Abstract: Objective To analyze radiotherapy setup errors of breast cancer sufferers immobilized by head-shoulder-truncus vacuum bags and calculate the clinical target volume （CTV） enlarging to the planning target volume（PTV）. Methods Fifty-three patients were immobilized by the head-shoulder-truncus vacuum bags. Setup errors were analyzed using kilovoltage cone beam computed tomography （CBCT）. The data of the linear （X， Y and Z） and rotational （Xr，Yr and Zr） setup errors were collected. The statistics were used to estimate the performance of head-shoulder-truncus vacuum bags. According to the formula Mptv=2.5∑+0.7δ， the Mptvwas calculated. Results A total of 265 CBCT scans from 53 patients were done. The average absolute value of 265 linear errors were 2.50 mm， 2.40 mm and 2.20 mm at X， Y and Z axes，rotation errors were 1.13°， 1.11°and 0.94°at Xr， Yr and Zr axes， and the errors of X and Y axes compared with Z axis had statistical significance （P<0.05）. For the linear setup errors， the group systematic errors were 2.50 mm， 2.30 mm and 1.90 mm， and the group random errors were 2.50 mm， 2.20 mm and 2.20 mm at X， Y and Z axes. For the rotational errors， the group systematic errors were 1.03°， 1.10°and 0.95°at Xr， Yr and Zr axes， and the random errors were 0.79°， 1.15° and 0.72° at Xr， Yr and Zr axes. In accordance with formula， the Mptv were recommended as 8.00 mm， 7.30 mm and 6.30 mm at X， Y and Z axes. Conclusion The head-shoulder-truncus vacuum bag has good setup accuracy. The data of setup errors which measured by CBCT provide references for CTV enlarging to PTV， and increase the precision in breast cancer radiotherapy.