Validation of 18F-rhPSMA-7 and 18F-rhPSMA-7.3 PET Imaging Results with Histopathology from Salvage Surgery in Patients with Biochemical Recurrence of Prostate Cancer

¹⁸F-rhPSMA-7, and its single diastereoisomer form, ¹⁸F-rhPSMA-7.3, are prostate-specific membrane antigen (PSMA)-targeting radiopharmaceuticals. Here, we investigated their accuracy for the assessment of lymph node (LN) metastases validated by histopathology. Methods: Data from 58 patients with biochemical recurrence of prostate cancer after radical prostatectomy receiving salvage surgery after PET imaging with ¹⁸F-rhPSMA-7 or ¹⁸F-rhPSMA-7.3 were retrospectively reviewed. Two nuclear medicine physicians reviewed all PET scans and morphologic imaging in consensus. Readers were masked from the results of histopathology. PET and morphologic imaging were correlated with histopathology from resected LNs. Results: In 75 of 150 resected regions in 54 of 58 patients, tumor lesions were present in histopathology. The template-based specificity of PET (¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 combined) and morphologic imaging was 93.3% and 100%, respectively. However, ¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 PET detected metastases in 61 of 75 histopathologically proven metastatic LN fields (81.3%) whereas morphologic imaging was positive in only 9 of 75 (12.0%). The positive predictive value was 92.4% for ¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 PET and 100% for morphologic imaging. ¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 PET performance was significantly superior to morphologic imaging (difference in the areas under the receiver-operating-characteristic curves, 0.222; 95% CI, 0.147-0.298; P < 0.001). The mean size of PET-positive and histologically confirmed LN metastases was 6.3 ± 3.1 mm (range, 2-15 mm) compared with a mean size of 9.8 ± 2.5 mm (range, 7-15 mm) on morphologic imaging. Conclusion: ¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 PET offer a high positive predictive value comparable to that reported for ⁶⁸Ga-PSMA-11 and represent a valuable tool for guiding salvage lymphadenectomy.

Utility of 18F-rhPSMA-7.3 positron emission tomography for imaging of primary prostate cancer and pre-operative efficacy in N-staging of unfavorable intermediate to very high-risk patients validated by histopathology

¹⁸F-rhPSMA-7.3, the lead compound of a new class of radiohybrid prostate-specific membrane antigen (rhPSMA) ligands, is currently in phase III trials for prostate cancer (PCa) imaging. Here, we describe our experience in primary PCa staging. Methods: We retrospectively identified 279 patients with primary PCa who underwent ¹⁸F-rhPSMA-7.3 PET/CT (staging cohort). A subset of patients (83/279) subsequently underwent prostatectomy with lymph node (LN) dissection without prior treatment (efficacy cohort). Distribution of tumor lesions was determined for the staging cohort and stratified by National Comprehensive Cancer Network (NCCN) risk score. Involvement of pelvic LN was assessed retrospectively by 3 blinded independent central readers, and a majority rule was used for analysis. Standard surgical fields were rated on a five-point scale independently for PET and for morphological imaging. Results were compared to histopathological findings on a patient-, right vs. -left, and template-basis. Results: For the staging cohort ¹⁸F-rhPSMA-7.3 PET was positive in 275/279 (98.6%), 106/279 (38.0%), 46/279 (16.5%), 65/279 (23.3%) and 5/279 (1.8%) patients for local, pelvic nodal, extrapelvic nodal, metastatic bone, and visceral metastatic disease. In the efficacy cohort, LN metastases were present in 24/83 patients (29%), located in 48/420 (11%) resected templates and in 33/166 (19.9%) hemi-pelvic templates in histopathology. Based on majority vote results, the patient-level sensitivity, specificity and accuracy for pelvic nodal metastases were 66.7% (95%CI, 44.7-83.6%), 96.6% (95%CI, 87.3-99.4%) and 88.0% (95%CI, 78.5-93.8%) for ¹⁸F-rhPSMA-7.3 PET and 37.5% (95%CI, 19.6-59.2%), 91.5% (95%CI, 80.6-96.8%) and 75.9% (95%CI, 65.0-84.3%) for morphological imaging, respectively. ¹⁸F-rhPSMA-7.3 showed higher interobserver agreement than morphological imaging (patient-level Fleiss' κ=0.54; 95%CI, 0.47-0.62 vs. 0.24; 95%CI, 0.17-0.31). A mean standardized uptake value ratio of 6.6 (95%CI, 5.2-8.1) documented a high image contrast between local tumors and adjacent low urinary tracer retention. Conclusion: ¹⁸F-rhPSMA-7.3 PET offers superior diagnostic performance to morphological imaging for primary N-staging of newly diagnosed PCa, shows lower inter-reader variation, and offers good distinction between primary tumor and bladder background activity. With increasing NCCN risk group an increasing frequency of extra-prostatic tumor lesions was observed.