The use of Ga-PET/CT PSMA to determine patterns of disease for biochemically recurrent prostate cancer following primary radiotherapy.

Background: Ga-PET/CT PSMA scan is being increasingly used for the staging of biochemically recurrent disease. Early identification of recurrent disease after radiotherapy is important in considering suitability for early salvage therapy to improve prognosis. The aim is to identify patterns of suspected prostate cancer recurrence in relation to post-radiotherapy PSA levels, especially below the accepted Phoenix definition of PSA failure (PSA nadir + 2).

Methods: This was a retrospective single tertiary institution cohort study of consecutive men between July 2014 and June 2018 who received a Ga-PSMA PET/CT for elevated PSA levels following radiotherapy as primary treatment of prostate cancer. The primary outcome measure was to determine the relationship between pre-scan PSA and the probability of identifying PSMA-avid disease suggestive of recurrent prostate cancer.

Results: Two hundred and seventy-six patients met criteria for inclusion. The median PSA was 3.60 ng/mL. The overall detection rate for suspected recurrent prostate cancer was 86.3%. Local recurrence was the most common site, occurring in 56.9% (157/276) of men, with isolated local recurrence in 32.6% (90/276). A total of 75.3% (55/73) of men below Phoenix criteria had scans suggestive of recurrent disease, with 52.1% of men having salvageable disease. The regions surrounding the iliac arteries were the most common areas of nodal metastatic disease, with 55.6% of recurrence occurring in the iliac regions.

Conclusions: Ga-PSMA PET/CT frequently identifies suspected recurrent disease prior to the accepted Phoenix definition of PSA nadir +2. Prospective outcome studies are required to determine if early identification of local recurrence improves outcomes by increasing the use of salvage local treatments and whether earlier identification of metastatic disease may improve outcomes with prompt initiation of multimodality therapies.