In liver cancer cases, the error was smallest with a small ROI. As shown in Fig. 7, if the ROI was large, the diaphragm, whose reproducibility is low, was often included within the ROI. This inclusion caused increased errors because the calculation was affected by different positions of the diaphragm. In contrast, in pancreatic cancer cases, the error was smallest with a large ROI. Kumagai et al. and Houweling et al. reported that the reproducibility of the volume or position of gas in the bowel surrounding the pancreas was very low.22, 23 Therefore, as shown in Fig. 8(c), when the ROI was small, the relative size of the low‐reproducibility regions within the ROI increased, consequently increasing the error.