The purpose of this study was to obtain information on the ?-particle dose-response relationship of <tex-math>${}^{244}{\rm Cm}$</tex-math> in rats. Rats were exposed briefly by inhalation to graded levels of monodisperse aerosols of 244 Cm 2 O 3 heat-treated at 1150°C. The initial lung burden (ILB) of each animal was determined by the use of the ?-ray-emitting radionuclide <tex-math>${}^{243}{\rm Cm}$</tex-math> in the aerosols. Seven groups of 84-day-old F344/Crl rats (a total of 637 males and 645 females) were exposed once to 244 Cm 2 O 3 or sham-exposed to filtered ambient air. Mean ILBs of all rats per group ranged from 0.51 ± 0.17 (±SD) to <tex-math>$240\pm 82\ {\rm kBq}\ {\rm kg}^{-1}$</tex-math> body weight. Mean lifetime ?-particle doses to the lungs per group ranged from 0.20 ± 0.069 (±SD) to 36 ± 6.5 Gy. After death, each rat was radiographed and necropsied. Dose-related increases occurred in incidences of benign and malignant lung neoplasms, except for the groups of rats with higher mean ILBs that were examined histologically (98 ± 18 and <tex-math>$240\pm 77\ {\rm kBq}\ {\rm kg}^{-1}$</tex-math> body weight) in which survival was markedly decreased. Also, average ?-particle doses of 0.0014 ± 0.00058 (±SD) to 0.17 ± 0.091 Gy and 0.018 ± 0.007 to 1.6 ± 1.1 Gy were also absorbed by the liver and skeleton, respectively, in the rats in the different exposure groups. Primary liver neoplasms occurred in several rats. However, the incidence of these lesions was not related to dose. Increased incidences of bone neoplasms occurred only in rats receiving higher doses to the skeleton. Excess numbers of rats with lung neoplasms per 10 4 Gy to the lung per group ranged from 760 ± 430 (±SE) at a mean dose of 0.48 Gy 84 ± 16 at a mean dose of 37 Gy. Risk factors for the lowest and highest <tex-math>${\rm ILB}\ {\rm kg}^{-1}$</tex-math> body weight groups were not considered reliable because of large errors associated with these calculations and the life-span shortening in the highest <tex-math>${\rm ILB}\ {\rm kg}^{-1}$</tex-math> group. Inhaled 244 Cm 2 O 3 appeared to be about 50% less effective as a lung carcinogen in rats compared to 239 PuO 2 at similar doses.

This study was conducted to examine the carcinogenic effects of inhaled β-particle-emitting radionuclides, particularly in lower dose regions in which there were substantial uncertainties associated with available information. A total of 2751 F344/N rats (1358 males and 1393 females) approximately 12 weeks of age at exposure were used. Of these, 1059 rats were exposed to aerosols of 144 CeO 2 to achieve mean desired initial lung burdens (ILBs) of 18 kBq (low level), 247 rats to achieve mean ILBs of 60 kBq (medium level) and 381 rats to achieve mean ILBs of 180 kBq (high level). Control rats (total of 1064) were exposed to aerosols of stable CeO 2 . Based on the 95% confidence intervals of the median survival times and the cumulative survival curves, there were no significant differences in the survival of groups of female and male exposed rats relative to controls. The mean lifetime β-particle doses to the lungs of the rats in the four groups were: low level, 3.6 ± 1.3 (± SD) Gy; medium level, 12 ± 4.5 Gy; and high level, 37 ± 5.9 Gy. The crude incidence of lung neoplasms increased linearly with increasing doses to the lungs (controls, 0.57%; low level, 2.0%; medium level, 6.1%; and high level, 19%). The estimated linear risk coefficients for lung neoplasms per unit of dose to the lung were not significantly different for the three dose levels studied. The risk coefficient at the lower level was 39 ± 14 (± SE) excess lung neoplasms per 10 4 rat Gy; at the medium level the risk was 47 ± 12; and at the higher level the risk was 50 ± 9.0. The relationship of β-particle dose to the lung and the crude incidence of lung neoplasms was described adequately by a linear function. We concluded that the risk of lung neoplasms in rats per unit of radiation dose did not increase with decreasing mean β-particle dose to the lung over the range of 3.6 to 37 Gy. The weighted average of these three values was 47 ± 6.4 (± SE) excess lung neoplasms per 10 4 rat Gy. To extend the risk coefficients for lung neoplasms to lower doses by experimentation will require much larger numbers of rats than used in this study.

