ExacTrac (BrainLAB) is an automatic patient positioning system used in many photon therapy facilities.9, 10, 11 Although this system achieves fast and highly accurate automatic patient positioning, it is incompatible with particle therapy, which requires visualization of bony structures, because all bony structures in the x‐ray image size of the ExacTrac system cannot be seen. Mori et al. reported an automatic patient positioning system for carbon ion radiotherapy.12, 13 The accuracy of the system was evaluated for tumors in three sites (pelvis, head and neck (H&N), and lung) and the authors reported the optimal metrics for the calculation. However, the system was not evaluated for use in other sites such as liver and pancreas. Additionally, the study did not mention the optimal region size for the calculation at each site. The positioning error could possibly be reduced by choosing the optimal region size for each target site.