Abstract
Most of the current dosimetry models of inhaled short-lived radon decay products assume uniform activity distributions along the bronchial airways. In reality, however, both deposition and clearance patterns of inhaled radon progenies are highly inhomogeneous. Consequently, a new deposition-clearance model has been developed that accounts for such inhomogeneities and applied together with biophysical models of cell death and cell transformation. The scope of this study was to apply this model which is based on computational fluid and particle dynamics methods, in an effort to reveal the effect of mucociliary clearance on the bronchial distribution of deposited radon progenies. Furthermore, the influence of mucociliary clearance on the spatial distribution of biological damage due to alpha-decay of the deposited radon progenies was also studied. The results obtained demonstrate that both deposition and clearance of inhaled radon progenies are highly non-uniform within a human airway bifurcation unit. Due to the topology of the carinal ridge, a slow clearance zone emerged in this region, which is the location where most of the radio-aerosols deposit. In spite of the slow mucus movement in this zone, the initial degree of inhomogeneity of the activity due to the nonuniform deposition decreased by a factor of about 3 by considering the effect of mucociliary clearance. In the peak of the airway bifurcation, the computed cell death and cell transformation probabilities were lower when considering deposition and clearance simultaneously, compared to the case when only deposition was considered. However, cellular damage remained clustered.
Highlights
Even if the macroscopic exposure level of an individual is known (ICRP 103 2007), the resulting dose distribution within the human body is difficult to quantify because technical and ethical barriers hamper its measurement
If the physical half-life of any deposited radioisotope is long enough that it is removed from their initial location of deposition but shorter than the time needed for the mucus to remove them from the airways, the resulting activity distribution in the airways is different from the primary activity distribution due to deposition alone
It is worth noting that the deposition fraction values are provided for the whole size distribution shown in Fig. 3; they represent combined values of unattached and attached progenies
Summary
Even if the macroscopic exposure level of an individual (e.g. due to radon in air) is known (ICRP 103 2007), the resulting dose distribution within the human body is difficult to quantify because technical and ethical barriers hamper its measurement. Numerical modelling can be an effective method to quantify the radiation dose at various levels of biological organization (whole body, organs, tissues, cells, subcellular entities). This kind of approach may be feasible especially when the distribution of the energy deposition due to ionising radiation is uneven, like in the case of. Based on current knowledge, shortlived radon progenies are transported by the mucus upward a few centimetres which means that the location of their decay will be different from the location of their deposition (Sturm and Hofmann 2007) In this context it is an important question to what extent mucociliary clearance can modify the highly non-uniform primary activity distribution due to deposition. The current standard model was refined in that it accounts for a realistic measured size distribution of radon progenies instead of assuming a simplified bimodal distribution, similar to the approach followed by (James et al 2004)
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