Risk of metastatic disease on 68gallium-prostate-specific membrane antigen positron emission tomography/computed tomography scan for primary staging of 1253 men at the diagnosis of prostate cancer

Objective To determine the number of men with (68)gallium-prostate-specific membrane antigen positron emission tomography/computed tomography (Ga-68-PSMA PET/CT) avid metastasis at diagnosis, as most data on Ga-68-PSMA PET/CT are for the evaluation of recurrent disease after primary treatment and to our knowledge this study is the largest series of primary prostate cancer staging with Ga-68-PSMA PET/CT. Patients and Methods A retrospective review conducted on 1253 consecutive men referred by urologists or radiation oncologists to our tertiary referral centre for Ga-68-PSMA PET/CT scan for staging at the initial diagnosis of prostate cancer between July 2014 and June 2018. The primary outcome measure was to determine the risk of metastasis based on Ga-68-PSMA PET/CT. Patients were risk stratified based on histological biopsy International Society of Urological Pathology (ISUP) grade, prostate-specific antigen (PSA) level, and staging with pre-biopsy multiparametric magnetic resonance imaging (mpMRI). Univariate and multivariate logistic regression were used to analyse results. Results The median PSA level was 6.5 ng/mL and median ISUP grade was 3, with high-risk disease in 49.7%. The prostate primary was PSMA avid in 91.7% of men. Metastatic disease was identified in 12.1% of men, including 8.2% with a PSA level of mL and 43% with a PSA level of >20 ng/mL. Metastases were identified in 6.4% with ISUP grade 2-3 and 21% with ISUP grade 4-5. Pre-biopsy mpMRI identified metastasis in 8.1% of T2 disease, increasing to 42.4% of T3b. Lymph node metastases were suspected in 107 men, with 47.7% outside the boundaries of an extended pelvic lymph node dissection. Skeletal metastases were identified in 4.7%. In men with intermediate-risk prostate cancer, metastases were identified in 5.2%, compared to 19.9% with high-risk disease. Conclusions These results support the use of Ga-68-PSMA PET/CT for primary staging of prostate cancer. Increasing PSA level, ISUP grade and radiological staging with mpMRI were all statistically significant prognostic factors for metastasis on both univariate and multivariate analysis.