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Objective: To compare the clinical positioning error of patient setup between the traditional positioning system with Optical Positioning System (OPS), and to evaluate the OPS based on our proposed approach of patient positioning.Materials and Methods: A phantom was used.We measured setup errors in left-toright (LR) and anterior-to-posterior (AP) directions by vernier caliper on a graph paper on Varian Linear accelerator, and then we shifted the couch height to make the source-to-surface distance (SSD)=100cm and recorded the height change which was displayed on monitor screen as the setup error in inferior-to-superior (IS) direction.Results: Average(Avg) setup errors for the CBCT guidance system were 0.42mm,0.50mm, and0.66mm in LR, IS and AP directions, respectively; the SD of it were0.24mm, 0.00mm and 0.52mm in LR, IS and AP directions, respectively.The OPS has an Avg setup error of 0.28mm, 0.40mm and 0.30mm in LR, IS and AP directions and SD of 0.08mm, 0.10mm and 0.07mm in LR, IS and AP directions, respectively.Conclusion: We demonstrated that OPS shows a comparable, fast and efficient positioning method compared to CBCT guidance system, and lives up to the actual need and will have a wide use in clinical application.