This study explores the feasibility of preoperative single-dose tumor-bed boost followed by oncoplastic breast-conserving surgery and ultra-hypofractionated postoperative radiotherapy in patients with early stage breast cancer. Patients less than 55 years old, who are diagnosed with breast cancer and are eligible to recieve breast-conserving surgery are enrolled. Patients who are older than 55 years, with suspected regional lymph node metastasis are excluded. The primary end point are the acute toxicities within 4 weeks after adjuvant radiation. The secondary endpoints are oncologic outcomes, surgical complications within 30 days, late toxicities, patients' quality of life and cosmetic outcomes.

This is a prospective, non-controlled, feasitility study. The tumor-bed boost part of the radiotherpay is put forward, 7-14 days before the definitive surgery. A single dose of 10Gy is prescribed to the tumor bed. In order to identify the tumor accurately, MRI guided radiotherapy is applied under the MR-linac. The breast surgery is planned after recovery from radiation induced acute toxicity, within 14 days. The surgeons would undertake lumpectomy only or an additional oncoplastic surgery, with axillary nodal evaluation with sentinal nodal biopsy or dissection. After wound healing, postoperative whole breast radiotherapy is administered, within 6 weeks following surgery. The dose is 26Gy/5.2Gy/5 fractions. Adjuvant systemic therapy is held until the end of the radiotherapy, no more than 12 weeks from surgery implementation.

Inclusion Criteria:

1. Patients diagnosed with invasive breast cancer;
2. cN0 based on clinical physical examination combined with at least two imaging tests, or axillary fine-needle biopsy proved pN0;
3. Patients who plan to receive breast-conserving surgery or breast-conserving oncoplastic surgery;
4. No distant metastasis;
5. The primary tumour> 5mm from the skin, without invasion of the ribs or intercostal muscles;
6. No neoadjuvant systemic therapy;
7. Patients who can tolerate MRI;
8. Life expectancy ≥5 years;
9. 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.);
10. Patients are willing to cooperate to follow up;
11. Patients should sign the informed consent;
12. Women of childbearing age need effective contraception.

Exclusion Criteria:

1. Concurrent active connective tissue disease;
2. Patients who had radiotherapy to the ipsilateral breast or adjacent areas before;
3. Other malignancies, which affect pateint life expectancy (except adequately treated basal cell carcinoma of the skin, carcinoma in situ of the cervix, superficial bladder neoplasms (no more than T1));
4. 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).