Assessment of self- and cross-absorbed SAF values for HDRK-man using Geant4 code: internal photon and electron emitters

Abstract

The ultimate need to account for the partial amount of energy deposited in target tissue/organ resulting from internal inhalation, ingestion, and injection intakes of radionuclides, defined by the Medical Internal Radiation Dosimetry committee as the specific absorbed fraction (SAF), has become obvious. In this study, we assessed the SAF values for self- and cross-absorption, which were calculated for a uniform distribution of monoenergetic photon and electron emitters with energies ranging from 15 keV to 3 MeV. The voxelized human phantom “High-Definition Reference Korean-man” (HDRK-man), which was implemented using the Monte Carlo simulation code Geant4 (version 10.1), was used for several combinations of target–source organs. The results were compared to those of the International Commission on Radiological Protection Reference (ICRP133) and Zubal phantoms. It was found that the SAF values of the three models have a similar trend. However, the SAF values for the HDRK-man phantom were higher than those of the other two models, with a relatively good agreement with those for the ICRP133 phantom (differences of 13.9 ± 2.8 and 12.1 ± 3.2 for photon and electron emitters, respectively). To analyze the differences in SAF values, we calculated the chord length distributions (CLDs) for selected target–source combinations. The parameters of organ mass (or volume) and CLDs, in addition to the adopted computational procedures, mainly cause such discrepancies. For realistic radionuclide emission spectra, an overall overestimation was observed when computing the S values for three radiopharmaceuticals studied (I-131, In-111, and Lu-177) and for liver–spleen intra- and inter-organ absorption when compared with published data. The new arrangement of S and SAF values is expected to add value for multidisciplinary research and clinical communities.