Specific absorbed fractions and radionuclide S values for adult and pediatric respiratory tracts within ICRP series of mesh-type reference computational phantoms

Abstract

Recently, the International Commission on Radiological Protection (ICRP) has released adult and pediatric mesh-type reference computational phantoms (MRCPs) through its Publications 145 and 156, which incorporate anatomically refined respiratory tract structures that overcome the limitations of earlier voxel and stylized models. In this study, a comprehensive dataset of specific absorbed fractions (SAFs) and radionuclide S values was generated for the respiratory tract across the entire age- and sex-specific series of ICRP MRCPs. The phantoms were implemented in the Geant4 Monte Carlo radiation transport code (version 11.3) to compute SAFs for photons, electrons, and alpha particles over the energy ranges of 0.001-10 MeV for photons and electrons and 1-12 MeV for alpha particles, with certain low-energy values supplemented by a limiting SAF interpolation approach. The calculated SAFs were subsequently combined with nuclear decay data from ICRP Publication 107 to derive S values for all relevant source regions following inhalation exposures to radionuclides. Photon and electron SAFs were obtained for 36 source-target combinations, and alpha SAFs for 18 combinations, while S values were produced for 1,252 radionuclides. The calculated SAFs exhibited clear age-dependent trends, with larger values in younger phantoms. Furthermore, the calculated SAFs and S values were generally greater than previously reported ICRP values. The complete dataset is available through an open-access repository, representing the first effort to provide SAFs and S values for the respiratory tract using the ICRP MRCPs. The calculations explicitly accounted for micrometre-scale source and target regions within anatomically realistic respiratory tract structures, while also incorporating inter-tissue irradiation cases, which had not been possible with previous models.