Characteristics of single beam collimated by Leksell Gamma Unit have been simulated using EGS4 Monte Carlo method. Results of the simulation indicated that non-scattered photons dominated in the aperture of final collimator and the number of non-scattered photons was 2 to 3 order bigger than the number of scattered Photons. Outside the aperture
the number of scattered photon could not be ignored. In addition
Compton scatter was the major interaction of photon with the final collitnator.