In this study, the calculation accuracy was evaluated under several initial positional conditions (2 mm, 0.2°; 5 mm, 0.5°; 10 mm, 1°; and 20 mm, 2°). In our previous study, the average of the absolute value of the initial deviation before positioning in prostate cancer cases was 3.0 ± 3.4 mm (maximum value, 14.8 mm).1 17 In the case of patient positioning at our facility, because laser alignment is performed before x‐ray image acquisition, there is no large error. Therefore, from the results in Fig. 5, we assume that the calculation in almost all cases of prostate, lung, and H&N cancer might be acceptable. However, the calculation results for three H&N cases were unacceptable when the initial value was 20 mm and 2°. If the initial positional error is greater than 10 mm, it is necessary to devise a calculation.