Abstract： Objective:To explore the target margin of different planning target volume (PTV) in patients with breast cancer after modified radical mastectomy.Methods:Retrospective analysis was conducted on 338 cone beam CT (CBCT) images of 38 female breast cancer patients (median age 50 years, range 34-74 years) who were treated at the Cancer Hospital, Chinese Academy of Medical Sciences from October 2022 to April 2023 after modified radical mastectomy and who were treated with radiotherapy using integral cervicothoracic thermoplastic mask fixation. CBCT images of all patients were registered for different regions of interest, including the chest wall target area, supraclavicular target area, and overall target area, and the setup errors in the six-dimensional direction of the three target areas were calculated (translation direction: left-right, head-foot, and abdomen-back; rotation direction: pitch, roll, and yaw). The target margin of the three target areas was calculated as follows: M=2.5∑+0.7σ. The differences in setup errors in the six-dimensional direction among the three target areas were compared using non-parametric tests (Friedman test), and their correlation was analyzedby Spearman method.Results:A significant difference in setup error was observed between the supraclavicular target area and the overall target area in the left-right direction ((2.53±2.46) mm vs. (1.82±2.18) mm, χ2=0.286, P=0.004), and significant differences in setup error were found between the chest wall target area and the overall target area ((1.10±0.95)° vs. (0.87±0.75)°), as well as between the supraclavicular target area and the overall target area ((1.24±1.05)° vs. (0.87±0.75)°) in the roll direction ( χ2=0.265, 0.241; P=0.001, 0.002). The target margins in the left-right, cranial-caudal, and anterior-posterior directions were 6.10, 6.21, and 5.90 mm, respectively, for the chest wall target area; 8.06, 6.22, and 6.70 mm, respectively, for the supraclavicular target area; and 6.09, 5.95, and 5.48 mm, respectively, for the overall target area. Compared with the supraclavicular target area and overall target area ( r=0.35-0.81, all P<0.001), the chest wall target area and overall target area show a stronger correlation in the six-dimensional direction ( r=0.76-0.93, all P<0.001). Conclusion:Different target margins should be used when planning the target area of the chest wall and supraclavicular area for patients with breast cancer after modified radical mastectomy.