Omission of Adjuvant Radiotherapy in Young Low-Risk Ductal Carcinoma In Situ Following 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 ≤1 cm on final pathology； 4. Negative surgical margins ≥3 mm, including margins obtained after re-excision； 5. Unifocal lesion； 6. Absence of comedo necrosis； 7. Screen-detected disease without bloody nipple discharge or palpable mass； 8. Clinically node-negative disease； 9. Central pathology review by the participating institution required； 10. Ability to understand and willingness to sign written informed consent. Exclusion Criteria: 1. Known BRCA1, BRCA2, or TP53 germline mutation carriers (genetic testing not mandatory)； 2. Bilateral breast cancer or prior contralateral breast cancer； 3. Severe uncontrolled comorbidities, including significant cardiac, pulmonary, hepatic, renal, hematologic, or psychiatric disorders； 4. History of other malignancies, including thyroid cancer, except adequately treated basal cell carcinoma of the skin or cervical carcinoma in situ； 5. Pregnant or breastfeeding patients； 6. Estimated life expectancy ≤5 years； 7. Poor compliance or inability to adhere to study follow-up requirements.