LIFE-SPAN STUDIES IN RATS EXPOSED TO (PUO2)-PU-239 AEROSOL

Spatial-temporal dose-distribution patterns in the lung continually change following inhalation of radionuclides. Lung clearance, microdosimetry, and radiation dose were examined in female Wistar rats exposed to high-fired (Yb2O3)-Yb-169-(PuO2)-Pu-239 aerosols. Whole-body counting for Yb-169 at 14 d postexposure provided an accurate (r = 0.99) estimate of Pu-239 lung content. Alpha irradiation of tracheal epithelium was at least 50 times less than for bronchiolar epithelium due principally to preferential retention of (PuO2)-Pu-239 in peribronchiolar alveoli as compared to other alveolar regions. The formation of large aggregates (>25 particles) increased linearly with initial lung burden starting at 0.4 kBq; mean dose rate to these focal alveolar regions was 120 Gy d-1. Concentration Of (PuO2)-Pu-239 in pulmonary macrophages and in aggregates, along with the limited penetration of alpha particles in tissue, resulted in a highly nonhomogeneous dose distribution pattern. Alveolar clearance was best represented by a biphasic clearance curve comprised of a rapid early phase (80% initial lung burden) and a slow late phase (20% initial lung burden). Studies with intratracheally instilled (PuO2)-Pu-237-(PuO2)-Pu-239 and with inhaled (PuO2)-Pu-239 showed that alveolar clearance was inversely proportional to initial lung burden. A single clearance function was derived from experimentally determined clearance curves for inhaled (PuO2)-Pu-239 that was used to accurately estimate lung dose at all initial lung burden levels. Lung doses were calculated for 2,105 exposed lifespan rats based on individually determined initial lung burden, survival time, and individually computed clearance function.