The current approach of the ICRP Task Group for modeling doses to respiratory tract tissues

For radiation protection purposes, the International Commission on Radiological Protection (ICRP) Task Group proposes to apportion radiation risk within the respiratory tract according to the tumor mortality rates observed in the different anatomical regions. This approach requires that doses absorbed by extrathoracic tissues must be considered, in addition to those in the lung. For the extrathoracic region, the tissues at highest potential risk are the pharyngeal parts of the nasopharynx and oropharynx and a part of the larynx. In the lung, all tissues are potentially at risk, and it is necessary to consider doses absorbed by bronchial tissues, the lung parenchyma, and lymph nodes. This paper outlines the methods proposed by the Task Group to evaluate the heterogeneous doses absorbed by sensitive cells in these tissues from radioactive decays of alpha-emitters. The objective is to evaluate doses to broad regions of the respiratory tract, where the regions are defined to reflect substantial differences in potential risk when taking into account deposition and clearance behavior. The Task Group proposes to represent the respiratory tract by three generic regions: an extrathoracic region and two thoracic regions, one clearing fast and one slowly. The models of aerosol deposition and clearance applied for each region are outlined. To illustrate the use of the model, doses are evaluated for the key cases of short-lived radionuclides and long-lived insoluble alpha-emitters and are discussed with regard to current ICRP recommendations.