A revised stylized model of the adult extrathoracic and thoracic airways for use with the ICRP-66 human respiratory tract model

The extrathoracic airways and lymph nodes have not yet been represented explicitly in mathematical or stylized models of the human body utilized in the transport of photons internally between source and target organs. Currently, the ICRP assumes that the extrathoracic airways are reasonably approximated by using the thyroid or brain as the surrogate source and target region within the ICRP 66 respiratory tract model. In the present study, a new mathematical model was created to explicitly consider the extrathoracic airways, as well as other respiratory structures in the thorax of the adult. The model incorporates the MIRD model of the adult head and neck, and the ORNL model of the adult torso/legs. Additional defining equations are established for the external nose, nasal cavity, nasal sinuses (frontal, ethmoid, sphenoid, and maxillary sinuses), oral cavity, larynx, pharynx, trachea, and main bronchi. Use of the thyroid as a surrogate source for photon emissions in the ET₁ and ET₂ tissues is shown to provide either close or conservative values of specific absorbed fraction to target organs such as the lungs of breasts at energies exceeding 50-100 keV. At lower energies, surrogate-region values of SAF underestimate dose to target organs in ways highly dependent upon the source/target configuration. The use of the brain as a surrogate source for ET₁ and ET₂ tissues irradiating the thyroid is shown to result in SAF values that are lower than values of SAF(thyroid↔ET ₁) by factors of ∼2-3, and lower than values of SAF(thyroid↔ET₂) by factors of ∼30 at photon energies >50 keV. At energies <50 keV, values of SAF(thyroid↔ET₂) are shown to be orders of magnitude higher than the ICRP 66 default given by SAF (thyroid↔brain).