Arc therapy is one of the treatment techniques for small, centrally located deep-seated tumors. However, care must be taken to remove any components that would interfere with the beam as the gantry rotates around the patient. One such component that may interfere with the beam is the patient support assembly (PSA) or treatment table. Beam attenuation factors due to the presence of the couch side-rails and the centerspine bar of the PSA are presented for both 6 MV and 18 MV photon beams of a Clinac 1800 during a 360 degrees rotation. Dose perturbations arising from these obstructions are displayed using an Alderson Rando phantom. A method is described to calculate the start and stop angles for the largest unobstructed arc for a given field size (FS), centerspine bar to the isocenter height (H), centerspine bar width (W), and SAD. As an illustration, for an SAD of 100 cm and a W of 4 cm, the start and stop angles for arc therapy with a FS of 10 x 10 cm2 and an H of 12.7 cm would be 29 degrees and 331 degrees.