Evaluation of risk factors for lung tumour induction in rats exposed to either NpO2 or PuO2 aerosols

Purpose: To compare the incidence of each lung tumour type after inhalation exposure of rats to either NpO2 or industrial PuO2 aerosols, which have a similar size. Materials and methods: Male Sprague-Dawley rats were exposed once and followed during their whole life span. At the end of their life, the whole lungs were fixed, embedded and cut into thin sections for histological analysis. The presence of tumours was evaluated on three distinct levels of the lobes for phenotype to establish dose-effect relationships. Results: In the range of lung doses studied (0.05 to more than 50 Gy), the general trend was an increased frequency of all types of tumours after inhalation exposure to neptunium compared with plutonium. The linearity of the lower part of the dose effect relationships for all malignant lung tumours leads to the conclusion that NpO2 is 3.3-fold more carcinogenic than PuO2. Conclusions: According to a linear extrapolation of the data on malignant lung tumour incidence collected among all studies reported on actinide oxide carcinogenesis, the risk of lung tumour appears to vary over a factor of about 10 depending on the nature and/or size of the aerosol. This variation has to be taken into account for a realistic assessment of tumour risk.