Charged particle equilibrium effects on the electron absorbed fraction in the extrathoracic airways

Estimates of the dose to the extrathoracic airway (nasal vestibule) from inhaled beta-emitting radionuclides, obtained using the respiratory tract model presented in Publication 66 of the International Commission on Radiological Protection, frequently predict that the basal cells in this region are the most highly irradiated tissues of the body. The dose to the basal cells is averaged over a layer of tissue 10 μm thick located at a depth of 40 μm into the airway assuming that charged particle equilibrium exists. Since the target (basal cell layer) is very small and thin (10 cm 2 area and 10 μm thickness), charged particle equilibrium does not exist. In this work the effect on the absorbed fraction of the lack of charged particle equilibrium is investigated.