Objective To quantify the setup errors measured with orthogonal megavoltage X-ray film in breast cancer patients immobilized in breast bracket. To probe into the margins from clinical target volume (CTV) to planning target volume (PTV) in 3 directions. Methods Repeat orthogonal megavoltage X-ray films were acquired for routine of fine setup verification in 29 breast cancer patients after conserving surgery, 17 received whole breast radiation and 12 received both whole breast and supraclavicular node radiation. All patients were immobilized in the supine position with both arms raised over their heads, using a personalized α-cradle (breast bracket). Registrations of the bony anatomy for megavoltage X-ray films to digitally reconstructed-radiographs from the planning CT were compared. Systematic and random setup errors were quantified, and the differences between groups with and without supraclavicular node radiation were compared by independent two-sample t-test. Results The mean set up deviation, systematic error and random error were (0.9±3.1) mm,(0.7±3.0) mm,(1.2±2.1) mm and 3.1 mm, 3.0 mm, 2.1 mm and 2.7 mm, 3.3 mm, 3.5 mm in the mediolateral, superoinferior and anteroposterior dimensions. There is no difference of set up deviation between patients received supraclavicular node radiation and those did not (t=0.02,0.20,0.20, P=0.98,0.85,0.85). The margins from CTV to PTV were 9.6 mm, 9.8 mm, 7.7 mm in mediolateral, superoinferior and anteroposterior directions. Conclusion For whole breast irradiation patients immobilized with breast bracket, the margins from CTV to PTV were recommended as 9.6 mm, 9.8 mm, 7.7 mm in mediolateral, superoinferior and anteroposterior directions.