Lung dosimetry for thorotrast patients: implications for inhalation of radon progeny

A peculiar feature of radiation-induced carcinogenesis in German Thorotrast patients is that no excess lung cancers have been observed so far in the epidemiological follow-up study. Two possibilities have been explored here to explain the apparent discrepancy between predicted and observed bronchial tumors: (1) Bronchial doses are smaller than presently predicted; and/or (2) lung cancer risk per unit exposure in Thorotrast patients is smaller than that derived from inhalation of (222)Rn progeny. First, the mean bronchial doses computed with an updated dosimetric model for the different radiation sources are indeed substantially smaller than previous dose estimates; however, the epidemiological findings still cannot be explained in terms of bronchial doses. Second, lung cancer risk estimates based on the revised dose estimates and relative risk coefficients specifically derived from chronic indoor exposures still yield a relative risk of 1.66 for the German Thorotrast patients. This suggests that either the lung cancer risk coefficient for inhalation of radon progeny cannot be applied to Thorotrast patients because of differences in dose distributions or exposure conditions, or the current tissue weighting factor should be applied primarily to the bronchial region of the lung in the case of exposure to radon progeny.