3238 - Early Outcomes from a Prospective Phase II Trial of Pencil Beam Scanning Proton SBRT for Prostate Cancer
Presenter(s)
D. J. Gorovets1, S. Hasan2, K. J. Mehta3, I. Yacoub2, A. M. Chhabra2, M. K. Garg4, E. Taborda2, T. Mistretta2, R. Holder2, and C. B. Simone II2; 1Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY, 2New York Proton Center, New York, NY, 3Department of Radiation Oncology, Albert Einstein College of Medicine and Montefiore Medical Center, Bronx, NY, 4Montefiore Einstein Comprehensive Cancer Center, Bronx, NY
Purpose/Objective(s): Patients with localized prostate cancer generally have long life expectancies, making it essential to minimize treatment-related toxicity and preserve quality of life. Photon-based stereotactic body radiation therapy (SBRT) is quicker and more cost-effective, but otherwise equivalent to longer courses of prostate radiation. Proton-based SBRT should also reduce costs and enhance access to proton therapy, although published data on its efficacy and toxicity are limited.
Materials/Methods: This prospective, single-arm, multicenter, Phase II non-inferiority trial enrolled 61 patients with prostate cancer (Grade group 1-3, PSA =20 ng/mL, cT1-2N0M0). Patients received 40Gy in 5 fractions every other day. Androgen deprivation therapy (ADT) was given at the treating physician's discretion. All patients underwent pre-treatment placement of three fiducial markers and a rectal spacer. Proton plans utilized two opposed lateral beams, PBS with single-field optimization, and robust optimization and evaluation. Image guidance included pre-treatment and intra-fraction kVs matched to fiducials, pre-treatment bladder scans, and either cone beam CTs (CBCT) or verification CTs. Patients were evaluated during SBRT and at 1, 3, 6, 12, 18, 24, and 36 months post-SBRT. The primary endpoint was the 24-month grade = 3 Genitourinary (GU) and Gastrointestinal (GI) toxicity-free rate, measured according to CTCAE v5.0.
Results: Enrollment was completed in June 2024. Data was locked in January 2025 for planned interim analysis. Median follow-up was 14 months (range: 6-38 months). The median age was 65, with 89% classified as cT1c and the majority favorable intermediate (59%) or unfavorable intermediate risk (39%). ADT was administered to 28% for a median of 6 months. Median baseline International Prostate Symptom Score (IPSS) was 6, peaking at 1 month (9.5, p<0.01) and returning to baseline levels beyond 3 months post-SBRT. Median Expanded Prostate Cancer Index Composite (EPIC)-26 scores for the GU and sexual/hormonal domains also decreased initially but returned to baseline after 3 months. There were no significant changes in median EPIC bowel domain scores or COST-FACIT scores (a measure of patient-reported financial toxicity). No grade 3 or worse GU/GI AEs were attributed to proton SBRT. Cumulative incidences of acute/late grade 2 GU and GI AEs were 15%/5% and 3%/2%, respectively. Biochemical recurrence (PSA nadir+2) rate was 1.6%.
Conclusion: This interim analysis suggests that image-guided PBS proton SBRT (40Gy in 5 fractions) with or without ADT results in transient changes in patient-reported GU and sexual/hormonal quality of life, low rates of financial toxicity, and low rates of physician-reported GU/GI AEs. Although longer follow-up is needed to evaluate the primary endpoint and other key secondary outcomes, these results suggest that proton SBRT is likely a safe and effective option that can expand access to proton therapy for prostate cancer.