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

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 for Ga-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 after Ga-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.