Four patterns of the initial parameters (x,y,z,θx,θy,θz) on the CT images were set: (2 mm, 2 mm, 2 mm, 0.2°, 0.2°, 0.2°), (5 mm, 5 mm, 5 mm, 0.5°, 0.5°, 0.5°), (10 mm, 10 mm, 10 mm, 1°, 1°, 1°), and (20 mm, 20 mm, 20 mm, 2°, 2°, 2°). The accuracy of each pattern was evaluated using the ROI size that had the smallest error, calculated as described in Section 2.D.3, to evaluate the dependence of accuracy on initial position parameters.