3370 - Urethra Sparing in Robotic SBRT for Localized Prostate Cancer with Simultaneous Focal Boost to MR-Defined High PI-RADS Score Intra-Prostatic Lesions: A Dosimetric Analysis
Presenter(s)
H. F. Xiang1,2, M. A. Frick1, X. Liang1, K. Berkenstock1, A. Ali1, T. P. Samson1, P. Boimel1,2, and S. Venigalla1,2; 1Department of Radiation Oncology, Penn Medicine Lancaster General Hospital, Lancaster, PA, 2Department of Radiation Oncology, University of Pennsylvania, Philadelphia, PA
Purpose/Objective(s): MR-defined high PI-RADS score dominant intra-prostatic lesions (DILs) are associated with residual cancer following prostate SBRT and where most recurrences occur. Novel prostate SBRT with simultaneous focal boost to DILs is a promising approach for improving oncologic outcomes, but can also increase dose to the prostatic urethra and lead to higher rates of acute and late grade 2+ GU toxicities. This study investigated the trade-off between dose coverage to the target volumes and steering higher dose off the urethra under robotic SBRT delivery.
Materials/Methods: Forty-six patients (N=46) with low or intermediate risk localized prostate cancer treated with SBRT on a frameless robotic radiosurgery system M6 between January 2021 and January 2025 were included in a retrospective dosimetry analysis. Fusions of planning MRI and diagnostic multi-parameter MRI to planning CT are performed sequentially by first aligning fiducial markers followed by refined alignment of prostate ROI soft-tissue profiles and SpacerOAR. Prostatic urethras are contoured on T2 MRI of 1mm resolution. For a group A of 31 patients (67.4%), each case was planned for a 5-fraction SBRT with 36.25 Gy to PTV, 40 Gy to CTV (prostate plus 0-2 cm proximal seminal vesicles) and simultaneous focal boost of 45-47.5 Gy to up-to-two MR-defined DILs of PI-RADS score 3 to 5 with biopsy-confirmation. MLC-based plans were generated using VOLO optimization with priority to meet the OAR constraints per PACE-B protocol. For a group B of 15 patients (32.6%), stricter dose constraints were met for urethra-sparing with D0.03cc < 40.5 Gy (EQD2 90 Gy) to the urethra and D0.03cc < 42 Gy to a 3 mm urethra PRV. Dose metrics for urethras are compared between the two groups using the Wilcoxon rank sum test, including the max-dose D0.03cc, mean dose and V40.5Gy.
Results: With prescribed dose to target volumes maintained, plans in Group B had significantly lower median urethra max-dose 39.76 Gy (IQR 39.27-40.34 Gy), median mean dose 39.23 Gy (IQR 38.59-39.50 Gy) and median V40.5Gy 0.0% (IQR 0.0%-0.6%) in comparison to plans in Group A at 41.63 Gy (41.29-41.87 Gy, A vs. B: p < .0001), 40.39 Gy (39.61-40.70 Gy, A vs. B: p<.0003) and 59.3% (38.5%-76.2%, A vs. B: p<.0001), respectively. For both groups, the median urethra max-dose is significantly lower than the reported median max-dose to the surrogate urethra for both a frameless robotic radiosurgery system (45.9 Gy, N=169) and conventional-LINAC (42.8 Gy, N=245) SBRT from a 2024 retrospective PACE-B planning analysis. The urethra median V42Gy and V45.6Gy in both group A (0.0% and 0.0%, N=31) and group B (0.0% and 0.0%, N=15) are significantly lower than those reported (30.5% and 0.35%, N=20) from SPARC in 2024.
Conclusion: With stricter urethra-sparing constraints and robotic SBRT delivery using MLC under intra-fraction prostate-motion-tracking, it is feasible to escalate dose to 45-47.5 Gy to the MR-defined DILs while keeping the maximum dose to urethra below EQD2 90 Gy. Further improvements in long-term oncologic outcomes and lower GU toxicities may be anticipated.