This is a prospective, multicenter, randomized, open-label, phase III non-inferiority trial designed to evaluate whether omission of postoperative radiotherapy is clinically acceptable in patients with low- and intermediate-risk ductal carcinoma in situ (DCIS) following breast-conserving surgery. Eligible patients include women aged ≥40 years with ER-positive, low- or intermediate-grade DCIS, tumor size ≤2.5 cm, negative surgical margins ≥3 mm, absence of comedo necrosis, and screen-detected unifocal disease. Patients will be randomized to postoperative observation alone or adjuvant radiotherapy. The primary endpoint is 5-year ipsilateral breast tumor recurrence (IBTR).
Female patients aged ≥40 years；
Histologically confirmed low- or intermediate-grade ductal carcinoma in situ (DCIS) without invasive carcinoma, including incidental DCIS identified within benign breast lesions；
Tumor size ≤2.5 cm on final pathology；
Negative surgical margins ≥3 mm, including margins obtained after re-excision；
Known BRCA1 or BRCA2 germline mutation carriers (genetic testing not mandatory)；
Contraindications to radiotherapy, including active connective tissue disease；
Active infection requiring systemic treatment；
Bilateral breast cancer or prior contralateral breast cancer；