Significance: Prostate cancer (PCa) is one of the most common types of malignant neoplasms in men and ranks second in morbidity and fifth in mortality around the world. In the USA and some European countries PC rates first in the structure of oncological diseases in males. In 2020, 248 530 cases were registered in the USA [Siegel et al., 2021]. Different radiotherapy techniques are currently used for localized prostate cancer. Hypofractionated radiotherapy may be radiobiologically favorable in the treatment of prostate cancer due to potentially greater sensitivity of high fraction dose. RT modality as provided by the robot-assisted technique Cyber Knife® (CK), which can deliver such radiation within a high fraction dose, is currently witnessing increased usage in the treatment of prostate cancer with low-to-intermediate risk.
Objectives: 
1.	To assess efficacy of the stereotactic body radiation therapy using the Cyber-Knife system in the treatment of patients with localized prostate cancer. 
2.	To detect early and delayed complications of the treatment.

Materials and methods.  In the period from August 2015 to December 2024, 497 patients with localized prostate cancer were treated at the Cyber-Knife Department of the Nuclear Medicine Center "PET-Technology" Ltd. in Ufa. 322 patients were low-risk, 175 patients were intermediate risk. Mean age was 65 (from 49 to 75 years). The initial PSA level amounted to 9,34 ng/ml (from 1,34 to 20,0 ng/ml). All patients received stereotactic body radiation treatment course based on Cyber-Knife system according to the Stanford University Protocol, total boost dose - 36,25 Gy for 5 fractions, single boost dose – 7,25 Gy. The mean time of observation was 20 months (from 1 to 36 months).

Results. All patients showed positive dynamics. The PSA level in 94,0% of patients in 6 months after the end of treatment didn’t exceed 2,0 ng/ml. In 9 months, the total level of PSA decreased less than 1,5 ng/ml in 91% of patients, and in 12 months the PSA level in 86,0% of patients was less than 1,0 ng/ml. In 24 months, the PSA level in 93% of patients was less than 1,0 ng/ml, two patients had the PSA level of no more than 2,0 ng/ml.

Radioreactions in the form of grade I rectitis were observed in 39 patients (7.9%), grade II rectitis - in 24 patients (4.9%). Cystitis grade I was detected in 65 patients (13.1%), cystitis grade II - in 23 patients (4.7%). One patient (0.2%) developed acute urinary retention. There were no delayed complications observed.

Conclusions. Stereotactic body radiation therapy using Cyber-Knife has proven to be a highly effective method for the treatment of patients with localized prostate cancer. A low toxicity of the Cyber-Knife system ensures high safety of the treatment.