68Ga-PSMA PET/CT IN PATIENTS WITH BIOCHEMICAL RECURRENCE AFTER RADICAL PROSTATECTOMY: HOW TO PREDICT A POSITIVE PSMA PET/CT RESULTS AND THE SITE OF RECURRENCE.

Objective: The aim of this study was to identify the clinical parameters that could predict positive findings at 68Ga-PSMA PET/TC for local, nodal or systemic recurrence in patients with biochemical recurrence (BCR) after radical prostatectomy (RP).

Materials and Methods: We retrospectively enrolled 276 patients submitted to RP for prostate cancer (PCa) and confirmed BCR. To assess the site of recurrence each patient was referred to 68Ga-PSMA PET/CT. The population was stratified according to the results of PSMA PET/CT scan (namely, positive vs. negative spot). The positive results of PSMA PET/CT was recorded according to site of recurrence: local, pelvic and retroperitoneal lymph nodes, skeletal and visceral. Finally, uni and multivariate logistic regression analyses were used to identify the predictors of positive PSMA PET/CT both on patient-based analysis and region-based analysis according to different site of recurrence (namely, local, nodal and skeletal).

Results: The characteristics of the population are summarized in Tab. 1. Overall, PSMA PET/CT was negative in 145 (52.25%) and positive in 131 (47.5%) individuals. Median PSA at PSMA PET/CT was 0.71ng/ml (IQR 0.37-1.40). Patients with positive finding at PSMA PET/CT had significantly higher pathologic stage and PSA at BCR and received more often adjuvant therapies, related to men with negative scan (all p≤0.05). The multivariate logistic regression revealed that PSA velocity was the only independent predictor of positive 68Ga-PSMA PET/TC (OR 12.35, p: 0.002; Table 2a). No significant parameters were found to be related to local recurrence detected by PSMA PET/CT (Table 2b). Finally, we found that only the PSA doubling time <3 months (OR 3.52, p: 0.04; Table 2c) and PSA velocity (OR 4.46, p=0.003; Table 2d) were independent predictors of nodal and bone metastases, respectively.

Conclusions: In our series, 68Ga-PSMA PET/TC demonstrate an important role in the setting of re-staging of PCa patients with BCR after RP, and can individuate the site(s) of metastasis, allowing a potential PSMA PET/CT- guided therapy even in case of low level of PSA. The PSA kinetic (PSA velocity and doubling time) are the most important predictors of positive 68Ga-PSMA PET/TC, especially in case of nodal and skeletal recurrence.