What is the Prostate Specific Antigen Cut-off Value to Detect Metastases in Ga-68 Prostate Specific Membrane Antigen Ligand Positron Emission Tomography/Computer Tomography Imaging for Intermediate and High-risk Prostate Cancer?

Objective: The aim of this study was to detect a prostate specific antigen (PSA) cut-off value for metastases, and prevent unnecessary use of Ga-68 prostate specific membrane antigen (PSMA) ligand positron emission tomography/computer tomography (CT) imaging. Materials and Methods: Between January 2016 and October 2017, patients with prostate cancer (PCa) who were staged using Ga-68 PSMA enrolled in this retrospective study. Patients were divided into two groups: group 1 (G1) had Ga-68 PSMA for primary staging, and group 2 (G2) had Ga-68 PSMA for detection of metastases after treatment (secondary staging). PSA cut-off values were calculated for general, bone, lymph node and visceral metastases. Results: A total of 181 patients were included in this study. PSA cut-off was 7.5 ng/mL for overall metastases in general. The PSA cut-offs for overall metastases using initial PSA were 8.98 ng/mL and 6.82 ng/mL for G1 and G2, respectively. For G2 patients the post-treatment PSA cut-off was 0.38 ng/mL. For bone metastases, the PSA cut-offs using initial PSA were 8.98 ng/mL and 10.7 ng/mL for G1 and G2, respectively. For lymph node metastases, the PSA cut-offs using initial PSA were 8.98 ng/mL and 6.71 ng/mL for G1 and G2, respectively. For visceral metastases, the PSA cut-offs using initial PSA were 16.4 ng/mL and 7.08 ng/ mL for G1 and G2, respectively. The same analyses could not be calculated in subgroups of G2 due to small sample size. Conclusion: These cut-off values do not suggest that Ga-68 PSMA is superior to using both CT and bone scintigraphy. It is an expensive test and its use is only reasonable when small lymph node metastases cannot be evaluated in CT.