Patient selection for primary staging with [99mTc]Tc-PSMA SPECT/CT: influence of combined PSA and ISUP grade on detection of metastatic disease.

OBJECTIVES: The aim of this study was to examine the combined influence of initial PSA level (iPSA) and ISUP grade on the detection rates of [99mTc]Tc-PSMA SPECT/CT in patients undergoing primary staging of prostate cancer and to identify selection criteria. PATIENTS AND METHODS: In this retrospective study, 91 patients underwent SPECT/CT following administration of 670 MBq (range, 470-792 MBq) [99mTc]Tc-MIP-1404. Detection rates of the primary tumour, pelvic lymph node metastases and distant metastases were determined together with imaging biomarkers. Univariate and multivariate nominal logistic regression analyses identified predictors of metastatic disease. RESULTS: Both iPSA level and ISUP grade were independent predictors of metastatic disease (pPSA=0.0004, pISUP=0.0015). SUVmax correlated strongly with ISUP grade (r = 0.61) but was not a reliable surrogate marker. Detection rates ≥ 50% were observed at ISUP grades 1-3 with iPSA > 50 ng/ml and at ISUP grades 4-5 with iPSA > 20 ng/ml. Metastases were detected in all patients with iPSA > 50 ng/ml, regardless of ISUP grade. Pelvic lymph node metastases predominated at ISUP grades 1-3 across all iPSA and at ISUP grades 4-5 with iPSA ≤ 20ng/ml. Conversely, distant metastases were more frequent at ISUP grades 4-5 with initial PSA > 50ng/ml. CONCLUSION: Patient selection for [99mTc]Tc-PSMA SPECT/CT may be improved by incorporating ISUP grade and iPSA, particularly ISUP grades 4-5 with iPSA > 20 ng/ml and ISUP grades 1-3 with iPSA > 50 ng/ml. Distinct patterns of metastatic spread were observed according to both ISUP grade and iPSA. Where PSMA PET/CT is unavailable, [99mTc]Tc-PSMA SPECT/CT may represent a valuable alternative for primary staging of selected patients with PCa.