68Ga-PSMA PET/CT for restaging recurrent prostate cancer: which factors are associated with PET/CT detection rate?

Purpose To assess the association between PSA levels, PSA kinetics and other factors and a pathological Ga-68-PSMA PET/CT scan in patients with recurrent prostate cancer (rPCa) with biochemical relapse (BR) after radical therapy. Methods Seventy consecutive rPCA patients referred for Ga-68-PSMA PET/CT, matching all the following criteria, were retrospectively evaluated: (a) previous radical prostatectomy or primary radiotherapy with curative intent; (b) BR or persisting high PSA levels after primary treatment; and (c) complete clinical and imaging information. The mean +/- SD PSA level was 3.5 +/- 5.3 ng/mL (median 1.7, range 0.2 - 32.2 ng/mL), the mean +/- SD PSA doubling time (PSAdt) was 6.5 +/- 5.5 months (median 5.5, range 1.3 - 31.6 months), and the mean +/- SD PSA velocity was 7.9 +/- 20.5 (median 2.1, range 0.2 - 147.5 ng/mL/year). Statistical analysis was performed to assess which factors were associated with the detection of rPCa on Ga-68-PSMA PET/CT. Results Ga-68-PSMA PET/CT was positive in 52 of 70 patients (74.2 %). In 30 patients (42.8 %) lesions limited to the pelvis were detected. Distant lesions were observed in 8 of patients (11.4 %). Local plus systemic lesions were detected in 14 patients (20 %). PSA level (p = 0.017) and PSAdt (p = 0.0001) were significantly different between PET-positive patients (higher PSA level, shorter PSAdt) and PET-negative patients (lower PSA, longer PSAdt). ROC analysis showed that PSAdt 6.5 months and PSA 0.83 ng/mL were optimal cut-off values. In multivariate analysis PSAdt was associated with Ga-68-PSMA PET/CT positivity. Ga-68-PSMA PET/CT was positive in 17 of 20 patients (85 %) with PSA < 2 ng/mL and PSAdt < 6.5 months, and in 3 of 16 patients (18.7 %) with PSA < 2 ng/mL and PSAdt >= 6.5 months. Conclusion The great potential of Ga-68-PSMA PET/CT in patients with rPCa and BR was confirmed. PSA and PSAdt were valuable predictors of pathological Ga-68-PSMA PET/CT findings.