Estimates of the absorbed fraction of energy for various electron source and target regions in the upper respiratory tract

Abstract

Estimates of radiation absorbed dose for internal emitters are needed for the evaluation of the risks associated with various intakes of radionuclides. When the intake route is inhalation, target tissues of interest will include the tracheal wall, the walls Of the main bronchi, and the tissue in the bifurcation zone where these airways interconnect. In the case of beta-emitting radionuclides, the source regions within the respiratory tract will include (1) the airway volume in the case of gases and nondepositing particulates, and (2) the lining of the airways in the case of particulates deposited via impaction, sedimentation, or interception processes. In considering the relatively large mean free paths of photons emitted from these airways, the primary component of radiation absorbed dose to the airway tissues will be that due to electron and beta particles. One key parameter in internal dosimetry calculations is the absorbed fraction of energy which is defined as the ratio of the energy absorbed in the target region to the energy emitted in the source region. In this research, electron radiation transport calculations were performed to estimate the absorbed fractions for various electron source and target regions in the upper respiratory tract. The source regions considered include the tracheal airway, the lining of the tracheal wall, the airways of the main bronchi and the bifurcation zone, and the lining of the bifurcation zone. Tissue at various depths within the tracheal wall, and within walls of the main bronchi and bifurcation zone were the target regions considered. The absorbed fractions were computed for monoenergetic sources covering the energy range of 15 keV to 4 MeV. Comparisons with results obtained by other calculational methods are presented.