Photon specific absorbed fractions for the ICRP family of mesh-type reference computational phantoms

Objective.To provide photon specific absorbed fractions (SAFs), which quantify the fraction of emitted energy absorbed in target tissues and form the basis for internal dose coefficients, using mesh-type reference computational phantoms (MRCPs).Approach.The International Commission on Radiological Protection (ICRP) has recently published MRCPs in Publications 145 and 156 for adult and pediatric reference individuals, respectively. These phantoms supersede the voxel-type reference computational phantoms used in ICRP Publications 133 and 155, overcoming key limitations such as the inability to explicitly define micrometer-scale radiosensitive target tissues, the reliance on supplemental stylized models, and known anatomical inaccuracies such as the lack of full coverage of trabecular spongiosa by cortical bone within the phantom's skeleton. Photon SAFs were computed using the Particle and Heavy Ion Transport code System Monte Carlo code for all 12 ICRP MRCPs, comprising male and female phantoms at each of six ages, covering 86 source regions and 62 target regions as defined within the ICRP internal dosimetric framework.Main Results.The MRCP-based photon SAF dataset demonstrates agreement with ICRP Publications 133 and 155 for most of the crossfire geometries and for the lowest photon energy self-absorption geometries. For photon energies exceeding 30 keV, differences are observed which are consistent with limitations associated with the ICRP reference organ mass scaling approach. SAF differences are also observed due to low-energy extrapolation methods, anatomical improvements including explicit modeling of thin tissue layers, and updated photon dose response functions for the skeletal targets of active marrow and bone endosteum. The datasets exhibit clear age-dependent trends, with younger reference individuals typically yielding higher SAF values.Significance.This study provides complete photon SAF datasets spanning all ICRP MRCPs with expanded source-target combinations, improved dosimetric accuracy, all within a unified computational framework to support the calculation of new reference dose coefficients for both tissue-specific equivalent dose and effective dose.