Because all cases of prostate, lung, and H&N cancer were acceptable with their optimal ROI sizes (Table 2), they were accurate enough to be feasible at the actual treatment site. However, more than half of liver cancer and pancreatic cancer cases were unacceptable. Here, we consider the cause of large error in one case of pancreatic cancer. The red box in Fig. 7(a) shows the position of one vertebra on the DR; the red line shows the position of the diaphragm. The blue boxes in Figs. 7(b) and 7(c) show the position of the vertebra on the DRR (corresponding to the red box in Fig. 7(a)); the blue lines show the position of the diaphragm (corresponding to the red line in 7 (a)). The positions of the red and blue boxes are almost the same in 7(a) and (c); however, the positions of the red and blue lines are different. Abe et al. reported that the average interfractional error in the marker position was 3.4 mm in the superior–inferior direction.20 Additionally, Kawahara et al. reported that the average interfractional error in the diaphragm position was 3.4 mm in the superior–inferior direction.21 As shown above, because the positional reproducibility of the liver in the abdomen is low, the patient positioning calculation was misled by the diaphragm position although patient positioning should be performed based on the vertebra position. Considering the correlation values on the images, the ZNCC on the position in Fig. 7(b) is 0.953, and that on the position in Fig. 7(c) is 0.757. The finding that the ZNCC on the calculation was higher than that on the reference position indicates that it is difficult to calculate the optimal patient positioning using the ZNCC alone. In contrast, the images in Figs. 7(a) and 7(c) show that the position of the diaphragm in the DR was different than that in the reference position, whereas the position of the vertebra was almost the same. We assume that the radiological technologist positioned the patient on the basis of the vertebrae, excluding low‐reproducibility regions such as the diaphragm.