Regarding the relationship between evaluation values and calculation errors ΔD shown in Fig. 6, no correlation was found in prostate, lung, or H&N cancer cases, which had relatively small errors. However, in the liver and pancreas cases, which had large errors, a correlation was found. We assume that in cases where the errors were large, the reproducibility of patient conditions (diaphragm position, gas position, and gas volume, for example) between treatment planning and actual treatment was low. Consequently, it is unlikely that the DR and DRR will accurately match on the bony structure for our system, even if the f(di)was minimized (ZNCCdiwas maximized). Radiological technologists expertly adjust their images on the bony structure by omitting the above‐described low‐reproducibility regions on the basis of their knowledge and experience. It is currently impossible to automatically perform such calculations with our system. To address this problem, it is necessary to add a technique for omitting low‐reproducibility regions from the calculation, as technologists do.