Objective:To investigate the skin positioning error in total breast radiotherapy after breast cancer surgery through image analysis.Methods:The study period was from January 2019 to June 2019. A total of 80 patients who received breast-conserving breast cancer surgery and adjuvant radiotherapy during this period in Zhejiang Cancer Hospital were selected. The CT positioning image for each patient in the treatment plan was created and the relevant cone beam computed tomography verification film after the patient positioning setting was obtained during radiotherapy, and the positioning map and the verification film to each patient through image processing software skin surface location were overlapped. The isocenter deviation of the nipple-lung ( X) and cranial tail ( Y) directions and the deviation of the X and Y axis rotation angle of the superimposed image were measured. Results:In the 80 patients, the system error ( μ, Σ) and random error ( σ) were calculated based on the X-axis and Y-axis deviation and the rotation angle deviation. The μ value of X-axis, Y-axis and rotation angle were (0.01 ± 0.01) mm, (-1.35 ± 0.14) mm and (0.06 ± 0.01)°. The Σ value of X-axis, Y-axis and rotation angle were (1.76 ± 0.72) mm, (1.49 ± 0.58)mm and (0.90 ± 0.12)°. The σ value of X-axis, Y-axis and rotation angle were (1.34 ± 0.96) mm, (1.93 ± 1.02) mm and (1.0 ± 0.2)°. The average value of the total vector error in the left and right patients were (3.02 ± 1.26), (2.88 ± 1.03) and (3.25 ± 1.38) mm, which had no clinical significance. Conclusions:In the routine breast radiotherapy after breast-conserving surgery, the smallest position error of the skin can be obtained by using the skin surface position combined with image processing software.