The microdosimetry of radon and its daughter products in the human respiratory tract

Abstract

Currently, there is a need for applying microdosimetry to radon and its daughters in the human respiratory tract. This need exists because of the inconsistencies of dose values reported in radon lung dosimetry studies. Moreover, microdosimetry can provide more information on the actual amount of energy deposited in target cells. Based on a review of the literature, a model of the human respiratory tract was developed to determine the spatial distribution of alpha source activity in the lung. Factors affecting deposition, retention and clearance of radon daughters were considered. Both basal and secretory cell nuclei were selected as target sites for lung cancer induced by alpha radiation from radon daughters. The source-target geometry was represented by alpha track-length distributions. Using the source-target geometry information obtained, microdosimetric calculations were completed for each airway generation, and specific energy distributions were obtained for a 30 year and a 30 day exposure. Results showed that most (70 - 90%) target cell nuclei are missed by alpha particles even for an exposure as long as 30 years. Secretory cell nuclei have about twice the probability of being hit compared to basal cell nuclei. Most target cell nuclei are hit only once. The lobar and segmental bronchi (generation 2 - 3) have the highest fraction of cells hit; these are also the airways reported to have highest incidence of lung cancer induction. The risk of lung cancer induction, therefore, depends on the probability of target cell nuclei being hit by alpha particles. The absorbed doses to basal and secretory cell nuclei in the segmental bronchi (generation 3) are about 20 rad (0.2 Gy) and 30 rad (0.3 Gy), respectively. For a 30 day short-term exposure, less than 0.1 % of target cell nuclei were hit. The absorbed doses to target cell nuclei are less than 0.1 rad (1 mGy). For the whole tracheobronchial tree, about 13 % of target cell nuclei are hit. A small fraction (4 % for 30 year exposure) of cell that are hit receive a small amount of energy, allowing for the possibility of misrepair and transformation leading to lung cancer. The absorbed dose to target cell nuclei in the tracheobronchial region is about 15 mGy/WLM.