Background and purpose: Correct target positioning is an important factor affecting precise radiotherapy in patients with breast cancer. We measured the setup variations respectively with breast bracket and arm support for radiation after breast-conserving surgery using electronic portal imaging device (EPID)

and analyzed the differences. Methods: Twelve breast cancer patients receiving intensity modulation radiation therapy after breastconserving surgery were enrolled in the study. Six patients were applied with breast brackets for positioning and another six with arm support. Electronic portal imaging (EPI) was performed for each patient before and after fractionated radiation at the position of the two main tangential fields. The EPI image was manually matched with the digitally reconstructed radiograph (DRR) derived from CT simulation image. And the setup variations in vertical and horizontal directions were measured. Results: The setup errors of arm support group in horizontal and vertical directions were 0 and 0 in 5 mm

6.6% and 4.9% in 3-5 mm

93.4% and 95.1% in 3 mm

respectively. The setup errors of breast bracket group were 6.7% and 3.3% in 5 mm

45.0% and 23.3% in 3-5 mm

48.3% and 73.3% in 3 mm

respectively. The differences of interfraction variation between two groups before fractionated radiation in both horizontal and vertical directions were statistically significant (P=0.000

P=0.006). The differences of interfraction variation between two groups in horizontal direction were statistically significant (P=0.003

P=0.008). And the difference of interfraction variation between two groups in horizontal direction was statistically significant (P=0.000). Conclusion: Breast brackets and arm support are both useful immobilization devices to ensure accuracy and reproducibility for radiation after breastconserving surgery. Arm support is superior to breast bracket and is recommended for position immobilization in whole breast radiation.