The goal of this clinical trial is to explore the application of online adaptive radiotherapy in patients who receive moderate hypofractionated or ultrafractionated radiotherapy after breast cancer surgery. The main questions it aims to answer are:

• Can online adaptive radiotherapy improve the accuracy of dose delivery?
• In patients undergoing online adaptive radiotherapy, how are the treatment-related toxicities and tumor control outcomes?

Participants will Receive moderate hypofractionated radiotherapy using online adaptive radiotherapy, a dose of 43.5Gy in 15 fractions or receive ultrafractionated radiotherapy using online adaptive radiotherapy, a dose of 26Gy in 5 fractions.

Online Adaptive Radiotherapy (ART) is an evolution of image-guided radiotherapy that compresses the entire process of image acquisition, plan design, verification, and radiation delivery into approximately 10-30 minutes during which patients do not need to leave the treatment bed. Online ART features rapid image acquisition, automatic contouring and plan optimization supported by artificial intelligence. Body position deviation, breast soft tissue deformation, and organ movement in different treatment fractions can affect the accuracy of radiotherapy for breast cancer patients. Online adaptive radiotherapy is expected to have dosimetric advantages in terms of target coverage and organ-at-risk (OAR) dose reduction, thereby reducing treatment-related side effects while ensuring local tumor control.

Moderate hypofractionation (40-43.5Gy/15-16f/3w) has been shown to achieve equivalent local tumor control and patient survival compared to traditional fractionationated regimens (50Gy/25f/5w), and may even reduce acute and late toxicities. Currently, moderate hypofractionationated radiotherapy has been a first-line recommendation for whole breast irradiation following breast cancer surgery. In addition, the FAST-Forward trial evaluated a shorter regimen, the ultrafractionated whole breast radiotherapy (26Gy/5f/1w) and found that local tumor control and normal tissue toxicity were comparable to the moderate hypofractionationated regimen. The Royal College of Radiologists (RCR) has endorsed 5-fraction ultrafractionationated radiotherapy as a standard option for postoperative breast cancer radiation treatment in 2020.

Inclusion Criteria:

• Female, aged ≥35 years, <70 years
• ECOG score: 0-2
• Having underwent breast-conserving surgery or Modified radical mastectomy with or without sentinel lymph node biopsy or axillary lymph node dissection
• Pathologically confirmed primary breast cancer
• For patients who did not receive neoadjuvant therapy: pathological staged pT0-2N0-1; for patients who received neoadjuvant therapy: staged ypT0-2N0-1
• Indicated for postoperative adjuvant radiotherapy, planned to undergo postoperative adjuvant radiotherapy
• Able to cooperate and tolerate the treatment

Exclusion Criteria:

• Pathologically confirmed metastasis in supraclavicular or infraclavicular lymph nodes, or distant metastasis
• History of radiotherapy to the neck or chest
• Contraindications or intolerance to radiation therapy (such as pregnancy or lactation, severe impairment of heart, lung, kidney, liver, or other vital organ functions; severe infection or hematologic abnormalities; brachial plexus nerve injury on the affected side; active connective tissue diseases, etc.)
• History of malignant tumors