• Janet H Pollard, Caleb Raman, Yousef Zakharia, Chad R Tracy, Kenneth G Nepple, Tim Ginader, Patrick Breheny, and John J Sunderland.
• Department of Radiology, University of Iowa Carver College of Medicine, Iowa City, Iowa janet-pollard@uiowa.edu.
• J. Nucl. Med. 2020 Aug 1; 61 (8): 1145-1152.

AbstractThe PET radiotracer 68Ga-PSMA (prostate-specific membrane antigen)-HBED-CC (N,N'-bis [2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N'-diacetic acid) shows potential as an imaging biomarker for recurrent and metastatic prostate cancer. The purpose of this study was to determine the repeatability of 68Ga-PSMA-HBED-CC in a test-retest trial in subjects with metastatic prostate adenocarcinoma. Methods: Subjects with metastatic prostate cancer underwent 2 PET/CT scans with 68Ga-PSMA-HBED-CC within 14 d (mean, 6 ± 4 d). Lesions in bone, nodes, prostate/bed, and visceral organs, as well as representative normal tissues (salivary glands and spleen), were segmented separately by 2 readers. Absolute and percentage differences in SUVmax and SUVmean were calculated for all test-retest regions. Repeatability was assessed using percentage difference, within-subject coefficient of variation (wCV), repeatability coefficient (RC), and Bland-Altman analysis. Results: Eighteen subjects were evaluated, 16 of whom demonstrated local or metastatic disease on 68Ga-PSMA-HBED-CC PET/CT. In total, 136 lesions were segmented in bone (n = 99), nodes (n = 27), prostate/bed (n = 7), and viscera (n = 3). The wCV for SUVmax was 11.7% for bone lesions and 13.7% for nodes. The RC was ±32.5% SUVmax for bone lesions and ±37.9% SUVmax for nodal lesions, meaning 95% of the normal variability between 2 measurements will be within these numbers, so larger differences are likely attributable to true biologic changes in tumor rather than normal physiologic or measurement variability. wCV in the salivary glands and spleen was 8.9% and 10.7% SUVmean, respectively. Conclusion: Repeatability measurements for PET/CT test-retests with 68Ga-PSMA-HBED-CC showed a wCV of 12%-14% SUVmax and an RC of ±33%-38% SUVmax in bone and nodal lesions. These estimates are an important aspect of 68Ga-PSMA-HBED-CC as a quantitative imaging biomarker. These estimates are similar to those reported for 18F-FDG, suggesting that 68Ga-PSMA-HBED-CC PET/CT may be useful in monitoring response to therapy.© 2020 by the Society of Nuclear Medicine and Molecular Imaging.

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