Specific absorbed fractions for a revised series of the UF/NCI pediatric reference phantoms: internal photon sources

Abstract

Assessment of radiation absorbed dose to internal organs of the body from the intake of radionuclides, or in the medical setting through the injection of radiopharmaceuticals, is generally performed based upon reference biokinetic models or patient imaging data, respectively. Biokinetic models estimate the time course of activity localized to source organs. The time-integration of these organ activity profiles are then scaled by the radionuclide S-value, which defines the absorbed dose to a target tissue per nuclear transformation in various source tissues. S-values are computed using established nuclear decay information (particle energies and yields), and a parameter termed the specific absorbed fraction (SAF). The SAF is the ratio of the absorbed fraction—fraction of particle energy emitted in the source tissue that is deposited in the target tissue—and the target organ mass. While values of the SAF may be computed using patient-specific or individual-specific anatomic models, they have been more widely available through the use of computational reference phantoms. In this study, we report on an extensive series of photon SAFs computed in a revised series of the University of Florida and the National Cancer Institute pediatric reference phantoms which have been modified to conform to the specifications embodied in the ICRP reference adult phantoms of Publication 110 (e.g. organs modeled, organ ID numbers, blood contribution to elemental compositions). Following phantom anatomical revisions, photon radiation transport simulations were performed using MCNPX v2.7 in each of the ten phantoms of the series—male and female newborn, 1 year old, 5 year old, 10 year old, and 15 year old—for 60 different tissues serving as source and/or target regions. A total of 25 photon energies were considered from 10 keV to 10 MeV along a logarithm energy grid. Detailed analyses were conducted of the relative statistical errors in the Monte Carlo target tissue energy deposition tallies at low photon energies and over all energies for source–target combinations at large intra-organ separation distances. Based on these analyses, various data smoothing algorithms were employed, including multi-point weighted data smoothing, and log–log interpolation at low energies (1 keV and 5 keV) using limiting SAF values based upon target organ mass to bound the interpolation interval. The final dataset is provided in a series of ten electronic supplemental files in MS Excel format. The results of this study were further used as the basis for assessing the radiative component of internal electron source SAFs as described in our companion paper (Schwarz et al 2021) for this same pediatric phantom series.

Absorbed dose rate coefficients for 134Cs and 137Cs with steady-state distribution in the human body: S-coefficients revisited

“…The advantage of our absorbed dose rate coefficients is that we can present them for males and females separately (table 4). However, these coefficients are primarily applicable to adults, and the elaboration of body-size-dependent absorbed dose rate coefficients that can be used for children and adolescents depends on future ICRP work on voxel phantoms for children (ICRP 2015, Schwarz et al 2021a, 2021b. We believe our organ dose coefficients readily can be used to calculate absorbed organ doses after protracted exposure both to occupationally exposed groups and members of the public after an accidental nuclear power plant release.…”

Fetal and Maternal Atomic Bomb Survivor Dosimetry Using the J45 Pregnant Female Phantom Series: Considerations of the Kneeling and Lying Posture with Comparisons to the DS02 System

“…Additionally, each phantom was voxelized and post-processed using UF’s muscle and lymph node generator (Stepusin 2016; Schwarz et al 2021). Upon completion, the total body mass of each kneeling phantom was verified to be within 1.5% of the total body mass of the original standing series for consistency in the radiation transport simulations (see below).…”

Stylized versus voxel phantoms: quantification of internal organ chord length distances

“…Recent studies by Schwarz et al (2021aSchwarz et al ( , 2021b, which employed the UF/NCI hybrid phantom series, demonstrated that organ distances play a significant role in the calculation of electron SAFs. Longer chord length distances that occur at greater pediatric ages have the effect of decreasing the estimated SAF (Schwarz et al 2021a).…”

“…The RORNL series is hermaphroditic, in that one phantom contains the sex-specific organs of both the male and female anatomy. The UF/NCI series is a hybrid phantom series which has previously been converted into voxel format for use in Monte Carlo simulation tools (Lamart et al 2011, Dewji et al 2018, Khamwan et al 2018, Schwarz et al 2021a, 2021b. Figure 1 highlights the contrasting geometries of the two series (Schwarz et al 2011).…”