To evaluate the accuracy dependence on ROI size at the five sites, three ROI sizes (small, medium, and large) were defined on the horizontal and vertical images. The length × width of small, medium, and large ROI on the vertical image were 41.2 × 46.5 mm, 61.7 × 69.7 mm, and maximum displayed DR, respectively; these values on the horizontal image were 43.0 × 38.4 mm, 64.4 × 57.6 mm, and maximum displayed DR, respectively. The center of both small and medium sizes was set at the isocenter. In most cases, the small size contained the planning target volume, which is an important matching target for the patient positioning, while the medium size usually contained the nearest bones. Examples of the three sizes are shown in Fig. 4. The initial positional values (x,y,z,θx,θy,θz) on the CT images were set to (5 mm, 5 mm, 5 mm, 0.5°, 0.5°, 0.5°) to replicate positioning in clinical use.