Introduction: The 60 Co beam emerging from the Gamma- Tomo source assembly was simulated in a previous study [1] and the authors reported the spectra of particles reaching the plane immediately (1mm) before the collimation system entrance. In the present work, we simulate the 60Co beam emerging from a novel Gamma-Tomo SBRT collimation system and calculate the output factors and dose rates for different source configurations and collimator sizes. Materials and Methods: A Gamma Tomo system includes 13 60 Co source capsules, source housing, a primary collimator and 4 different changeable collimators. The sources (6.22 mm of diameter) are located in 2 rows with different angles and distances to the longitudinal axis and the beams can be collimated to obtain four different circular field sizes at the isocenter (35 mm, 16 mm, 7mm, and 3.5mm). The BEAM- Monte Carlo [2] code is used to realistically model the collimation system geometry, including primary collimator and 4 different changeable collimators. The previously calculated phase space file [1] is used to transport particles throughout the collimation system to the patient plane for all of the changeable collimators of the Gama-Tomo System, and the respective phase space files of particles are stored at isocenter for each of the circular fields to be used as input of the GEPTS [3] to perform dose calculations. A newly designed geometry module is used to determine the dose distributions in a spherical polystyrene phantom with an 8cm radius centered at the isocenter. Results: The characteristics of the particle spectra emerging from the collimation assembly are determined and the effect of the source-collimator position on the spectra reaching the isocenter found to be dependent on field size. We observed differences up to 5% between the energy spectra and fluence distributions calculated for different configurations of the source-collimator assembly. The effect of the source-collimator position is observed (up to 6%) on dose distributions calculated at isocenter and is dependent upon the size of the circular field. Conclusions: The resulting data from the present study supports the machine design process and is valuble in building a well-represented source model for all sources to perform the dose calculations for all of the changeable collimators for this novel Gamma-Tomo system.