Pulmonary carcinogenesis was compared in female F344 and Wistar rats after inhalation of high-fired 239 PuO 2 . Plutonium particle aggregation, as determined by quantitative light and scanning electron microscopic autoradiography, was greater for the F344 strain than for the Wistar strain. The median survival times were similar in control and low-dose (0.8-1.0 Gy) groups of both strains, but were significantly decreased in the high-dose (34-37 Gy) groups of both strains. Squamous metaplasia was not found in control or low-dose groups of either strain, but was found in 62-65% of high-dose groups of both strains. Adenomatous metaplasia was considerably higher in control and low-dose groups of F344 rats than in Wistar rats. A total of 87 lung tumors were found in 140 exposed F344 rats and 46 lung tumors in 176 exposed Wistar rats. The incidence of lung tumors in F344 rats was 1.7% in controls, 20% in the low-dose group and 82% in the high-dose group. The incidence of lung tumors in Wistar rats was 0.1% in controls, nil in the low-dose group and 68% in the high-dose group. About half of all lung tumors in both strains were considered to be the primary cause of death. The median survival times of rats of both strains in the high-dose groups that died with lung tumors were greater compared with rats in these groups that died without lung tumors. In contrast, these differences did not occur among rats in the low-dose groups. The absolute risk was 1900 lung tumors per 10 4 Rat-Gy for F344 rats receiving low doses and nil for Wistar rats receiving low doses, but about 210 lung tumors per 10 4 Rat-Gy for high-dose groups of both strains. The adenomatous tumor phenotype predominated in the F344 strain, while the squamous tumor phenotype predominated in the Wistar strain. Risk of squamous tumors was similar for both strains. Overall, the F344 strain appears to be more "sensitive" than the Wistar strain to formation of lung tumors at low to moderate doses from inhaled 239 PuO 2 due mostly to an increased incidence of adenomatous phenotype tumors.

Light microscopy, morphometry, and cytokinetic techniques were used to examine the dynamics of plutonium-induced pulmonary proliferative lesions and neoplasms in rats at several intervals to 450 days after inhalation exposure to aerosols of 239 PuO 2 . Maximal increases in alveolar and bronchiolar epithelial cell labeling were seen at 30 days; decreasing subsequently, the levels remained elevated above control indices. Focal proliferative epithelial lesions developed in the lung by 180 days and before the onset of pulmonary neoplasms. Pulmonary neoplasms, predominantly adenocarcinomas and squamous cell carcinomas, were initially observed at 308 days. The proliferative lesions progressed through a succession of morphological changes leading to the development of neoplasms. The volume density (fraction) and epithelial surface area of foci of alveolar epithelial hyperplasia increased progressively between 180 and 450 days after exposure, in contrast to the other proliferative lesions. We conclude that plutonium-induced pulmonary neoplasms develop through a succession of focal proliferative lesions that represent developmental preneoplastic lesions. Progressive increases in volume and epithelial surface area of the alveolar epithelial hyperplasias suggest that they may be more at risk for neoplastic transformation than the other histological types of proliferative foci.

Groups of 94-day-old F344/Crl rats were exposed repeatedly to aerosols of ${}^{144}{\rm CeO}{}_{2}$ to reestablish desired lung burdens of 1.9, 9.2, 46, or 230 kBq of ${}^{144}{\rm Ce}$ every 60 days for 1 year (seven exposures). Other 94-day-old rats were exposed once to achieve similar desired initial lung burdens of ${}^{144}{\rm Ce}$ . Older rats were exposed once to achieve desired initial lung burdens of 46 or 230 kBq when 500 days of age, the same age at which rats had the last of the repeated exposures. Control rats were either unexposed, sham-exposed once or repeatedly, or exposed once or repeatedly to stable ${\rm CeO}_{2}$ . Approximately equal numbers of male and female rats were used. The median survival time and cumulative percentage survival curves were significantly decreased only in male and female rats exposed repeatedly to reestablish a 230-kBq lung burden and among the 94-day-old male rats exposed once to achieve a 230-kBq lung burden of ${}^{144}{\rm Ce}$ . The crude incidences of primary lung cancers (well described by a single Weibull distribution function), time to death with lung tumors, and risk of lung cancer per unit of β-radiation dose to the lungs were correlated with the cumulative β-radiation dose rather than the rate at which the dose was accumulated. A linear function, 70 (±7.3) + -0.15 (±0.056) × dose (±SD), adequately described the excess numbers of rats with lung cancers over a β-radiation dose range to the lungs of 6.8 to 250 Gy for two groups of rats with the highest doses to the lungs after a single exposure and for two groups with the highest doses after repeated exposure.

