The aim of this retrospective study was to assess the adverse effects and outcomes of salvage re-irradiation with stereotactic body radiotherapy (sSBRT) for local recurrence of prostate cancer (PCa) after definitive radiotherapy (RT). The study was focused on the adverse effects and prognostic factors for treatment toxicity, followed by an analysis of patterns of failure and survival. Patients treated with sSBRT between 2012 and 2020 at a tertiary institution were included. The exclusion criteria were a primary or salvage radical prostatectomy or a palliative sSBRT dose. Patients with oligorecurrence were eligible if all metastatic lesions were treated locally with curative intent. The Kaplan-Meier method was used to estimate time to grade ≥ 3 toxicity, local control (LC), freedom from distant metastases (FFDM), progression-free survival (PFS), biochemical control (BC) and overall survival (OS). The differences between groups (focal vs. whole-gland sSBRT) were compared using the log-rank test. The Cox proportional hazards model was used to assess prognostic factors for the listed endpoints. A total of 56 patients with a median age of 70.9 years and a median follow-up of 38.6 months were included in the analysis. The majority of them received local sSBRT only (45; 80.4%), while the rest were simultaneously treated for oligometastases (11; 19.6%). Overall, 18 (32.1%) patients experienced any grade ≥ 3 toxicity, including 1 (6.7%) patient who received focal sSBRT, and 17 (41.5%) patients treated with whole-gland sSBRT. The Planning Target Volume (per cc; HR 1.01; 95% CI 1-1.02; p = 0.025) and use of ADT (yes vs. no; HR 0.35; 95%CI 0.13-0.93; p = 0.035) were independent prognostic factors for the risk of grade ≥ 3 toxicity. The estimated rate of grade ≥ 3 adverse events was significantly higher (43.8% vs. 7.1% at 2 years; p = 0.006), and there was no improvement in the LC (92.9% vs. 85.3% at 2 years; p = 0.759) in patients treated with whole-gland sSBRT compared to focal sSBRT. The 2- and 5-year LC were 87.6% and 47.9%, respectively; the 2- and 5-year FFDM were 72.7% and 42.8%, respectively; and the 2- and 5-year PFS were 67.9% and 28.7%, respectively. The primary pattern of failure was distant metastasis. The sSBRT for local recurrence of PCa after definitive RT was associated with a high risk of severe grade ≥ 3 toxicity, which significantly increased with the volume and extent of re-irradiation.

3rd Radiotherapy and Chemotherapy Department Maria Skłodowska Curie National Research Institute of Oncology Wybrzeże Armii Krajowej 15 44 102 Gliwice Poland

Brachytherapy Department Maria Skłodowska Curie National Research Institute of Oncology Wybrzeże Armii Krajowej 15 44 102 Gliwice Poland

Department of Biostatistics and Translational Medicine Medical University of Łódź 90 419 Łódź Poland

Department of Radiation Oncology Comprehensive Cancer Center Medical University of Vienna Währinger Gürtel 18 20 1090 Vienna Austria

Department of Urology 2nd Faculty of Medicine Charles University 12108 Prague Czech Republic

Department of Urology Comprehensive Cancer Center Medical University of Vienna 1090 Vienna Austria

Department of Urology Medical University of Silesia 40 752 Zabrze Poland

Department of Urology The Jikei University School of Medicine Tokyo 105 8461 Japan

Department of Urology University of Texas Southwestern Medical Center Dallas TX 75230 USA

Department of Urology Weill Cornell Medical College New York NY 10065 USA

Hourani Center for Applied Scientific Research Al Ahliyya Amman University Amman 19328 Jordan

Institute for Urology and Reproductive Health Sechenov University 119435 Moscow Russia

Karl Landsteiner Institute of Urology and Andrology 1090 Vienna Austria

Radiotherapy Department Centre Antoine Lacassagne 33 Avenue Valombrose CEDEX2 06189 Nice France

Radiotherapy Department Maria Skłodowska Curie National Research Institute of Oncology Wybrzeże Armii Krajowej 15 44 102 Gliwice Poland

Radiotherapy Planning Department Maria Skłodowska Curie National Research Institute of Oncology Wybrzeże Armii Krajowej 15 44 102 Gliwice Poland

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