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；
Parallel
Adjuvant Radiotherapy
RadiationOne of the following protocol-approved radiotherapy regimens may be delivered: * Partial breast irradiation (PBI): 26 Gy in 5 fractions； * Partial breast irradiation (PBI): 40.05 Gy in 15 fractions. * Whole breast ultrahypofractionated radiotherapy: 26 Gy in 5 fractions； * Moderate hypofractionated whole breast radiotherapy: 40.05 Gy in 15 fractions. Radiotherapy should start within 3 months after surgery.
tamoxifen
DrugAdjuvant endocrine therapy with tamoxifen according to institutional practice.
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 randomization to death due to breast cancer.
Overall Survival (OS)
Defined as time from randomization to death from any cause.
Distant Metastasis-Free Survival (DMFS)
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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