To develop a better understanding of the influence of cumulative radiation dose and dose rate to the lungs on the biological responses to inhaled radionuclides, several studies are in progress at this institute in which laboratory animals have been exposed once or repeatedly to aerosols of insoluble particles containing ${}^{144}{\rm Ce}$ or 239 Pu. In the study reported here, F344 rats were exposed repeatedly to aerosols of ${}^{144}{\rm CeO}{}_{2}$ beginning at 94 days of age to reestablish desired lung burdens of 1.9, 9.2, 46, or 230 kBq of ${}^{144}{\rm Ce}$ every 60 days for 1 year (seven exposures). Other 94-day-old rats were exposed once to achieve similar desired initial lung burdens of ${}^{144}{\rm Ce}$ . Older rats were exposed once to achieve desired initial lung burdens of 46 or 230 kBq when 500 days of age, the age of the repeatedly exposed rats when exposed for the last time. Control rats were either unexposed, sham-exposed once or repeatedly, or exposed once or repeatedly to stable ${\rm CeO}_{2}$ . Approximately equal numbers of male and female rats were used. The cumulative β-radiation doses to the lungs, liver, and skeleton of rats exposed repeatedly were similar to those of rats with similar total lung burdens of ${}^{144}{\rm Ce}$ from a single inhalation exposure. The average β-radiation dose rate to the lungs of the rats exposed repeatedly was about one-fifth of that in rats with similar total lung burdens after a single exposure.

The potential exists for humans to be repeatedly or chronically exposed by inhalation to radionuclides in relatively insoluble forms; however, only limited experimental data are available on the effects of such exposures. Female mice were repeatedly exposed by inhalation at approximately 60-day intervals for 1 year to <tex-math>${}^{144}{\rm CeO}{}_{2}$</tex-math> to reestablish lung burdens of either 0.2, 1.0, or 4.5 μCi of <tex-math>${}^{144}{\rm Ce}$</tex-math> to determine the effect of repeated exposure on <tex-math>${}^{144}{\rm Ce}$</tex-math> retention and dosimetry. The long-term effective retention half-times of <tex-math>${}^{144}{\rm Ce}$</tex-math> in the lung, liver, and skeleton after the last repeated inhalation exposure were similar to those in mice exposed once by inhalation to <tex-math>${}^{144}{\rm CeO}{}_{2}$</tex-math>. A mathematical simulation model was developed as an alternative method of describing the pulmonary clearance of <tex-math>${}^{144}{\rm Ce}$</tex-math> after repeated exposures by inhalation to aerosols of <tex-math>${}^{144}{\rm CeO}{}_{2}$</tex-math>. The cumulative radiation doses to the lungs, livers, and skeletons of the repeatedly exposed mice were similar to those which would have occurred had the total lung burden been achieved by a single exposure. The repeatedly exposed mice also had temporal radiation dose patterns to the lung similar to beagle dogs exposed once.

Groups of mice (C57Bl/6J strain) were repeatedly exposed by inhalation to aerosols of <tex-math>${}^{144}{\rm CeO}{}_{2}$</tex-math> at 60-day intervals for seven consecutive exposures to reestablish lung burdens of <tex-math>${}^{144}{\rm Ce}$</tex-math> of 0.2, 1.0, or 4.5 μCi. Additional groups of mice were exposed only once when 70 days old to achieve desired initial lung burdens of 0.2, 1.0, or 4.5 μCi. Control mice consisted of those exposed once or repeatedly to stable <tex-math>${\rm CeO}_{2}$</tex-math>, sham-exposed once or repeatedly, and unexposed mice. Protraction of the absorbed radiation dose by repeated inhalation exposures resulted in a sparing from the life-shortening effects of <tex-math>${}^{144}{\rm Ce}$</tex-math> pulmonary irradiation. The protraction of dose had no effect on the total number of lung tumors seen or their time of onset. The total incidence of lung tumors, benign and malignant, was correlated with cumulative dose, not dose rate.

