The photon absorbed fraction $\phi$ is simply the fraction of the emitted photon energy by the source that is absorbed in the region of interest [1,2]. The absorbed fraction can only be determined after specifying the source and the geometry of the absorber, the photon energy, and the material medium [1].
The absorbed fraction can be expressed as the proportion of the photon energy emitted by the source that is absorbed by the target; this relationship is given by the equation below [1]:
Once the absorbed fraction is obtained, it is possible to determine the Specific Absorbed Fraction (SAF) given the mass of the target; thus, the SAF is the ratio of the absorbed fraction to the mass of the target in kg⁻¹ [3]. This ratio is given by the equation below:
Where $\phi(r_T \leftarrow r_S, E_i, t)$ is the absorbed fraction (defined as the fraction of radiation energy $E_i$ emitted within the source tissue $r_S$ at time $t$ that is absorbed in the target tissue $r_T$), and $M(r_T, t)$ is the time-dependent mass of the target tissue $r_T$ in the reference individual [3].
The SAF values in this study were calculated based on the deposited energy per unit mass using the t-deposit tally available in PHITS [1]. Since the results are returned in Gy/source, which corresponds to J·kg⁻¹, it is therefore possible to calculate the SAF using the equation below:
Where $D_{\text{PHITS},\, r_T \leftarrow r_S}$ is the absorbed dose obtained using the PHITS 3.34 Monte Carlo code in J·kg⁻¹, for a source-target organ pair, and $E_{i,\, r_S}$ is the energy emitted by the source organ in J.
With this approach it is possible to calculate SAF values for various scenarios; however, some organs and tissues have more than one segmented region, such as the kidneys (cortex, medulla and pelvis). In these cases, the SAFs were obtained using the equation below, which represents a weighted average of the SAF values of each subregion of the corresponding organ/tissue:
Where $M_{\text{Organ}}$ is the total mass of all subregions of the organ or tissue, $m_i$ is the mass of the subregion, and $\text{SAF}_{i,\, r_T \leftarrow r_S}$ are the SAF values obtained for the subregion. This approach was used for all organs/tissues that have subregions, or bilateral organs.
In radiation dosimetry and radiological protection, a computational phantom is a computerized, anatomical representation of the human body designed for use with Monte Carlo radiation transport simulation codes (such as PHITS, Geant4, MCNP, and FLUKA) [1,2].
Computational phantoms provide the spatial geometry, volume, density, and elemental composition (H, C, N, O, etc.) of organs and tissues. They enable physicists to simulate ionizing radiation interactions, energy deposition, and calculate Specific Absorbed Fractions (SAFs), organ absorbed doses, and effective dose quantities that cannot be measured directly inside living human beings [1,3].
In 2009, the International Commission on Radiological Protection (ICRP), in a joint effort with the International Commission on Radiation Units and Measurements (ICRU), released ICRP Publication 110: Adult Reference Computational Phantoms [1]. This landmark publication established the official international voxel models for the:
These models were derived from high-resolution CT scans of real individuals and then systematically modified voxel-by-voxel so that organ masses and body dimensions strictly matched the standard physiological reference parameters recommended in ICRP Publication 89 [1,2]. Over 140 segmented tissues and organs were defined with specific elemental compositions and densities, providing the baseline for reference dose conversion coefficients in ICRP Publications 116, 119, and 133 [1].
Although ICRP 110 provided a monumental leap in anatomical realism compared to stylized models, voxel-based geometry has fundamental constraints [1,4]:
To overcome the fundamental limitations of voxel-based representations while maintaining total consistency with ICRP 89 anatomical reference data and ICRP 110 organ masses, the ICRP Task Group 103 developed the Adult Mesh-Type Reference Computational Phantoms (MRCPs), published in ICRP Publication 145 (2020) [1].
The MRCPs convert the voxel structures of the ICRP 110 phantoms into high-definition polygon surface meshes and internal volume-filling tetrahedral solid meshes (MRCP-AM for the Reference Male and MRCP-AF for the Reference Female) [1,2].
The Specific Absorbed Fractions (SAFs) displayed in this explorer were simulated using the ICRP 145 Adult Male Mesh Phantom (MRCP-AM) [1,3].
By tracking radiation transport directly through tetrahedral mesh elements in the PHITS 3.34 Monte Carlo code [3], energy deposition from monoenergetic photons and secondary electrons is calculated with high spatial fidelity. For organs with multiple segmented regions (such as kidneys with cortex, medulla, and pelvis), the total organ SAF is evaluated via a mass-weighted average of the subregion SAF values [3].