The purpose of this thesis is to evaluate the influence of patient weight and couch angle on the treatment accuracy of brain SRS guided by the AlignRT system integrated with the TrueBeam-IGRT couch system. Two phantoms were used during our study. First, a Winston-Lutz-type phantom with a facemask on top was used for our initial testing. Second an AlignRT 6D QA phantom was used for the remainder of our tests. Both were used to simulate a brain SRS treatment with AlignRT monitoring. The phantoms contained small fiducial markers that guided the initial setup using kV orthogonal and CBCT imaging. A CT scan of the phantoms were taken and used to form a plan that included 15 sets of different combinations of couch rotations and gantry angles. Once the phantomw were set up, a reference image for AlignRT was acquired and motion monitoring was initiated. The gantry and couch were then moved to their respective positions and an EPID image was acquired. A motion report was generated on the AlignRT system. EPID image registration was also performed using the OBI system with DRR image and the shifts were recorded. The method was repeated with the AlignRT 6D QA phantom for evaluation of patient weight with five different weights. The weight range of this simulation was 19.7 kg - 118.4 kg. Examination of the AlignRT data suggests that the accuracy of the TrueBeam-IGRT couch system is slightly compromised after applying couch rotation during the tracking. The accuracy did not seem to be affected by varying patient weight if couch sagging is compensated for during the initial setup. All of the deviations of the EPID image registration record showed that the true accuracy of the TrueBeam system was within 0.5 millimeters proving that the true isocenter was held in a constant sphere. Our data suggests that the TrueBeam-IGRT couch system maintains geometric accuracy of up to 118.4 kg of weight. We know that improvements could be made various couch angle positions, however, the deviations AlignRT reported is still within tolerance. The data suggests that the system is reliable for use with Brain SRS treatments. Addition of AlignRT for intra-fraction monitoring helps to ensure that the treatment remains accurate.