This is a phase II study to investigate the feasibility of Ultra-hypofractionated radiotherapy with or without simultaneous integrated boost for low risk breast cancer patients who have received breast conservative surgery of mastectomy.

During the study, the patients would undergo radiation of 5.2Gy for 5 fractions to the prophylactic radiation volumes and and a 6Gy per-fraction simultaneous boost to the tumor bed or other high-risk volumes. Acute toxicity of grade 2 or higher in the following 12 weeks after radiotherapy is the primary end point.

Inclusion Criteria:

1. Patients diagnosed with invasive or non-invasive breast cancer;
2. The patients have undergone breast-conserving surgery or mastectomy with axillary sentinel nodal biopsy or dissection;
3. Stage ypT0-2N0-1 (if receive neoadjuvant therapy) or stage pT0-2N0-1 (if receive upfront surgery).
4. No distant metastasis;
5. Life expectancy ≥6 months;
6. Organ function is fine (Hemoglobin ≥100g/L, leukocyte ≥2×109/L, neutrophil ≥1×109/L, platelet ≥80×109/L; Creatinine 1.5 mg/dl or less; Alanine aminotransferase/aspartate aminotransferase ≤2.5×UNL.);
7. Patients are willing to cooperate to follow up;
8. Patients should sign the informed consent;
9. Women of childbearing age need effective contraception.

Exclusion Criteria:

1. Biopsy proven ipsilateral supraclavicular, infraclavicular or internal mammary nodal involvement.
2. Concurrent or previous neuropathy overlapping with the radiation volume or brachial plexus injury;
3. Patients who had radiotherapy to the ipsilateral breast, lymph-drainage regions or adjacent areas before;
4. Concurrent active connective tissue disease;
5. Other malignancies, which affect patient life expectancy (except adequately treated basal cell carcinoma of the skin, carcinoma in situ of the cervix, superficial bladder neoplasms (no more than T1), early stage thyroid carcinoma);
6. Severe comorbidities or active disease (Poorly controlled heart disease: New York Classification of Cardiac Function ≥ Grade 2, active coronary heart disease, unstable angina pectoris, arrhythmia requiring medical treatment/persistent refractory hypertension; Myocardial infarction, stroke within six months; Poorly controlled diabetes persists. Fasting blood glucose ≥ 10mmol/L, 2 hours postprandial blood glucose ≥ 13 mmol/L. Poorly controlled psychosis develops or worsens within six months; Active infection; Positive for antibodies to HIV).
7. Pregnant or breast-feeding.