Outcomes of a Phase II Interventional Clinical Trial of Prostate Bed Stereotactic Body Radiation Therapy for Prostate Cancer With High-Risk Features Following Radical Prostatectomy.

[PURPOSE] This study reports the safety and efficacy outcomes of a phase II prospective trial evaluating stereotactic body radiation therapy (SBRT) to the prostate bed in patients with high-risk features following radical prostatectomy.

[METHODS AND MATERIALS] Between May 2018 and May 2020, 49 patients were enrolled and stratified into 3 groups based on clinical context: group I (adjuvant SBRT, PSA < 0.2 ng/mL with adverse features such as positive margins, seminal vesicle invasion (SVI), or extraprostatic extension), group II (salvage SBRT, PSA ≥ 0.2 and <2.0 ng/mL), and group III (oligometastatic disease, ≤5 sites). Patients with PSA ≥ 0.2 ng/mL underwent staging with C11 positron emission tomography (PET); those with biochemical failure following treatment were restaged with PSMA PET. All patients received SBRT to the prostate bed to a total dose of 30 to 32 Gy in 5 fractions administered every other day. Patients in the salvage and oligometastatic groups received 6 and 18 months of androgen deprivation therapy (ADT), respectively. The primary endpoint was freedom from failure (FFF), defined as a PSA < 0.5 ng/mL without clinical progression or reinitiation of ADT. Secondary endpoints included FFF with a PSA < 0.2 ng/mL, toxicity rates (graded per CTCAE v5.0), clinical failure, and FFF in early salvage patients (PSA 0.2 to <0.5 ng/mL).

[RESULTS] The median follow-up was 60 months. High-risk features included extraprostatic extension in 49%, SVI in 10%, positive margins in 55%, and Gleason score 8 to 10 in 22%; no patients had nodal involvement at surgery. The median PSA at enrollment was 0.35 ng/mL (0.11 for group I, 0.42 for group II, and 0.39 for group III). SBRT was well tolerated, with only 2 (4.1%) cases of grade ≥2 toxicity (one case of hematuria in group I and one of rectal bleeding in group II); no significant adverse events were observed in group III. Five-year FFF rate was 60.9% (95% confidence interval [CI], 48.3%-76.9%). Improved FFF was observed in early salvage patients, at 5 years it was 74.7% (95% CI, 62.7%-89.0%); hazard ratio [HR], 0.33; 95% CI, 0.04-3.17). Overall survival was 95.5% (95% CI, 89.7%-100%) for all cases.

[CONCLUSIONS] SBRT to the prostate bed was feasible and associated with a low incidence of treatment-related toxicity over long-term follow-up. The observed FFF outcomes-particularly among patients treated in the early-salvage setting-were promising and appear comparable with historical results from conventionally fractionated postprostatectomy radiation therapy.