Background: Stereotactic radiosurgery is an important clinical tool for the treatment of small lesions in the brain, including benign conditions, malignant and localized metastatic tumors. A dosimetry study was performed for Elekta ‘Synergy S ’ as a dedicated Stereotactic radiosurgery unit, capable of generating circular radiation fields with diameters of 1-5 cm at isocentre using the BEAM/EGS4 Monte Carlo code. Materials and Methods: The linear accelerator Elekta “Synergy S” equipped with a set of 5 circular collimators from 10 mm to 50 mm in diameter at isocentre distance was used. The cones were inserted in a base plate mounted on the collimator linac head . A PinPoint chamber and Wellhofer water tank chamber were selected for clinical dosimetry of 6 MV photon beams. The results of simulations using the Monte Carlo system BEAM/EGS4 to model the beam geometry were compared with dose measurements . Results: An excellent agreement was found between Monte Carlo calculated and measured percentage depth dose and lateral dose profiles which were performed in water phantom for circular cones with 1, 2, 3, 4 and 5 cm in diameter. The comparison between calculation and measurements showed up to 0.5 % or 1mm difference for all field sizes. The penumbra (80-20%) results at 5 cm depth in water phantom and SSD=95 ranged from 1.5 to 2.1 mm for circular collimators with diameter 1 to 5 cm . Conclusion: This study showed that BEAMnrc code has been accurate in modeling Synergy S linear accelerator equipped with circular collimators.