Biological Effects of Inhaled ²³⁹PuO₂ in Beagles

Seven groups of 8–24 Beagle dogs, exposed to ²³⁹PuO₂ aerosols by inhalation [mean initial lung depositions (ILD) of 0.0, 0.14, 0.63, 3.2, 13, 44 and 210 kBq] were observed throughout their lives to determine tissues at risk and dose-effect relationships. The mean average pulmonary retention half-time of ²³⁹Pu was 1,192 days. Most (70%) of the plutonium recovered at death in dogs surviving >10 years after exposure was found in the thoracic lymph nodes with ∼15% in lung, ∼10% in liver and ∼2% in bone. Eight dogs at the highest exposure levels died from radiation pneumonitis prior to a minimal 3-year latency period after exposure for the observation of lung tumors, with the first succumbing 337 days after exposure. Of 108 plutonium-exposed Beagles with ILD <100 kBq, 51 (47%) had lung tumors with significantly increased incidence in those dogs with total lung dose of ≥1.1 Gy at death. The primary non-neoplastic effects observed were lymphopenia, atrophy and fibrosis of the thoracic lymph nodes, radiation pneumonitis and pulmonary fibrosis, and bacterial pneumonia. Lesions of the thoracic lymph nodes were observed in 98 of 108 exposed dogs, but there were no primary neoplasms of the lymph nodes. Bacterial pneumonia was observed in 13 plutonium-exposed dogs and was the most notable non-neoplastic cause of death, with survival nearly the same as that of controls. Setting of dose limits on the basis of detrimental effects commonly considers and differentiates between stochastic and deterministic effects, raising the question of whether the non-neoplastic effects found in this study were deterministic. The International Commission on Radiation Protection (ICRP), National Council on Radiation Protection & Measurements (NCRP), and similar organizations generally consider effects that increase in incidence and severity to meet the definition of deterministic. We demonstrated the radiation dose-related nature of effects such as pneumonitis and fibrosis graphically and lymphopenia numerically, rather than by quantified estimates. It is clear, however, that both incidence and severity increased with ILD and radiation dose and should be considered as deterministic effects.