Comparative Dosimetry for ⁶⁸Ga-DOTATATE: Impact of Using Updated ICRP Phantoms, S Values, and Tissue-Weighting Factors

Abstract

The data that have been used in almost all calculations of MIRD S value absorbed dose and effective dose are based on stylized anatomic computational phantoms and tissue-weighting factors adopted by the International Commission on Radiological Protection (ICRP) in its publication 60. The more anatomically realistic phantoms that have recently become available are likely to provide more accurate effective doses for diagnostic agents. Ga-DOTATATE is a radiolabeled somatostatin analog that binds with high affinity to somatostatin receptors, which are overexpressed in neuroendocrine tumors and can be used for diagnostic PET/CT-based imaging. Several studies have reported effective doses forGa-DOTATATE using the stylized Cristy-Eckerman (CE) phantoms from 1987; here, we present effective dose calculations using both the ICRP 60 and more updated formalisms. Whole-body PET/CT scans were acquired for 16 patients afterGa-DOTATATE administration. Contours were drawn on the CT images for spleen, liver, kidneys, adrenal glands, brain, heart, lungs, thyroid gland, salivary glands, testes, red marrow (L1-L5), muscle (right thigh), and whole body. Dosimetric calculations were based on the CE phantoms and the more recent ICRP 110 reference-voxel phantoms. Tissue-weighting factors from ICRP 60 and ICRP 103 were used in effective dose calculations for the CE phantoms and ICRP 110 phantoms, respectively. The highest absorbed dose coefficients (absorbed dose per unit activity) were, in descending order, in the spleen, pituitary gland, kidneys, adrenal glands, and liver. For ICRP 110 phantoms with tissue-weighting factors from ICRP 103, the effective dose coefficient was 0.023 ± 0.003 mSv/MBq, which was significantly lower than the 0.027 ± 0.005 mSv/MBq calculated for CE phantoms with tissue-weighting factors from ICRP 60. One of the largest differences in estimated absorbed dose coefficients was for the urinary bladder wall, at 0.040 ± 0.011 mGy/MBq for ICRP 110 phantoms compared with 0.090 ± 0.032 mGy/MBq for CE phantoms. This study showed that the effective dose coefficient was slightly overestimated for CE phantoms, compared with ICRP 110 phantoms using the latest tissue-weighting factors from ICRP 103. The more detailed handling of electron transport in the latest phantom calculations gives significant differences in estimates of the absorbed dose to stem cells in the walled organs of the alimentary tract.

Accuracy in dosimetry of diagnostic agents: impact of the number of source tissues used in whole organ S value-based calculations

“…This assumption dedicating a specific TIAC to the ovaries was only made to have a source-target for gonads (testes and ovaries) in the calculations, which has a dedicated tissue weighting factor in both ICRP Publications 60 and 103. This increased the effective dose coefficient by 1.7% and 0.1% for the CE stylized and ICRP voxelized reference phantoms, respectively relative to the values published by Josefsson et al [8].…”

“…A common way to perform dosimetric calculations for tumors or small organs (e.g., pituitary gland or salivary glands) has been to use the sphere model, which is included in the OLINDA/EXM version 1.0 [11] software. The pituitary gland has high uptake and the second-highest calculated absorbed dose coefficient previously reported by Josefsson et al [8]. The pituitary gland did not notably affect the calculated effective dose coefficient for the ICRP voxelized reference phantoms (Tables 2, 3, and 4, Fig.…”

“…The effective dose coefficient reference value mentioned by Bodei et al [15] in current concepts of 68 Ga-DOTA-TATE PET/CT imaging was that of Sandström et al [14]. Josefsson et al [8] calculated the effective dose coefficient 23 ± 3 μSv/MBq using the recent ICRP voxelized reference phantom series [5,6] and tissue weighting factors from ICRP Publication 103 [7]. This effective dose coefficient was slightly higher, but not significant, and will most likely not have an impact on the 68 Ga-DOTA-TATE PET/ CT imaging protocols used today.…”

“…These differences between the individual patient and respective reference phantom size will not give individual correct S values regarding self-or reciprocal contributions to neighboring organs/tissues. To compensate for the difference in size between the patient and their respective reference phantom, Josefsson The order of the remainder organs/tissues were according to the impact shown in Table 3 Table [8] normalized the TIACs using the reference phantom and patient total body weight ratio. The next step is to evaluate the non-uniform rational Bspline (NURBS) and polygon mesh (PM) surfacebased computational hybrid phantoms developed by the University of Florida [16,17].…”

“…The patient data, dosimetric methodologies, and results in this study have been previously described by Josefsson et al [8]. Abbreviated descriptions are provided below.…”