Non-linear relationship of cell hit and transformation probabilities in a low dose of inhaled radon progenies

Abstract

Cellular hit probabilities of alpha particles emitted by inhaled radon progenies in sensitive bronchial epithelial cell nuclei were simulated at low exposure levels to obtain useful data for the rejection or support of the linear-non-threshold (LNT) hypothesis. In this study, local distributions of deposited inhaled radon progenies in airway bifurcation models were computed at exposure conditions characteristic of homes and uranium mines. Then, maximum local deposition enhancement factors at bronchial airway bifurcations, expressed as the ratio of local to average deposition densities, were determined to characterise the inhomogeneity of deposition and to elucidate their effect on resulting hit probabilities. The results obtained suggest that in the vicinity of the carinal regions of the central airways the probability of multiple hits can be quite high, even at low average doses. Assuming a uniform distribution of activity there are practically no multiple hits and the hit probability as a function of dose exhibits a linear shape in the low dose range. The results are quite the opposite in the case of hot spots revealed by realistic deposition calculations, where practically all cells receive multiple hits and the hit probability as a function of dose is non-linear in the average dose range of 10-100 mGy.

3D-modelling of radon-induced cellular radiobiological effects in bronchial airway bifurcations: Direct versus bystander effects

“…Our earlier results, however, prove that the radiation burden originating from low doses of radon inhalation is distributed among mainly a very small percentage of the biological targets of the bronchial epithelium (Sz ő ke et al 2008, 2009, Bal á sh á zy et al 2009, Farkas et al 2011, Farkas et al 2006. Moreover, this burden is non-uniformly distributed even among the targets that are actually hit during exposure, i.e., values of the calculated microdosimetric parameters in some cells or cell nuclei may considerably exceed the means of these parameters (for the whole bifurcation), even if only the hit targets are taken into consideration.…”

Radon Exposure and the Definition of Low Doses—The Problem of Spatial Dose Distribution

“…Since radon progeny are incorporated in the same way, the spatial distribution of the activity is similarly heterogeneous (Balásházy et al 2009). Mucociliary clearance smooths the degree of heterogeneity to some extent; however, its effectiveness is decreased by the short half-life of radon progeny and the low mucus velocity in the peak of the bronchial bifurcations, which are the most exposed parts of the lungs (Farkas and Szőke 2013).…”

“…If the particle enhancement ratio is different from one, then it depends highly on the size of the surface elements where particle density is measured. Neglecting mucociliary clearance, particle enhancement ratio can reach 344 (at home) or 461 (in a mine) in the bronchial airways if the patch size is 0.17 mm 2 (Balásházy et al 2009). The effect of mucociliary clearance on the maximum particle enhancement factors for different patch sizes has been estimated by Farkas and Szőke (2013) in the case of particle sizes of 1 and 10 mm.…”

Radon induced hyperplasia may provide an explanation for inverse exposure rate effect

“…Inhalation of radon progeny is characterized by highly heterogeneous dose distribution within the bronchial airways [3]. While the spatial and temporal distribution of dose does not modulate biological effects which show linear dose dependence, they may have significant impact in case of non-linear biological responses to ionizing radiation such as radioadaptive responses.…”

“…

higher than the average over the airway generation [3]. Besides average cell nucleus hits and dose, distribution of hit numbers and doses of cell nuclei were determined.

…”