1050 - Five-Year Outcomes from a Prospective Trial of MR-Guided SBRT with Focal DIL Boost for Intermediate-Risk Prostate Cancer
Presenter(s)
A. G. Goglia1, N. Toumbacaris2, M. A. Kollmeier1, S. M. McBride1, B. R. Mychalczak1, R. M. Gewanter1, B. A. Mueller1, M. B. Bernstein1, H. Nagar1, Z. Zhang2, M. J. Zelefsky1,3, and D. J. Gorovets1; 1Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY, 2Department of Epidemiology and Biostatistics, Memorial Sloan Kettering Cancer Center, New York, NY, 3Department of Radiation Oncology, NYU Langone Health, New York, NY
Purpose/Objective(s): To report 5-year toxicity and oncological outcomes from a prospective phase I study evaluating stereotactic body radiation therapy (SBRT) with simultaneous integrated boost (SIB) to MRI-defined dominant intraprostatic lesions (DILs) in intermediate-risk prostate cancer.
Materials/Methods: This single-institution prospective trial enrolled 27 men with NCCN intermediate-risk prostate cancer and =1 MRI-visible PIRADS 4-5 DIL =0.5 cm (=2 additional foci permitted). Patients underwent MR-only simulation with synthetic CT generation and received SBRT to 40 Gy in 5 fractions with a 45 Gy SIB to DILs, delivered on alternate days over ~2 weeks. PTV margins were 5 mm (3 mm posteriorly) and =2 mm for DILs. No patients received androgen deprivation therapy. All patients underwent rectal spacer placement. The primary endpoints were technical feasibility and dose-limiting toxicity (DLT; CTCAE grade =3 GU/GI within 3 months). Secondary endpoints included late toxicity, IPSS/IIEF changes, PSA kinetics, and Phoenix-defined biochemical progression-free survival (bPFS). Survival outcomes were estimated using Kaplan–Meier methods.
Results: Median age was 68 years, median prostate volume 39 mL, and median baseline PSA 7.36 ng/mL. Most men had unfavorable intermediate-risk disease (78%, 21/27). Median follow-up was 60.3 months (range, 60-60.8). Planning feasibility was 100% with no acute DLTs observed. Late grade =3 toxicity was rare (1/27 grade 3 rectal pain; no late grade =3 GU events). Median IPSS was 7 at baseline, 8 at 3-month follow-up (p=0.22), and 8 at last follow-up (p=0.28). Median IIEF declined from 17 at baseline to 12 at last follow-up (p=0.045). Median PSA nadir was 0.23 ng/mL with median time to nadir of 48 months. Two patients experienced biochemical failure. Estimated 5-year bPFS was 93% (95% CI, 77-98%). No prostate cancer–specific deaths were observed. Protocol-recommended 2-year surveillance biopsy was performed in 11 patients and demonstrated residual GS 3+4 disease in 2 cases (both within the boosted DIL). Both patients had continued PSA decline without biochemical recurrence.
Conclusion: At 5 years, MR-guided SBRT with focal DIL boost demonstrates durable biochemical control and low rates of high-grade toxicity in predominantly unfavorable intermediate-risk patients treated without ADT. These prospective data support continued evaluation of MR-based focal dose escalation strategies in larger multi-institutional trials.