Quantitative Test-Retest Measurement of 68 Ga-PSMA-HBED-CC in Tumor and Normal Tissue.

The PET radiotracer ⁶⁸ Ga-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 ⁶⁸ Ga-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 ⁶⁸ Ga-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 SUV _max and SUV _mean 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 ⁶⁸ Ga-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 SUV _max was 11.7% for bone lesions and 13.7% for nodes. The RC was ±32.5% SUV _max for bone lesions and ±37.9% SUV _max 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% SUV _mean , respectively. Conclusion: Repeatability measurements for PET/CT test-retests with ⁶⁸ Ga-PSMA-HBED-CC showed a wCV of 12%-14% SUV _max and an RC of ±33%-38% SUV _max in bone and nodal lesions. These estimates are an important aspect of ⁶⁸ Ga-PSMA-HBED-CC as a quantitative imaging biomarker. These estimates are similar to those reported for ¹⁸ F-FDG, suggesting that ⁶⁸ Ga-PSMA-HBED-CC PET/CT may be useful in monitoring response to therapy.
(© 2020 by the Society of Nuclear Medicine and Molecular Imaging.)