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；
Single group
Observation
Other interventionNo postoperative radiotherapy will be administered. Patients will receive close surveillance after surgery.
Endocrine Therapy (Optional)
DrugAdjuvant endocrine therapy may be administered according to physician discretion and patient preference.
Ipsilateral Breast Tumor Recurrence (IBTR)
Defined as recurrence of invasive carcinoma or DCIS in the ipsilateral breast.
Local-Regional Recurrence (LRR)
Defined as recurrence in the ipsilateral breast, chest wall, supraclavicular region, axilla, or internal mammary region.
Breast Cancer-Specific Survival (BCSS)
Defined as time from study enrollment to death due to breast cancer.
Overall Survival (OS)
Defined as time from study enrollment to death from any cause.
Distant Metastasis-Free Survival (DMFS)
Distant Metastasis defined as recurrence outside local-regional sites.
Tumor-Infiltrating CD8-Positive T-Cell Density
Density of tumor-infiltrating CD8-positive T cells in pretreatment tumor tissue, assessed by IHC or mIF.
Tumor PD-L1 Expression
PD-L1 expression in pretreatment tumor tissue assessed by IHC and quantified using the Combined Positive Score (CPS).
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