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The main goal of this study was to evaluate tolerance and effectiveness of Stereotactic Ablative Radiotherapy (SABR) for prostate cancer. The secondary aim was an attempt to define predictive factors based on the analysis of treatment failures. The three main aspects regarded in the study are curability (defined as biochemical control and local/distant control), adverse effects, and feasibility of this method of treatment (including patients' convenience and applicability). Materials/Methods: The study group consists of 500 (264 low risk e LR and 236 intermediate risk e IR) consecutive patients (median age-69) treated with robotic radiosurgery between 2011 and 2017 at a single institution. The treatment protocol included one fractionation schedule (5x7.25Gy to a Total Dose of 36.25Gy) and a precise definition of target delineation. The mean PSA concentration prior to radiotherapy was 7.59. Androgen Deprivation Therapy (ADT) was applied in 277 cases for a median time of 5 months. Control visits were scheduled at 1, 4, and 8 months after completion of SABR and every 6 months afterwards (14, 20, 26,.). Each consecutive visit included assessment of patient's adverse effects, ADT usage and PSA concentration. The adverse effects were assessed using RTOG/EORTC scale. Results: The median follow up was 31.3 and ranged up to a maximum of 76 months. In most the cases, PSA level was decreasing for about 20 months before reaching a plateau. PSA decline was slowest for IR patients without ADT and its concentration after 19th month remained higher compared to other groups. The percentage of patients without gastro-intestinal (GI) toxicity was above 90%. Three cases of G3 and one case of G4 GI toxicity were observed. As much as 75% of the patients reported no genito-urinary (GU) toxicity in the first month after treatment, and this index increased up to 90% in following months. There was one case of G3 GU toxicity. Adverse effects were more frequent and severe among patients with higher PSA levels. 15 failures (3%) including 8 local recurrences and 7 disseminations (8 in LR, 7 in IR patients) were observed after a median time of 19.9 months. Out of these patients, 6 were receiving ADT. PSA concentration levels correlated with occurrence of treatment failure, including both PSA concentration below Phoenix criteria threshold and early measurements, months before clinical manifestation of treatment failures. Conclusion: CK based SABR of LR and IR prostate cancer patients is an effective and well tolerated modality of treatment. PSA levels before SABR and during follow up are the most important predictive factor for treatment failure and occurrence of adverse effects. A thorough analysis allows forming a hypothesis that ADT usage in IR patients group could improve long term treatment results.

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