Pu vs. 11.90 +/- 0.36 y for controls. We reported previously that differences in mammary cancer occurrence had been identified between this same group of control dogs and 57 female beagles given 226Ra as young adults. The present study appears to support the earlier conclusion that something other than alpha irradiation of the skeleton (both 226Ra and 239Pu deposit in bone) seems to affect the appearance of mammary cancers, since internally deposited 226Ra does appear to induce these malignancies, possibly from initial deposition in mammary tissue of the parent radionuclide or the subsequent concentration in sensitive tissue of its radioactive progeny, 22Rn or isotopes of polonium, lead, and bismuth, which are absent in the case of 239Pu.

Relative effectiveness of 239Pu and some other internal emitters for bone cancer induction in beagles

The toxicity ratio (relative effectiveness per gray of average skeletal dose) has been estimated for bone cancer induction in beagles injected as young adults for a number of bone-seeking internal emitters. These experiments yielded calculated toxicity ratios (+/- SD) relative to 226Ra = 1.0 of 239Pu = 16 +/- 5 (single exposure to monomeric Pu) and 32 +/- 10 (continuous exposure from an extraskeletal deposit in the body), 224Ra = 16 +/- 5 (chronic exposure) and approximately 6 +/- 2 (single exposure), 228Th = 8.5 +/- 2.3, 241Am = 6 +/- 0.8, 228Ra = 2.0 +/- 0.5, 249Cf = 6 +/- 3, 252Cf = 4 +/- 2, 90Sr = 1.0 +/- 0.5 (for high doses) and 0.05 +/- 0.03 (for low doses) and 0.01 +/- 0.01 (for extremely low doses). Because no skeletal malignancies were observed among beagles given only 253Es, the toxicity ratio is undefined.

Occurrence of mammary tumors in beagles given radium-226

A total of 128 primary mammary tumors (66 of them malignant) occurred in 35 female beagles injected with 226Ra at eight dose levels ranging from 0.2 to 440 kBq/kg body mass as young adults, while a total of 156 mammary tumors (57 of them malignant) were seen in 46 female control beagles not given any radioactivity. Sixty-three of 65 control dogs and 59 of 61 dogs given 226Ra survived the minimum age for diagnosis of mammary tumors of 3.75 years. Based on the observed age-dependent tumor incidence rates in the controls and on the corresponding number of dog-years at risk, the total number of observed malignant tumors in the radium group was statistically greater than the number of expected malignant tumors (66 observed vs 34 expected, P < 0.005). There was no such difference for the benign tumors. Cox regression analysis indicated no increased risk for the first tumor occurrence in irradiated dogs. Cox regression analysis of the multivariate risk sets showed no significantly increased risk for the occurrence of benign tumors but a statistically higher risk of 1.66 with a confidence interval of 1.15-2.40 for the occurrence of malignant tumors. The increased risk was dependent on dose, but a dependence on the frequency of previous occurrence of mammary tumors could not be confirmed. Censoring ovariectomized dogs at time of surgery decreased the relative risks slightly but did not alter the significance. Exposure to diagnostic X rays with cumulative exposures below 0.2 Gy had no effect on tumor formation. It is unknown whether the increased risk for malignant mammary tumors was due to some initial deposition of radium in sensitive tissue, a possible irradiation of fatty mammary tissue from transient radon-->polonium deposition, or a general effect of the overall radium deposition on the immune system of the dogs that lowered their resistance to formation of mammary tumors. Results of this study are potentially useful in understanding risks of radium-induced breast cancers in humans.

Distribution of skeletal malignancies in beagles injected with 239Pu citrate

The distribution of skeletal malignancies among our beagles injected with 239Pu as young adults roughly seems to follow the distribution of skeletal mass and skeletal 239Pu. These findings are similar to those we reported previously for a group of dogs given 26Ra. Although there were differences in tumor distribution between the animals given 226Ra and those given 239Pu, most of them were not statistically significant; however, the radium dogs seemed to show a greater sensitivity to bone tumor origin in the tibia, while there may have been a tendency among the plutonium dogs toward increased relative sensitivity in the scapula, lumbar vertebrae, sacrum, and ribs. In contrast, the most common site for the formation of naturally-occurring bone malignancy in the dog is the distal radius. Perhaps there were too few tumors and too few dogs to establish statistical significance. A correlation between tumor location and at least two anatomical-physiological factors in the skeleton indicated that these two factors (site-specific bone turnover rate and percent of red marrow at the site, which is correlated with vascularity) may influence the appearance of malignancies both individually and in combination. Except for the femur, there appeared to be no difference between the relative distribution of skeletal malignancies of low-level (30 Bq-2 Bq kg-1 injected) and high-level (3-122 kBq kg-1) dogs. Distribution of bone tumors between the axial and appendicular skeleton was 50% vs. 50% for 239Pu (42 and 42), but it was 39% axial vs. 61% appendicular (22 and 35, respectively) for dogs given 226Ra. This difference was not significant (p > 0.2).

Eye tumors and other lesions among beagles given 90Sr or 226Ra

Analysis of eye tumors and other eye lesions among beagles given either 90Sr or 226Ra, and among control animals, indicated that intraocular tumors in excess of the rate for our control animals were not associated with radiation from incorporated 90Sr + 90Y. It is unequivocal that eye melanomas were produced by injected 226Ra. Intraocular neoplasia, hyperplasia, hyperpigmentation, and melanosis in the eye all occurred in our control beagles given no radioactivity; however, tumor experience as currently reported for different beagle colonies may not be directly comparable because of differing rates of discovery, nonuniform nomenclature, and varying criteria for classification of lesions with their discordant interpretation by different pathologists.

