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Abstract
BackgroundDosimetry for diagnostic agents is performed to assess the risk of radiation detriment (e.g., cancer) associated with the imaging agent and the risk is assessed by computing the effective dose coefficient, e. Stylized phantoms created by the MIRD Committee and updated by work performed by Cristy-Eckerman (CE) have been the standard in diagnostic dosimetry. Recently, the ICRP developed voxelized phantoms, which are described in ICRP Publication 110. These voxelized phantoms are more realistic and detailed in describing human anatomy compared with the CE stylized phantoms. Ideally, all tissues should be represented and their pharmacokinetics collected for an as accurate a dosimetric calculation as possible. As the number of source tissues included increases, the calculated e becomes more accurate. There is, however, a trade-off between the number of source tissues considered, and the time and effort required to measure the time-activity curve for each tissue needed for the calculations. In this study, we used a previously published 68Ga-DOTA-TATE data set to examine how the number of source tissues included for both the ICRP voxelized and CE stylized phantoms affected e.ResultsDepending upon the number of source tissues included e varied between 14.0–23.5 μSv/MBq for the ICRP voxelized and 12.4–27.7 μSv/MBq for the CE stylized phantoms. Furthermore, stability in e, defined as a < 10% difference between e obtained using all source tissues compared to one using fewer source tissues, was obtained after including 5 (36%) of the 14 source tissues for the ICRP voxelized, and after including 3 (25%) of the 12 source tissues for the CE stylized phantoms. In addition, a 2-fold increase in e was obtained when all source tissues where included in the calculation compared to when the TIAC distribution was lumped into a single reminder-of-body source term.ConclusionsThis study shows the importance of including the larger tissues like the muscles and remainder-of-body in the dosimetric calculations. The range of e based on the included tissues were less for the ICRP voxelized phantoms using tissue weighting factors from ICRP Publication 103 compared to CE stylized phantoms using tissue weighting factors from ICRP Publication 60.
Highlights
Dosimetry for diagnostic agents is performed to assess the risk of radiation detriment associated with the imaging agent and the risk is assessed by computing the effective dose coefficient, e
Using a previously published comprehensive data set consisting of sixteen patients receiving 68Ga-DOTA-TATE, we examine the impact of source tissue number on estimates of the patient’s effective dose coefficient
Using only the whole body time-integrated activity coefficients (TIAC) as the remainderof-body TIAC, the respective effective dose coefficients were 13.9 ± 1.6 μSv/MBq and 14.0 ± 1.7 μSv/MBq for the CE stylized and the International Commission on Radiological Protection (ICRP) voxelized reference phantoms, respectively
Summary
Dosimetry for diagnostic agents is performed to assess the risk of radiation detriment (e.g., cancer) associated with the imaging agent and the risk is assessed by computing the effective dose coefficient, e. We used a previously published 68Ga-DOTA-TATE data set to examine how the number of source tissues included for both the ICRP voxelized and CE stylized phantoms affected e. In the Medical Internal Radiation Dose (MIRD) committee S value methodology, the absorbed dose to a particular tissue is given by the sum, over all source regions, of the time-integrated activity (TIA) assigned to each region (i.e., the source tissue, rS), multiplied by the corresponding source to target S value, S(rT ← rS) [1, 2]. There is, a trade-off between the number of source tissues and the time and effort required to measure the time-activity curve (TAC) for each tissue needed to compute the TIA. The analysis was performed using radionuclide S values based on Cristy-Eckerman stylized reference phantoms (CE stylized reference phantoms) [3] using tissue-weighting factors, wT, from the International Commission on Radiological Protection (ICRP) Publication 60 [4], which were compared to the recent ICRP Publication 110 voxelized reference phantom series (ICRP voxelized reference phantoms) and specific absorbed fractions (SAF) from ICRP Publication 133 [5, 6], using wT from ICRP Publication 103 [7]
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