In this study,the eligible breast cancer patients will be randomized divided into two groups. the study group receive the accelerated hypo-fractioned whole breast irradiation with a concurrent boost to the tumor bed (WBI 2.5Gy x18 and SIB 2.88Gy x18) and the control group receive conventional fractioned whole breast irradiation with a concurrent boost to the tumor bed(WBI 1.8Gy x28 and SIB 2.15Gy x28). All radiotherapy will be delivered by external beam-intensity modulated radiotherapy. the purpose of this study is to assess whether the accelerated hypo-fractioned is non-inferior to the conventional fractionated radiotherapy. The outcomes evaluation will include acute toxicity , late toxicity including breast fibrosis and cardiac disease, cosmetic outcome ,local control and survival outcome.

Inclusion Criteria:

1. Female patients has had breast conserving surgery
2. age ≥ 18 and ≤ 65 years
3. with a histological diagnosis of invasive carcinoma of the breast
4. with pathological stage of T1-2N0-1M0
5. multidisciplinary decision of adjuvant WBI with a boost to the tumor bed
6. informed consent obtained, signed and dated before specific protocol procedures

Exclusion Criteria:

1. patients treated with Mastectomy
2. Need for lymph node irradiation
3. positive or close(≤ 1 mm) surgical margin
4. treated with neoadjuvant chemotherapy before surgery
5. Bilateral malignancy of the breast (synchronous or metachronous)
6. Pregnant or breastfeeding
7. Mental condition rendering the patient unable to understand the nature, scope and possible consequences of the study
8. Patient unlikely to comply with the protocol; i.e. uncooperative attitude, inability to return for follow-up visits, and unlikely to complete the study