At our facility, CT images are acquired with x‐ray CT (Aquilion LB, Self‐Propelled, Toshiba Medical Systems); treatment planning is performed with the XiO‐N system (Mitsubishi Electric and Eleckta). In the treatment room, horizontal and vertical x‐ray tubes, flat panel detectors (DAR – 8000f, Shimadzu), and carbon beam irradiation nozzles are positioned as shown in Fig. 1. Patient positioning is performed using orthogonal DRs acquired with the flat panel detectors and DRRs reconstructed from CT data during treatment planning.