Omission of Adjuvant Radiotherapy in Low- and Intermediate-Risk Ductal Carcinoma In Situ After Breast-Conserving Surgery

Inclusion Criteria: 1. Female patients aged ≥40 years； 2. Histologically confirmed low- or intermediate-grade ductal carcinoma in situ (DCIS) without invasive carcinoma, including incidental DCIS identified within benign breast lesions； 3. Tumor size ≤2.5 cm on final pathology； 4. Negative surgical margins ≥3 mm, including margins obtained after re-excision； 5. Estrogen receptor-positive disease (ER ≥10%) with planned endocrine therapy using tamoxifen； 6. Unifocal lesion； 7. Absence of comedo necrosis； 8. Screen-detected disease without bloody nipple discharge or palpable mass； 9. Clinically node-negative disease； 10. Patients randomized to the radiotherapy arm must initiate radiotherapy within 3 months after surgery； 11. Central pathology review by the participating institution required； 12. Ability to understand and willingness to sign written informed consent. Exclusion Criteria: 1. Known BRCA1 or BRCA2 germline mutation carriers (genetic testing not mandatory)； 2. Contraindications to radiotherapy, including active connective tissue disease； 3. Active infection requiring systemic treatment； 4. Bilateral breast cancer or prior contralateral breast cancer； 5. Severe uncontrolled comorbidities, including significant cardiac, pulmonary, hepatic, renal, hematologic, or psychiatric disorders； 6. History of other malignancies, including thyroid cancer, except adequately treated basal cell carcinoma of the skin or cervical carcinoma in situ； 7. Prior thoracic radiotherapy； 8. Pregnant or breastfeeding patients； 9. Estimated life expectancy ≤5 years； 10. Poor compliance or inability to adhere to study follow-up requirements.