To illustrate the dependence of accuracy on ROI size at each site (Table 2), Fig. 8 shows DRR examples of calculation results in a prostate cancer case with a large ROI, a lung cancer case with a small ROI, and a pancreatic cancer case with a small ROI. First, we consider the size dependence of prostate, lung, and H&N cancer cases, which all were acceptable under the optimal ROI size condition. The errors were smallest with the small ROI in the prostate cancer case. This finding can be attributed to the fact that the small ROI did not contain joints such as the hips, whose reproducibility is low. If the center of a small ROI moves the accuracy would be changed; however, the ROI center would not move significantly because the patient positioning is performed using the isocenter (center of the PTV), and therefore the accuracy may be little changed. With a large ROI, as shown in Fig. 8(a), the calculation errors increased because the large ROI often included low‐reproducibility areas such as joints and the bones just below the joints. However, the errors with the small ROI were largest in the case of lung cancer [Fig. 8(b)]. In these cases, the small ROI contained only low‐density regions such as ribs, but not high‐density regions such as vertebrae and shoulder blades. We found that the errors were large if the reproducibility of the shoulder blades was low. In contrast, calculation accuracy was not dependent on ROI size in H&N cancer cases.