Ga-labeled Prostate-specific Membrane Antigen Ligand-positron-emission Tomography: Still Just the Tip of the Iceberg.

OBJECTIVES: To assess the performance of Ga-labeled prostate-specific membrane antigen ligand-positron-emission tomography (Ga-PSMA PET) for positive lymph nodes on imaging after curatively intended radical prostatectomy.

PATIENTS AND METHODS: Seventeen patients with biochemical recurrence after radical prostatectomy undergoing robot-assisted salvage lymphadenectomy for positive lymph nodes on imaging were included in this single surgeon study. The performance of Ga-PSMA PET was assessed on per patient, per lesion, per landing site and per laterality level using sensitivity, specificity, and negative and positive predictive value analysis.

RESULTS: A total of 34 positive nodes were detected on Ga-PSMA PET with a median of 2 nodes per patient (IQR 1-3 nodes per patient). Sixty six nodes were pathologically disease positive from 14 patients, with a median of 2 positive nodes per patient (IQR 1-6). Three patients had no pathologically detectable disease. On a per patient basis, the positive predictive value was 82%. Sensitivity, specificity, and negative predictive value were not able to be calculated as all patients had disease recurrence with a detectable prostate-specific antigen.On a "per lesion" basis, the sensitivity, specificity, positive predictive value, and negative predictive value were 36.7%, 96.9%, 73.5%, and 86.7%, respectively.

CONCLUSION: Our study indicates that sensitivity of Ga-PSMA PET in the salvage setting is not yet sufficient to detect all sites of metastasis. Therefore, imaging-guided metastasis targeted treatment is likely to fail given the likely concomitant imaging negative more widespread disease.