Abstract
We have conducted radon exposure experiments with mice to clarify the mechanism of radon therapy. Radon, an inert gas, is mostly exhaled even if inhaled, but radon progeny of solids is deposited on respiratory airways. In the present study, we calculated fractional deposition and absorbed dose of inhaled radon progeny in mice lung. The results were compared with those of rats and humans. A lung model of mice (Oldham et al., Anat. Rec., 2007) divided a lung into 23 airway generations. The generation 1 (trachea) to 16 is a tracheobronchial (TB) region, and the other generation are an alveolar region (from generation 17 to 23). First, we calculated inhalability and deposition in the nasopharyngeal (NP) region. Factional depositions in mice lung were then computed, taking into account the deposition mechanism of inertial impaction, gravitational sedimentation and diffusion. Finally, absorbed doses were assessed for after calculating the particle clearance and absorbed fraction of alpha-particle energy emitted in target cells. The condition of radon progeny was assumed as follows: equilibrium factor 0.4, unattached fraction 0.01, particle-size distribution of geometric mean diameter 250 nm (geometric standard deviation 2.5) for attached radon decay products and of geometric mean diameter mean 1 nm (geometric standard deviation 1) for unattached ones. There were two peaks of fractional depositions at a particle size of 20 nm to 5 μm in the tracheobronchial region, and the two peaks also occur at 80 nm and 3 μm in the alveolus region. Absorbed dose rates in the tracheobronchial airway and alveolus region was 51.6 and 4.6 nGy/h/(Bq/m3)), respectively, which was comparable to 35.9 nGy/h/(Bq/m3)) in the whole lung. Here, this concentration means the radon equilibrium equivalent radon concentration. The same dosimetry was performed for rats and humans. As a result, absorbed doses in mice were two and seven times higher than those in rats and humans, respectively.
