The Impact of Baseline PSMA PET/CT Versus CT on Outcomes of223Ra Therapy in Metastatic Castration-Resistant Prostate Cancer Patients

Imaging before 223 Ra-dichloride ( 223 Ra) therapy is crucial for selecting metastatic castration -resistant prostate cancer (mCRPC) patients with bone -only disease. The purpose of this study was to evaluate if baseline prostate -specific membrane antigen (PSMA) PET/CT (bPSMA) versus CT is associated with outcomes of 223 Ra therapy. Methods: A secondary analysis of the data of a prospective observational study (NCT04995614) was performed. Patients received a maximum of 6 223 Ra cycles and were retrospectively divided into the bPSMA or baseline CT (bCT) groups. All patients received baseline bone scintigraphy. Primary endpoints were alkaline phosphatase and prostate -specific antigen response. Secondary endpoints were overall survival (OS) and radiologic response. Results: Between 2017 and 2020, 122 mCRPC patients were included: 18 (14.8%) in the bPSMA group and 104 (85.2%) in the bCT group. All baseline characteristics were comparable. No significant differences in alkaline phosphatase or prostate -specific antigen response were found. The bCT group showed an OS significantly shorter than that of the bPSMA group (12.4 vs. 19.9 mo, P 5 0.038). In 31 of 76 patients (40.1%) in the bCT group who also received posttherapy CT, lymph node or visceral metastases (soft -tissue involvement [STI]) were detected after 223 Ra therapy, compared with 0 of 15 patients in the bPSMA group who received posttherapy PSMA PET/CT or CT. No significant difference in OS was found between patients in the bCT or posttherapy CT subgroup without STI (46/76) and the bPSMA group. Conclusion: bPSMA versus CT does not seem to impact biochemical response during 223 Ra therapy in mCRPC patients. Nevertheless, patients in the bCT group had a significantly shorter OS, most likely due to underdetection of STI in this group. Therefore, replacing bCT with PSMA PET/CT appears to be a valuable screening method for identifying patients who will benefit most from 223 Ra therapy.