Abstract: ObjectiveTo evaluate the benefits of intensity-modulated radiotherapy （IMRT）with active breathing coordinator technique（ABC）compared with free breathing （FB） during treatment with conventional wedge-based whole breast irradiation (WBI) for the early stage breast cancer after breast-conserving surgery. MethodsEight patients (4 left-sided and 4 right-sided lesions) with stage T1~2N0M0 breast cancer were selected for this study. All patients had CT scans at FB and ABC. A dosimetric comparison of ABC-IMRT with FB-WBI was performed on each patient using three dimensional treatment planning. Dose volume histograms （DVH） and isodoses were used to compare the planning target volume（PTV）and organs at risks （OARs）,such as ipsilateral lung, and heart for the four left sided lesions. ResultsThe 95%PTV received 47.5 Gy/25 fractions; But V90% was 98.29% in ABC-IMRT method and 97.71% in FB-WBI method (P＝0.041); V105%was 54.54% vs. 72.22% (P＝0.035); V110% was 11.71% vs. 30.69% (P＝0.014) respectively in two methods. The percentage of volume receiving more than 20 Gy（V20）of ipsilateral lung for ABC-IMRT decreased from 22.20% to 17.89% (P＝0.000) compared with FB-WBI; and V30 from 18.15% to 14.39% (P＝0.000); The mean lung dose from 12.67 Gy to 9.81 Gy (P＝0.004). The percentage of V5 of heart for the four left sided patients for ABC-IMRT decreased from 15.23% to 9.40% (P＝0.014) compared with FB-WBI; V30 from 5.95% to 0.58% (P＝0.074); The thickness which was involved by the isodose of 30 Gy from 1.35 cm to 0.45 cm (P＝0.003).The mean heart dose from 4.35 Gy to 2.36 Gy (P＝0.044). ConclusionABC-IMRT improves dose homogeneity and significantly reduces the lung and heart irradiated volume and dose, thus possibly lessens the radiation injury to lung and heart.