Comparison of internal dosimetry factors for three classes of adult computational phantoms with emphasis on I-131 in the thyroid

Abstract

S values for 11 major target organs for I-131 in the thyroid were compared for three classes of adult computational human phantoms: stylized, voxel and hybrid phantoms. In addition, we compared Specific Absorbed Fractions (SAFs) with the thyroid as a source region over a broader photon energy range than the x- and gamma-rays of I-131. S and SAF values were calculated for the International Commission on Radiological Protection (ICRP) reference voxel phantoms and the University of Florida (UF) hybrid phantoms by using Monte Carlo transport method, while the S and SAF values for the Oak Ridge National Laboratory (ORNL) stylized phantoms were obtained from earlier publications. Phantoms in our calculations were for adults of both genders. The 11 target organs and tissues that were selected for the comparison of S values are: brain, breast, stomach wall, small intestine wall, colon wall, heart wall, pancreas, salivary glands, thyroid, lungs, and active marrow for I-131 and thyroid as a source region. The comparisons showed, in general, an underestimation of S values reported for the stylized phantoms compared to the values based on the ICRP voxel and UF hybrid phantoms and a relatively good agreement between the S values obtained for the ICRP and UF phantoms. Substantial differences were observed for some organs between the 3 types of phantoms. For example, the small intestine wall of ICRP male phantom and heart wall of ICRP female phantom showed up to 8-fold and 4-fold greater S values, respectively, compared to the reported values for the ORNL phantoms. UF male and female phantoms also showed significant differences compared to the ORNL phantom, 4.0-fold greater for small intestine wall and 3.3-fold greater for heart wall. In our method, we directly calculated the S values without using the SAFs as commonly done. Hence, we sought to confirm the differences observed in our S values by comparing SAFs among the phantoms with the thyroid as a source region for selected target organs - small intestine wall, lungs, pancreas and breast as well as illustrate differences in energy deposition across the energy range (12 photon energies from 0.01 to 4 MeV). Differences were found in SAFs between phantoms in a similar manner to the differences observed in S values but with larger differences at lower photon energies. To investigate the differences observed in S and SAF values, the chord length distributions (CLDs) were computed for the selected source-target pairs and compared across the phantoms. As demonstrated by the CLDs, we found that the differences between phantoms in those factors used in internal dosimetry were governed to a significant degree by inter-organ distances which are a function of organ shape as well as organ location.

Internal dosimetry estimates using voxelized reference phantoms for thyroid agents

“…Figure 1 indicates that the S values calculated in this study (using F6 and *F8 tally) were in good agreement with those from Lamart et al [6]. The results for each radiopharmaceutical come in the following sections.…”

“…The S value in turn, is defined as the mean absorbed dose to the target organ ( r T ) per unit of nuclear transition of the relevant radionuclide in the source region considered [5, 6]: \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$$D\left( {r_T } \right){\rm \; }=\mathop \sum \limits_{r_S } S\left( {r_T \leftarrow r_S } \right)\tilde A\left( {r_S } \right) $$\end{document} …”

The contributions of source regions to organ doses from incorporated radioactive iodine

“…The thyroid is the major contributor to the lung dose; hence the shorter distances between the thyroid and lungs for voxel phantoms would result in a greater lung dose. Lamart et al (2011) reported the CLDs between the lungs and thyroid in female phantoms which have different mean distances; (14.5 and 20.4 cm) for the ICRP voxel and ORNL phantom, respectively. This shift demonstrates the approximately fourfold greater lung dose for the voxel phantoms.…”

Computational lymphatic node models in pediatric and adult hybrid phantoms for radiation dosimetry

“…1): $$Sfalse(r_T\leftarrow r_{S}false)=1.602\times {10}^{-10}\frac{1}{Mfalse(r_{T}false)}\left[E_{\gamma }\left({\gamma }_T\right)Y_{\gamma }+E_{\beta }\left(r_T\right)Y_{\beta }\right]$$ where 1.602 × 10 −10 is the number of joules per MeV. We previously reported the details of the calculation method (Lamart et al 2011). …”