Objective To explore the effects of improvements on setup error ( SE ) and clinical target ( CTV ) margin of supraclavicular field ( SCF ) by using moisture-cured resin cushion and breast bracket for lower neck fixation in breast cancer patients who underwent post-mastectomy radiotherapy. Methods Totally 13 patients with breast cancer who underwent post-mastectomy radiotherapy were enrolled. All patients were immobilized by breast bracket and moisture-cured resin cushion. Firstly, each patient′s lower neck and head was fixed well by moisture-cured resin cushion, filling the gap between the neck and breast bracket. Secondly,each patient underwent the cone-beam CT ( CBCT) at the first, tenth and twentieth treatment after positioning. Then these CBCT images were registered to the planning CT to determine setup errors in translational and rotational directions, and SCF CTV margins by the systematic and random errors were evaluated. Results The setup errors in x (left-right), y (superior-inferior), z (anterior-posterior) translational directions were (2. 16 ± 1. 25), (1. 50 ± 1. 28), (1. 94 ± 1. 12) mm and (1. 76 ±1. 87)°, (1. 82 ±1. 12)°, (0. 99 ±0. 58)°, respectively in θ (pitch degree),Ф(roll degree),ψ( yaw degree) rotational directions. Non-parametric rank test ( Mann-Whitney U test) was performed with previous data, the differences of the setup error in y, z,θ directions were statistically significant ( Z =4. 152, 3. 415, 2. 053, P<0. 05). The margins from CTV were 4. 07, 4. 03 and 3. 73 mm in x, y and z directions, respectively. Compared with the previous study on SCF, CTV margin required 8, 8 and 6 mm in x, y and z axis directions, the volume of CTV to PTV were decreased by 32. 73% on average. Conclusions Compared with plastic circular pillow alone, moisture-cured resin cushion with breast bracket locating method could reduce setup errors in SCF target region. The margins from CTV to PTV weredecreased to 4. 07, 4. 03 and 3. 73 mm in x, y and z axis directions at least.