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

ABSTRACT

OBJECTIVE: To determine the number of men with 68 gallium-prostate-specific membrane antigen positron emission tomography/computed tomography (68 Ga-PSMA PET/CT) avid metastasis at diagnosis, as most data on 68 Ga-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 68 Ga-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 68 Ga-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 68 Ga-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 <10 ng/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 68 Ga-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.