Early identification of recurrent local recurrence after radiotherapy or brachytherapy is vitally important in considering suitability for early salvage therapy to improve prognosis or long term local control. 68Ga-PET/CT PSMA scan is increasingly replacing standard radiology techniques for the staging of biochemical failure after primary prostate radiotherapy. The aim is to evaluate the ability of 68Ga-PET/CT PSMA to identify patterns of suspected prostate cancer recurrence in relation to post radiotherapy PSA levels, especially below the accepted Phoenix definition of PSA failure of PSA nadir +2. To our knowledge this is the largest dataset in the literature on 68Ga-PET/CT PSMA for post radiotherapy biochemical failure.
This was a retrospective single tertiary referral institution cohort study of consecutive men between July 2014 and July 2017 who received a 68Ga-PSMA PET/CT scan for elevated PSA levels following radiotherapy (either external beam radiotherapy or brachytherapy) for primary treatment of prostate cancer. The primary outcome measure was to determine the relationship between pre-scan PSA and Gleason score / ISUP grade and the probability of identifying recurrent cancer. For diagnosis of a metastasis, a concordant moderate or intense PSMA uptake in a CT anatomical lesion was required. Local recurrence was defined as a moderate or intense PSMA avid lesion within the prostate with an SUV ≥3.
236 patients received a 68Ga-PSMA PET/CT scan for investigation of suspected recurrent prostate cancer due to an elevated PSA following primary radiotherapy in the study period. The median post radiotherapy PSA was 3.7ng/mL. The detection rate of 68Ga-PSMA PET/CT for suspected recurrent prostate cancer was 40.0% for PSA 0 – <0.5ng/mL, 63.6% for PSA 0.5 - <1.0ng/mL, 78.1% for PSA 1.0 - <2.0 ng/mL and 89.2% for PSA >2.0ng/mL. Local recurrence was the most common site identified, occurring in 53.4% of men. Lymph node metastasis was identified in 42.8% of men with suspected disease recurrence. The most likely sites for nodal metastases was in the retroperitoneum (22%), common iliac (20%) and external iliac (20%) regions.
68Ga-PSMA PET/CT frequently identifies suspected recurrent disease prior to the accepted Phoenix definition of PSA nadir +2. Local recurrence is the most common site for recurrent disease in PSA levels below the Phoenix definition. Early identification of local recurrence allows the option of salvage local treatments, while timely identification of metastatic disease may improve outcomes via prompt initiation of multimodality therapies.