1005 - A Prospective Pilot Study Investigating ¹⁸F rhPSMA-7.3 PET/MRI to Detect Recurrent Disease and Guide Radiotherapy Planning in Patients with Biochemically Recurrent Prostate Cancer Post-Pros

D. S. Surasi¹, T. Bathala¹, S. Choi², S. J. Shah³, Q. N. Nguyen³, K. E. Hoffman³, L. L. Mayo⁴, H. Mok³, S. J. Frank¹, A. Fang¹, R. Sheth¹, K. P. Hwang⁵, O. R. Mawlawi⁶, H. Macapinlac¹, P. Troncoso⁷, M. Zhang¹, S. Pasyar¹, R. Bassett Jr.¹, B. Chapin⁸, and C. Tang²; ¹The University of Texas MD Anderson Cancer Center, Houston, TX, ²Department of Genitourinary Radiation Oncology, Division of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, ³Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, ⁴Department of Radiation Oncology, Division of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, ⁵Department of Imaging Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, ⁶University of Texas MD Anderson Cancer Center, Houston, TX, ⁷MD Anderson Cancer Center, Houston, TX, ⁸Department of Urology, The University of Texas MD Anderson Cancer Center, Houston, TX

Purpose/Objective(s): Simultaneous PET/MRI imaging has the advantage of combining metabolic information from PET with the high-spatial resolution of MRI to identify disease with greater accuracy than conventional imaging. F-18 rhPSMA 7.3 (rhPSMA) ligands are a new class of diagnostic/therapeutic PSMA-targeting agents in prostate cancer (PCa). We hypothesized that F-18 rhPSMA 7.3 PET/MRI accurately detects recurrent PCa to direct field design in salvage radiation therapy (RT) planning even at low PSA levels. Materials/

Methods: This is a prospective phase II pilot study enrolling men with biochemical recurrence (BCR) after prostatectomy for PCa (NCT04978675) who underwent rhPSMA 7.3 PET on a simultaneous 3T PET/MRI scanner. The radiation oncologists answered surveys to document changes to RT plan based on the PET/MRI results. All patients underwent standard fractionated RT with at least 6 months of hormonal therapy (HR). Patients with positive scan returned 6-18 months after the first scan for a second timepoint PET/MRI scan after treatment. Standard of truth was established by pathology when feasible or a combination of confirmatory imaging showing radiographic and PSA response after treatment. The primary aim is to evaluate the positive predictive value (PPV) of rhPSMA PET/MRI in detecting disease. The secondary aims included change in RT plan after rhPSMA PET/MRI and treatment response.
Results: 29 patients with a median age of 66 years (IQR: 47-76) at the time of imaging were enrolled between Aug 2021 to Jan 2023, of which 28 patients underwent rhPSMA PET/MRI scan. The Gleason score at diagnosis was >7 with a median PSA of 7.0 ng/mL (IQR: 0.9-29.5) before surgery. Median PSA was 0.3 ng/mL (IQR: 0.2-1.5) at BCR presentation with 24 (86%) patients having PSA <0.5. Twenty patients (71%) had rhPSMA positive findings. 4/20 patients with rhPSMA positive lesions did not receive follow up imaging as they chose to undergo treatment elsewhere. Of the 16 patients who underwent a confirmatory scan and/or biopsy, 15/16 (94%) patients were found to be true positive while 1/16 (6%) was a false positive. RT plan was changed in 22/28 (79%) with major changes including extension of clinical target volumes to cover PSMA positive pelvic lesions or cancellation of RT due to polymetastatic disease in 8/22 (36%), minor changes including dose escalation to gross disease or dose de-escalation to the rest of prostate fossa in 9/22 (41%) and both major and minor changes in 5/22 (23%) patients. All 14 patients who underwent a combination of RT and HT had complete response on the second timepoint rhPSMA PET/MRI. Median PSA was <0.1 ng/mL (IQR: <0.1-0.1) ng/mL after treatment before the second scan.
Conclusion: Even at low PSA levels, F-18 rhPSMA 7.3 PET/MRI resulted in a high detection rate of true positive lesions. Furthermore, incorporation of this technology led to changes in 79% of RT plans. Simultaneous F-18 rhPSMA 7.3 PET/MRI imaging can potentially serve as a “one stop shop” to stratify patient treatment and tailor salvage radiation fields.