Distribution of inhaled 239PuO2 in rat and hamster lung.

The distribution of inhaled 239PuO2 in rat and hamster lung was investigated up to one year postexposure in order to determine whether differences in pulmonary dose distribution could account for the apparent difference between the two species in lung tumor response. In previous life-span studies, rats developed up to 70% lung tumors at cumulative doses less than 2000 rads, whereas hamsters developed tumors in about 3% of animals at comparable or higher doses. Morphometric methods were used to quantitate the particle distribution in different lung regions of both species following exposure at levels that produced tumors in rats. The PuO2 particles were counted on autoradiograms of lung cross-sections and classified as to their location in either subpleural or internal parenchyma, lung vasculature, or major airways. Rat lung showed a greater long-term association of particles with the major airways than hamster lung. The particles also showed a greater tendency to occur in groups for rats, the difference being more pronounced in the subpleural region than in the remainder of the lung. In previous experiments, both bronchioles and subpleural fibrotic scars appeared to be the sites of origin for lung tumors. Different dose distributions within these regions may therefore affect the tumor response to inhaled 239PuO2.