68Ga-PSMA PET/CT for primary NM staging of high-risk prostate cancer.

: With the largest high-risk prostate cancer (PCa) cohort to date undergoing 68Ga-prostate-specific membrane antigen (PSMA) PET/CT primary staging, we aimed to 1) characterize the metastatic spread of PCa in relation to tumor 68Ga-PSMA-uptake and the D\u0027Amico classification, and 2) compare 68Ga-PSMA PET/CT findings with radical prostatectomy (RP) with pelvic lymph node dissection (PLND) histopathology. Methods: A total of 691 consecutive newly diagnosed, biopsy-proven, treatment-naive, D\u0027Amico high-risk PCa patients primary staged by 68Ga-PSMA PET/CT were included. PSMA maximum standardized uptake value (SUVmax) and metastatic findings were compared to PSA level, International Society of Urologic Pathology (ISUP) grade, and clinical stage as traditional risk stratification parameters. Moreover, 68Ga-PSMA PET/CT findings were compared with histology in RP patients undergoing PLND. Undetected lymph node metastases (LNMs) underwent immunohistochemical PSMA staining. Results: Advanced disease (N1/M1) was observed in 35.3% of patients (244/691) and was associated with increasing PSA levels, ISUP grades, and clinical stages. LNMs (N1/M1a) were detected in 31.4% (217/691) and bone metastases (M1b) in 16.8% (116/691). Advanced disease frequencies in patients with ISUP grade 2 and 3 were 10.8% (11/102) and 37.1% (33/89), respectively. Risk of advanced disease for cT2a/cT2b/cT2c tumors were almost equal (24.2%, 27.9%, and 22.4%, respectively). We observed a weak correlation between SUVmax and biopsy ISUP grade (ρ = 0.21; P \u003c 0.001) and a modest correlation between SUVmax and post-prostatectomy ISUP grade (ρ = 0.38; P \u003c 0.001). Sensitivity, specificity, positive and negative predictive value, and accuracy for LNMs detection on 68Ga-PSMA PET/CT in the PLND cohort were 30.6%, 96.5%, 68.8%, 84.5%, and 83.1%, respectively. Undetected LNMs were either micrometastases located in the lymph node border or without PSMA expression. Conclusion: In this high-risk PCa cohort, we identified advanced disease in about one-third at diagnosis. ISUP grade was the superior predictor for advanced disease at diagnosis. We found a significant difference in frequency of advanced disease between ISUP grade 2 and 3, which supports the Gleason Score 7 subdivision. Interestingly, we observed no significant differences in risk of advanced disease when comparing the different cT2 stages. The undetected LNMs were either PSMA-negative or micrometastases.