Soft-tissue tumors among beagles injected with 226Ra

A total of 409 primary soft-tissue tumors (189 malignant) occurred among 87 of 120 young adult beagles (72.5%) injected with 226Ra in eight dose levels ranging from 0.2-440 kBq kg-1 body mass, while a total of 565 primary soft-tissue tumors (208 of them malignant) were seen among 117 of 133 control beagles not given radioactivity (88%). Because the p-value for the difference in these two percentages was > 0.05, further comparisons were not made of all tumor locations or types taken together but only of the individual tumor locations or types. There was a clear excess of malignant tumors and all tumors (benign plus malignant) in the eye among dogs injected with radium (p < 0.05, p < 0.01, respectively), but the occurrence of all the other types of soft-tissue tumors was not greater in irradiated vs. control dogs (p > 0.05). This was also true for hematopoietic tumor types (including just one leukemia in a control and none in irradiated dogs) in which there was no difference between controls and dogs given radium. The following total tumors (benign plus malignant) occurred in control dogs but not in radium dogs: brain = 3, peritoneum = 1, and pituitary = 4. Malignant tumors other than leukemia appearing in control animals and not among radium dogs were brain = 2, lymph nodes = 1, adrenal = 3, uterus = 1, and pancreas = 5. Tumors that occurred in dogs given radium and not in controls were 3 mast cell sarcomas and 2 tumors of the thymus (1 malignant). Age at first tumor diagnosis for corresponding tumor types did not seem to differ (p > 0.10 or p > 0.05) between radium dogs and controls except for the eye (p < 0.05), with radium dogs being somewhat younger than controls at first diagnosis, at death, or at loss from the colony. Cox regression indicated differences between radium dogs and controls in risk of dying with specific tumors. The following tumors had p values of < 0.05 and risk ratios of > 2.2:eye, mouth (mostly melanomas), and thyroid for malignant tumors and for malignant and benign tumors together. When all sarcomas were considered as a group, there was no difference between controls and radium dogs but there was a difference for all carcinomas taken together, even when mammary tumors and eye tumors were excluded and when eye tumors alone were excluded.

Duration and dose-related effects of an orally administered, partially lipophilic polyaminocarboxylic acid on the decorporation of plutonium and americium

A recently developed, orally administered, partially lipophilic polyaminocarboxylic acid-based chelator, docosyl-triethylenetetraminepentaacetic acid (C22TT), was tested for its ability to promote decorporation of 239Pu and 241Am. The effects of dose and duration of treatment were determined in rats injected with 239Pu or 239Pu/241Am 2 weeks before the initiation of C22TT treatment and compared with untreated controls. In the dose-effects study, significant reductions in total body Am content were seen within 3 days after the initiation of C22TT treatment. After 30 days of treatment, there were dose-related reductions in the Pu and Am content of soft tissues and bones. All doses of C22TT resulted in substantial reductions in Pu and Am content of the liver. In the time-response study, there were rapid reductions in total body Am content in the C22TT-treated animals. The greatest reductions occurred within the first 30 days of treatment. Significant decreases in Pu content of soft and hard tissue were observed in the treated animals at 30, 60 or 90 days compared with untreated controls. The greatest reductions in organ Pu content occurred within the first 30 days of treatment, particularly in the liver, but it continued throughout the experiment. Neutron-induced autoradiography showed that C22TT greatly reduced the incorporation of Pu into new bone and substantially reduced the Pu content of the bone marrow. There was no evidence of overt toxicity in either experiment. This study demonstrates that orally administered C22TT is effective in reducing soft and hard tissue content of internally deposited Pu and Am.

241Am-induced thyroid lesions in the beagle

One of the features of 241Am metabolism in the beagle dog is a relatively high selective deposition in the thyroid glands, equal to approximately 0.06% of the injected dosage per gram. Retention is prolonged and principally in the interstitial connective tissue. The resulting average dose to the thyroid glands is about 1.42 and 0.76 times that delivered to the skeleton and the liver, respectively. This thyroid dose is much higher than has been reported in people. Although significant morphological changes and reductions in serum thyroxine occurred in this experiment, a significant increase in thyroid neoplasia was not observed.

Bone cancer occurrence among beagles given 239Pu as young adults

The occurrence of skeletal malignancies has been documented among 234 young adult beagles given single intravenous injections of monomeric 239Pu citrate. Occurrence has also been documented among 132 comparable control group animals surviving the minimum latent time period of 2.79 y for radiation-induced bone cancer, who were maintained for lifespan observation. Injected amounts ranged from about 0.02-106 kBq kg-1 body mass with factors of 2 or 3 between dose levels. There were 84 radiographically apparent bone tumors in 76 plutonium-injected dogs and one tumor in a control group dog. Most of these were osteosarcomas except for seven chondrosarcomas, one liposarcoma, and one plasma cell myeloma of bone. The relationship between percent of dogs at any dose level with bone malignancy and average skeletal dose at the presumed time of tumor initiation of 1 y before death appeared to be linear below about 1.3 Gy average skeletal dose. The observed data can be approximated by the expression A = 0.76 + 75 D, where A = percent of dogs with bone cancer at any dose level, D = average skeletal dose in Gy (for doses up to 1.3 Gy) at tumor initiation, and 0.76 represents the percent tumor response in the control animals not given plutonium. Similar analysis of our corresponding data for beagles given 226Ra, excluding the two highest dose levels (approximately 100% occurrence), yielded the expression A = 0.76 + 4.7 D, where D = the average skeletal dose in Gy (for doses up to 20 Gy) at 1 y before death. The ratio of coefficients indicates the effectiveness for bone cancer induction of 239Pu relative to 226Ra, or [(75 +/- 22.5)(4.7 +/- 0.47)-1] = 16 +/- 5 for a single, brief intake of either nuclide into blood.