This is a single-center, prospective phase II clinical study designed to evaluate whether omission of postoperative radiotherapy is clinically acceptable in young patients with low-risk ductal carcinoma in situ (DCIS) following breast-conserving surgery. Eligible patients include women younger than 40 years with low- or intermediate-grade DCIS, tumor size ≤1 cm, negative surgical margins ≥3 mm, absence of comedo necrosis, and unifocal screen-detected disease. Patients in the radiotherapy omission cohort will undergo close surveillance after surgery, with endocrine therapy permitted according to physician discretion. 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 ≤1 cm on final pathology；
Negative surgical margins ≥3 mm, including margins obtained after re-excision；
Known BRCA1, BRCA2, or TP53 germline mutation carriers (genetic testing not mandatory)；
Bilateral breast cancer or prior contralateral breast cancer；
Severe uncontrolled comorbidities, including significant cardiac, pulmonary, hepatic, renal, hematologic, or psychiatric disorders；
History of other malignancies, including thyroid cancer, except adequately treated basal cell carcinoma of the skin or cervical carcinoma in situ；