Objective:To analyze the difference of positioning accuracy by cone beam CT(CBCT) between modified head neck shoulder thermoplastic mask and breast bracket in intensity modulated radiotherapy for breast cancer patients after modified radical mastectomy.Methods:A total of 68 cases of breast cancer treated with IMRT after modified radical mastectomy were selected from August 2015 to December 2018. According to the different immobalization methods, the patients were divided into two groups: modified head neck shoulder thermoplastic mask group (body mask group, 42 cases) and breast bracket group (bracket group, 26 cases). After the first, sixth, 11th, 16th and 21st treatment, the patients were scanned by CBCT. The positioning errors in left and right (RL), anterior and posterior (AP) and head and foot (SI) directions were obtained, and the duration of radiotherapy positioning was recorded. The positioning error, distribution ratio and positioning duration were compared between two groups, and the respective MPTV margin were calculated. The influence of various factors on the setup error of patients was analyzed in the mask group. Results:There were 210 scans in the body mask group and 130 in the bracket group. The setup errors of the body mask group and bracket group in RL, AP and SI directions were (2.12±2.01) and (2.38±1.92) mm, (3.29±2.46) and (3.88±2.76) mm, (3.47±2.29) and (4.11±3.15) mm, respectively, and the differences in AP and SI directions were statistically significant ( t=-2.05, -2.16, P<0.05). The proportion of setup errors less than or equal to 3 mm in the direction for body mask group was higher than that of bracket group ( χ2=4.97, P<0.05). The proportion of setup errors more than 5 mm in AP and SI directions for body mask group was lower than that of bracket group ( χ2=5.21, 9.29, P<0.05). The positioning duration of mask group was shorter than of bracket group ( t=-2.16, P<0.05). The MPTV margins of the mask group in RL, AP and SI directions were smaller than those of the bracket group. The modified head, neck and shoulder thermoplastic mask immobalization method led to large setup errors in AP and / or SI directions for the elderly (≥ 60 years old), the patients with body mass index (BMI) ≥ 24 kg/m 2 and the patients who were relatively unfamiliar with the radiotherapy process (treatment time≤ 2 weeks). The setup error for patients aged < 60 years old in SI direction was smaller than that for the elderly (≥ 60 years old) patients ( t=-2.43, P<0.05). The setup error for patients with BMI <24 kg/m 2 in AP and Si directions was smaller than that with BMI ≥24 kg/m 2 ( t=-2.21, -2.04, P<0.05). The setup error for treatment time > 2 weeks in AP direction was smaller than that for treatment time ≤ 2 weeks ( t=2.23, P<0.05). Conclusions:In IMRT radiotherapy for breast cancer patients after modified radical mastectomy, the application of modified head neck shoulder thermoplastic mask can reduce the setup error in the anterior-posterior and superior-inferior directions, and shorten the positioning duration. For the elderly (≥ 60 years old), BMI ≥ 24 kg/m 2 and the patients who are not familiar with the radiotherapy process (treatment time ≤ 2 weeks), attention should be paid to the setup of anterior-posterior and superior-inferior directions to ensure the treatment effect of radiotherapy.