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.

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