3195 - Stereotactic Body Radiation Therapy to Only Prostate and Proximal Seminal Vesicles is a Viable Option for a Favorable Subset of High-Risk Prostate Cancer Patients

Purpose/Objective(s): High-risk prostate cancer represents a heterogeneous disease spectrum, with treatment paradigms increasingly incorporating pelvic nodal irradiation (PNI). However, within this high-risk category, a subset of patients may have more favorable prognoses, warranting consideration for treatment de-escalation. We aimed to identify and compare a favorable high-risk cohort, specifically patients with Gleason score 4+4 and PSA <20 ng/mL meeting AJCC IIC criteria, to those with Gleason score 4+3 and PSA <20 ng/mL in the same prognostic risk group. The current report seeks to determine whether a subset of high-risk patients can be safely treated with SBRT without PNI.

Materials/Methods: Patients from two large institutional databases were combined for this analysis. Biochemical Relapse Free Survival (BRFS) in patients with unfavorable intermediate and “favorable” high risk disease was analyzed using Kaplan Meier Estimate, and Log Rank test was utilized to determine differences in BRFS based on subgroups. PSA failure was defined based on Phoenix Criteria. All patients received 36.25 Gy to the PTV with Robotic SBRT with real time fiducial tracking. All patients in this study received androgen deprivation therapy (ADT).

Results: In this multi-institutional study, 196 patients, treated between March 2009 and October 2021, were evaluated and categorized into two risk groups: 81 as favorable high risk (FHR) and 115 as intermediate risk (UIR). Baseline characteristics were well balanced between the groups. Specifically, the median age was 74.6 years for the FHR cohort and 70.9 years for the UIR cohort, while the mean initial PSA values were 9.0 and 8.3, respectively. The median follow-up was 60.6 months for the FHR group and 52.9 months for the UIR group. The estimated 5-year biochemical recurrence-free survival (BRFS) rates were not statistically different at 89.2% for FHR patients and 90.4% for UIR patients (p-value 0.80).

Conclusion: Our findings indicate that PSA outcomes are comparable between patients with Gleason score 4+4 and PSA <20 (AJCC IIC) and those with Gleason score 4+3 and PSA <20, all of whom were treated with SBRT to the prostate and proximal seminal vesicles along with ADT. This suggests the existence of a favorable high-risk prostate cancer cohort that may not necessitate pelvic nodal irradiation and can be safely treated with SBRT. Furthermore, these patients may be candidates for treatment de-escalation strategies, such as reduced-intensity ADT, to minimize treatment-related morbidity without compromising disease control.