Test-Retest Reproducibility of Conventional Quantitative Parameters on PSMA-targeted ¹⁸F-DCFPyL PET/CT in Patients with Metastatic Prostate Cancer

Objectives: Prostate-specific membrane antigen (PSMA)-targeted ¹⁸F-DCFPyL has proven to detect metastatic prostate cancer (PC) with high accuracy. In this study, we evaluated ¹⁸F-DCFPyL test-retest reproducibility of conventional quantitative parameters in widespread disease.

Methods: In a prospective clinical trial (NCT03793543), 23 patients with histologically proven PC underwent two ¹⁸F-DCFPyL PET scans within 7 days (mean 3.7, range 1 to 7d). Lesions in bone, lymph nodes and other organs were manually segmented on both scans and quantitative parameters were assessed (mean [SUV_mean] and maximum [SUV_max] standardized uptake values, PSMA-tumor volume [PSMA-TV] and total lesion PSMA [TL-PSMA, SUV x TV]). Repeatability of quantification was determined using correlations and within-subject coefficient of variation (wCOV, in %; <10% indicating excellent, 10-20% good, 20-30% acceptable and >30% unacceptable reproducibility).

Results: In total, 229 lesions (176 bone, 38 lymph nodes, 15 others) were delineated in both scans. For all investigated parameters, a high inter-scan correlation was found (R²≥0.98). Analyzing all lesions, the wCOVs for SUV_max and SUV_mean were 12.1% and 7.3%, respectively, indicating good to excellent reproducibility for PSMA expression derived by SUV. The wCOV of LN, however, were significantly lower relative to those derived from osseous lesions (SUV_mean: LN, 3.8% vs. skeleton, 7.8%, p<0.0001; SUV_max: LN, 8.8% vs. skeleton, 12.0%, p<0.03), supporting the notion that higher reproducibility for SUVs can be assessed in LN. Relative to SUV, TV-based features of all lesions demonstrated acceptable reproducibility (PSMA-TV, 23.5%; TL-PSMA, 24.0%). No significant difference could be assessed when comparing the wCOV of volumetric parameters in LN and osseus lesions (PSMA-TV: LN, 24.1% vs. skeleton, 22.8%, p=0.63; TL-PSMA: LN, 23.5% vs. skeleton, 23.3%, p=0.90). Conclusions: Relative to volumetric parameters, SUVs demonstrated a better reproducibility on ¹⁸F-DCFPyL PET, in particular for lymph node disease. Thus, SUV may be better suited as a reliable response assessment tool, for both existing studies and future trials.