Reirradiation of Local Recurrences of Prostate Cancer: PROSTARE (PROstate Cancer STereotActic REirradiation) Early Safety Analysis of a Phase 2 Study with a Limited Cohort.

Objective: This study aims to report the early safety outcomes from an ongoing single-center, non-randomized phase 2 trial on focal salvage stereotactic radiotherapy (s-SBRT) for local prostate cancer recurrence. Materials and methods: This prospective phase 2 study includes patients with local recurrence after conventional or hypofractionated radiotherapy, ultrahypofractionated radiotherapy, or post-prostatectomy radiotherapy. The present analysis includes an initial subset of 21 out of 55 planned patients. All patients undergo mpMRI and PSMA-PET; biopsy is not required if imaging results are unambiguous. Focal s-SBRT is delivered to the recurrent lesion with a dose of 5 × 6.75 Gy. The primary endpoint is the rate of treatment-related CTCAE v5.0 grade ≥ 3 genitourinary (GU) or gastrointestinal (GI) toxicity. Secondary endpoints include early biochemical response (BR), defined as any PSA decline at 3 months. Results: With a median follow-up of 14 months (range: 4.5-25), one patient (4.8%) experienced both early and persistent late Grade 3 GU toxicity (bladder bleeding). Late Grade 2 GU and GI toxicities occurred in five (23.8%) and one (4.8%) patients, respectively. In exploratory univariable analysis, PTV volume 13 cc was identified as a marginal predictor for increased GU/GI radiation reactions (p < 0.1). Regarding efficacy, all 21 patients (100%) demonstrated an early biochemical response, with 15 patients (71.4%) achieving a PSA reduction of 50%. Conclusions: Focal s-SBRT demonstrates a favorable early safety profile and consistent biochemical response, supporting the preliminary safety of this ongoing study.