Objective:To investigate the setup errors of postoperative radiotherapy immobilized with integrated cervicothoracic board (mask) system in breast cancer patients.Methods:Thirty-two breast cancer patients treated with postoperative radiotherapy immobilized with integrated cervicothoracic board (mask) system were prospectively recruited in this study. Breast/chest wall (cw) and supra/infraclavicular nodal region (sc) were irradiated with intensity-modulated radiotherapy. CBCT location verification in radiotherapy and target areas of the breast/chest wall and upper and lower collarbone were carried out, respectively. The consistency between setup errors and the position of the upper and lower target areas of 239 CBCT images was analyzed.Results:The translational setup errors of the breast/chest wall in the X-cw (left-right), Y-cw (superior-inferior), Z-cw (anterior-posterior) directions were (1.84±2.36) mm, (1.99±2.48) mm, and (1.75±1.86) mm, respectively. The translational setup errors of the supra/infraclavicular nodal region in the X-sc (left-right), Y-sc (superior-inferior), Z-sc (anterior-posterior) directions were (1.98±2.44) mm, (1.98±2.48) mm, and (1.71±1.79) mm, respectively. The differences of translational setup errors between the breast/chest wall and supra/infraclavicular nodal region in the X, Y, Z directions were (0.38±0.66) mm, (0.07±0.41) mm, and (0.45±0.92) mm, respectively. Conclusion:For the breast cancer patients treated with postoperative radiotherapy covering breast/chest wall and supra/infraclavicular nodal region, the integrated cervicothoracic board (mask) immobilization system provides good reproducibility and yields Sfew setup errors.