Partial breast irradiation is currently a hot topic in clinical research, but the appropriate population and appropriate irradiation The technology and dose fractionation mode are still inconclusive. Foreign studies have mostly selected patients with low risk of recurrence. Breast-conserving surgeries for early-stage breast cancer are increasing year by year in China. It is necessary to conduct prospective clinical studies to explore the Chinese population suitable for partial breast irradiation and the appropriate radiotherapy dose fractionation model. A preliminary study in our center has compared the safety and efficacy of a 2-week schedule of partial breast irradiation (40Gy/10f) and a 3-week schedule of whole-breast radiotherapy (43.5Gy/15f) in patients with early-stage low-risk breast cancer after breast-conserving surgery. , preliminary results show that the 2-week partial breast irradiation regimen is safe and effective. This study plans to continue to select low-risk breast-conserving patients, use extracorporeal intensity-modulated radiotherapy technology, refer to the radiotherapy dose fractionation model of the FAST-Forward study, and give partial breast irradiation 5.2 Gy/f/d, a total of 5 times, with a total dose of 26Gy, to further shorten the time The radiotherapy time is 1 week, and the acute and late adverse reactions of radiotherapy and the local control of the tumor are prospectively observed. Provide evidence for conducting a national multi-center phase III clinical randomized study.

Inclusion Criteria:

1. ≥45 years old, <75 years old women

2. Life expectancy >5 years

3. Start radiotherapy within 12 weeks after breast-conserving surgery (or within 8 weeks from the end of chemotherapy)

4. Invasive ductal carcinoma grade 1-2, or mucinous carcinoma, papillary carcinoma, canalicular carcinoma, medullary carcinoma; primary tumor

   ≤3.0cm; both pN0 (axillary dissection or sentinel lymph node biopsy)

5. Low-risk DCIS: tumor ≤2.5cm, low to medium grade; axillary surgery is not required or pN0

6. Single focus (with MRI diagnosis)

7. Vascular tumor thrombus negative

8. ER and/or PR positivity (defined as strong positivity in >1% of tumor cell nuclei)

9. Negative pathological margin≥2mm

10. Place metal markers on the tumor bed

11. Sign the informed consent form

Exclusion Criteria:

1. Multiple primary tumors
2. Invasive ductal carcinoma grade 3
3. Invasive micropapillary carcinoma
4. Lobular carcinoma in situ
5. Invasive lobular carcinoma
6. Simple nipple paget's disease
7. Oncoplastic surgery
8. Neoadjuvant chemotherapy or neoadjuvant endocrine therapy
9. Previous or simultaneous contralateral breast cancer
10. History of ipsilateral chest wall radiotherapy
11. Have active collagen vascular disease