Dosimetry model for bronchial and extrathoracic tissues of the respiratory tract

The Task Group to Review the Lung Model for the International Commission on Radiological Protection (ICRP) is proposing to revise the dosimetry model for the respiratory tract on the basis of the relative radiosensitivities of the various tissue components. The task group considers that if all tissues were to receive the same radiation dose, approximately 64% of fatal cancers would be likely to arise in the bronchi, about 12% in the bronchioles, about 20% in extrathoracic tissues (divided between the naso-- and oropharynx and larynx), and only about 4% in the alveolar-interstitial tissue, with a very small fraction of all cancers (less than 0.1%) in lymph nodes. A new and detailed treatment of dose received by epithelia in the tracheo-bronchiolar and extra-thoracic regions of the respiratory tract is therefore required. This paper describes the approach proposed by the task group to evaluate doses to presumed target cells in each of these tissues at risk in the respiratory tract from non-uniform irradiation by α-, β- and electron-emitters. The task group's approach takes into account the impact of observed phenomena that may cause long-lived radionuclides to be retained in epithelial tissues: the uptake of particles by the airway wall; the chemical binding of sparingly soluble nuclides in epithelial tissues; and, especially in the bronchi and bronchicles, the slow clearance of particles by mucus. Values of dose, weighted by the task group's proposed relative risk factors are calculated for several types of α- and β- emitters. These weighted doses are compared with the committed effective dose equivalent calculated using the current ICRP lung model, in which the simplifying assumption is made that all of the activity retained in the thorax uniformly irradiates the 1 kg mass of a ''composite'' lung. 11 refs., 14 figs., 7 tabs