Fig. 5.

The 3D positions of the stationary marker at the MV beam isocenter determined by kV/MV images during the arc delivery. The marker was aligned at the isocenter by acquiring anterior–posterior MV and left-lateral kV images. The mean offset in each direction from the isocenter, indicated by the center of dotted circle, was caused by the uncertainty of this alignment process. The radius of the dotted circle, ~± 0.25 mm (1 pixel), represents the uncertainty of marker extraction.