S1199
Clinical - Urology
ESTRO 2026
7 Department of Radiotherapy and Brachytherapy Planning, Maria Sklodowska Curie National Research Institute of Oncology, Gliwice, Poland. 8 Department of Radiodiagnostics, Maria Sklodowska Curie National Research Institute of Oncology, Gliwice, Poland Purpose/Objective: The early safety analysis of a prospective phase 2 study evaluating the tolerance and efficacy of focal salvage stereotactic radiotherapy (s-SBRT) for local recurrences of prostate cancer following radiotherapy (NCT06201078). Material/Methods: The single-institution study with enrollment period ranging from 2023 and 2029 and a planned accrual of 55 patients was designed. Patients who experienced local recurrence following conventional/hypofractionated, ultrahypofractionated or post-prostatectomy radiotherapy are eligible. All patients undergo mpMRI and PSMA-PET; the biopsy is not required if the imaging results are unambiguous. Androgen deprivation therapy (ADT) and oligometastatic disease (if treated with MDT) are permitted. The s-SBRT is delivered focally to the tumor with 5 x 6.75 Gy. The rate of CTCAE ≥ G3 genitourinary (GU) or gastrointestinal (GI) treatment-related adverse events is the primary end-point. In this report the early (3- months post-SBRT) and late (beyond 3-months post- SBRT) adverse events are presented, as well as early biochemical response (BR) i.e., PSA decrease from baseline to 3 months after s-SBRT. Results: From 2023 to 2025, 30 patients were enrolled of whom 21 patients underwent at least one follow-up visit beyond 3 months after s-SBRT, making them evaluable for the early safety analysis. Local recurrence was detected in 20/21 patients based on mpMRI and in 19/21 patients based on PSMA-PET imaging. Eight patients underwent a biopsy, eight had oligometastases, and ten patients were on ADT. Median pre-s-SBRT PSA concentration was 1.32 ng/ml (IQR: 0.59-2.56) and median GTV was 1.8 cc (IQR: 1.1- 4.6). Early ≥ G3 adverse events were observed only in one patient (bladder bleeding). No early G2 GU side effects were reported, and there was one case of early G2 GI adverse events. With the median follow-up of 14 months (min-max: 4.5-25), late G2 GU and GI adverse events were reported in five and one patient, respectively. Late G3 GU persistent symptoms were recorded in one patient, the same who experienced early G ≥ 3 adverse events. All patients exhibited an early biochemical response following s-SBRT with a median decrease in PSA of 0.60 ng/ml (IQR: 0.20-1.53). Conclusion: The value of s-SBRT is implied by the early biochemical response in all patients and a satisfactory treatment
tolerance profile. The preliminary analysis indicates the safety of this ongoing study. Keywords: prostate cancer, SBRT, salvage
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Single-Center RetrOspective Study of 5F SBRT for localized PROstate Cancer (SiRO-SB-PRO) with C- based, Robotic-, and MR-Linac Naphat Komenake, Pittaya Dankulchai, Akrapol Suppasedtanon, Tissana Prasartseree, Ratchapas Romrattaphan, Wajana Thaweerat, Wiwatchai Sittiwong, Supamon Nitipitch, Natthanicha Sauenram, Naritsa Rotmuenwai Division of Radiation Oncology, Department of Radiology, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand Purpose/Objective: This retrospective study, SiRO-SB-PRO, aimed to evaluate the real-world effectiveness and safety of five-fraction SBRT in localized prostate cancer Material/Methods: SiRO-SB-PRO retrospectively analyzed localized prostate cancer patients treated with five-fraction SBRTbetween 2018-2023 at a single institute with MR- guided adaptive radiotherapy, Robotic-based, or C- based linac with gold fiducial marker implantation. The whole prostate gland and proximal seminal vesicles received 36.25 Gy, while the rest of the seminal vesicles received 30 Gy. Dominant intraprostatic lesion (DIL) boost was varied depending on the discretion of the individual clinician; if used, it was delivered 40 Gy simultaneous integrated boost (SIB). Patient and tumor characteristics were summarized descriptively. Biochemical recurrence-free survival (bRFS) was defined using the Phoenix criteria. Acute and late GU and GI adverse events were graded using CTCAE version 5.0. The oncologic outcomes and late adverse events were analyzed using Kaplan-Meier survival analysis. Results: 182 patients with localized prostate cancer were treated with five-fraction SBRT. Median follow-up was 24.5 months. Median age was 71.5 (IQR 67-77) years, and median baseline PSA was 8.555 (IQR 6.090-11.740) ng/mL. 75.3% was grade group 1-2, while 24.2% was grade group 3-5. Most patients were intermediate-risk (61.6%), followed by high-risk (23.1%) and low-risk (15.4%). MR-Linac was used in 80.2%, Robotic linac in 14.8%, and C-based linac in 4.9%. DIL boosts were delivered in 30.2%. The 2-year bRFS was 99.40% (95% CI, 95.80-99.91). Median nadir PSA was 0.042 ng/mL (IQR 0.006-0.426) with a median time to nadir of 14 months. Acute GU grade ≥ 2 adverse events occurred in 11 patients (6%), including one case (0.5%) of grade 3
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