The role of ⁶⁸Ga-PSMA-PET/CT after a negative ¹⁸F-Choline PET/CT in biochemical relapse of prostate cancer: Initial experience

Introduction: For many years ¹⁸F-Choline PET/CT has been used to stage and restage prostate cancer while recently PSMA-PET/CT is more and more used in this situation. The aim of this study was to establish the detection rate of PSMA-PET/CT after a negative ¹⁸F-Choline PET/CT in patients at biochemical relapse of prostate cancer. Materials and Methods: From July 2017 to December 2018, 50 consecutive patients (mean age: 70.8 years old ± 15.6) underwent ⁶⁸Ga-PSMA-PET/CT in our centre, at maximum 3 months after a negative ¹⁸F-Choline PET/CT. All of them were in biochemical relapse after curative therapy (prostatectomy and/or radiotherapy and/or brachytherapy). Images were analysed visually in order to localise the recurrence. Findings on ⁶⁸Ga-PSMA-PET/CT were correlated with PSA level for different thresholds and with PSA doubling time (PSAdt).

Results: Mean PSA level was 7.2 ng/ml (± 17.8). Overall, ⁶⁸Ga-PSMA-PET/CT was positive in 42 cases for an overall detection rate of 84% (42/50). In patients with PSA <0.5 ng/ml (group 1) ⁶⁸Ga-PSMA-PET/CT showed a detection rate of 42.9% (3/7), in patients with 0.5&#8804;PSA &#8804; 1.5 ng/ml (group 2) a detection rate of 85.7% (12/14) and in patients with PSA > 1.5ng/ml a detection rate of 93.1% (27/29). ⁶⁸Ga-PSMA-PET/CT showed a higher detection rate in group 2 compared to 1 (p=0.046) as well as in group 3 compared to 1 (p=0.016), while no significant difference was seen between groups 2 and 3 (p=0.43). Doubling time was not significantly different between groups (p>0.05).

Conclusions: In biochemical relapse, patients with a negative Choline PET/CT should undergo PSMA-PET/CT given its high expected detection rate. Noteworthy, in cases with PSA higher than 0.5 ng/ml PSMA-PET/CT gives the possibility to lead towards targeted therapies, while efforts should still be done to increase the detection rate in patients with PSA lower than 0.5 ng/ml.