The rate of pulmonary clearance of inhaled Staphylococcus aureus in mice was determined at intervals after inhalation exposure to either 144 CeO 2 or 239 PuO 2 . In mice with mean initial lung burdens between 0.6 and 4.7 μCi 144 Ce the pulmonary clearance of S. aureus was suppressed up to 12 weeks after inhalation of 144 CeO 2 . In mice with mean initial lung burdens between 1.3 and 29.0 nCi 239 Pu the pulmonary clearance of S. aureus was suppressed up to 26 weeks after inhalation of 239 PuO 2 . The suppressed pulmonary clearance of S. aureus appeared to correlate with the radiation dose rate to the lungs at the time of exposure to bacteria but not with the cumulative radiation dose to the lungs. The changes in bacterial clearance did not appear to be correlated with changes in body weight, hematological parameters, or radiation-induced histopathological changes. Altered bacterial clearance may be related to radiation damage to pulmonary macrophages. It was concluded that irradiation of the lung from radionuclides inhaled in relatively insoluble forms may result in increased bacterial invasion of the lungs.

The effect of ${}^{90}{\rm Y}$ inhaled in fused clay particles on the pulmonary clearance of inhaled Staphylococcus aureus in mice was investigated to provide an improved understanding of the influence of localized irradiation from inhaled radionuclides on infectious processes. Pulmonary clearance of inhaled S. aureus was suppressed in mice with initial lung burdens of 20 μCi ${}^{90}{\rm Y}$ or greater at 2, 3, and 4 weeks after inhalation exposure to ${}^{90}{\rm Y}$ . Suppressed clearance rates were accompanied by radiation-induced lifespan shortening, retarded increases in average body weight, suppression of blood lymphocyte count, and pulmonary lesions. Only equivocal suppression of bacterial clearance was observed in mice with initial lung burdens of less than 20 μCi ${}^{90}{\rm Y}$ that were tested from 1 through 52 weeks after inhalation exposure. An initial lung burden of 1 μCi ${}^{90}{\rm Y}$ was estimated to result in 400 rad to the lung delivered within 24 days after exposure.

One hundred and seventy-eight mice, exposed by inhalation to an aerosol of <tex-math>${}^{144}{\rm CeO}{}_{2}$</tex-math> particles, were observed for their lifespan and their survival compared with those of 200 unexposed control mice. Approximately 91% of the initial whole-body burden of <tex-math>${}^{144}{\rm Ce}$</tex-math> was cleared within 6 days. The whole-body burden at 6 days was estimated to be equivalent to the initial lung burden (ILB) of which 32% was retained with an effective half-time of 7 days, 49% with a half-time of 28 days, and 19% with a half-time of 145 days. Cerium-144 retained in the lungs as a percentage of whole-body burden at death was determined through day 454 postinhalation exposure to be 84%. Mean survival time and cumulative percentage of survival were related to the estimated ILBs; ILBs of 4-5 μCi or greater resulted in 62% or more shortening of lifespan and ILBs of 3-4 μCi resulted in a 24% shortening of lifespan. The dose of beta radiation to the lung to the day of death ranged from 10,000 to 32,000 rads. No malignant primary pulmonary tumors were observed in the mice that had inhaled <tex-math>${}^{144}{\rm CeO}{}_{2}$</tex-math> particles.

The toxicity of <tex-math>${}^{90}{\rm Y}$</tex-math> in the beagle is being investigated as part of a program to evaluate the biological effects of inhaled radionuclides. Thirtythree beagles were exposed to aerosols of <tex-math>${}^{90}{\rm Y}$</tex-math> in fused clay resulting in initial lung burdens (ILB) of 80-5200 μCi <tex-math>${}^{90}{\rm Y}/{\rm kg}$</tex-math> body weight. Cumulative beta radiation dose to the lungs to infinity or death ranged from 990 to 55,000 rads. Twenty-one dogs with ILBs from 670 to 5200 μCi/kg and beta radiation doses to lung ranging from 8400 to 55,000 rads died between 7.5 and 163 days postexposure. Clinical signs included progressive increase of respiratory rates, abnormal lung sounds on auscultation, progressive weight loss, lymphopenia, and eventual cyanosis. Principal pathological findings were pulmonary and pleural fibrosis, occlusive pulmonary vascular lesions, metaplasia and/or hyperplasia of terminal bronchiole and alveolar epithelium, right-heart dilatation and hypertrophy. Infarcts of the right atria were found in some animals. All dogs with ILBs of 460 μCi/kg or less and cumulative dose to lung of 5700 rads or less are still alive at 402-465 days postexposure and show no detectable clinical effects and no changes in either thoracic radiographs or blood gas and pulmonary function parameters. These dogs will be observed for their lifespan.