Eight patterns of DR sets and DRRs of a head and neck phantom (Whole Body Phantom PBU‐50, Kyoto Kagaku) were used to verify that our calculation system worked normally. The eight patterns of DR sets are shown in Table 1. The length × width of ROI on the vertical image was 41.2 × 46.5 mm, and these values on the horizontal image were 43.0 × 38.4 mm. The reference DRs are shown in Fig. 3.