Eligibility Criteria:
Inclusion Criteria:
* Females aged 18 years or older;
* Postoperative pathological stage I early breast cancer: histologically confirmed invasive carcinoma with a maximum diameter ≤2 cm and node-negative (N0);
* Immunohistochemistry (IHC) shows ER-positive (ER ≥50%), HER2 IHC score of 0, 1+, or 2+ with no amplification confirmed by FISH, and Ki-67 ≤20%;
* Presence of at least one of the following potential low-risk factors:
1)Tumor size ≤1 cm, 2)21-gene recurrence score \<11, 3)Fudan digital pathological subtype classified as SNF1, 4)Age ≥65 years;
* ECOG performance status of 0 or 1;
* Patients with bilateral synchronous invasive lesions are eligible if both lesions are ER-positive, HER2-negative, and meet the tumor size criteria;
* Normal major organ function, meeting the following criteria:
1. Hematological: HB ≥90 g/L (no transfusion within 14 days), ANC ≥1.5×10⁹/L, PLT ≥100×10⁹/L;
2. Biochemical: TBIL ≤1.5×ULN, ALT and AST ≤3×ULN, serum Cr ≤1×ULN, and creatinine clearance \>50 mL/min (Cockcroft-Gault formula);
* Participants voluntarily enroll, sign informed consent, demonstrate good compliance, and cooperate with follow-up.
Exclusion Criteria:
* Primary tumor size \>2 cm in maximum diameter and/or axillary lymph node positivity;
* Prior neoadjuvant therapy, any systemic therapy, or local therapy (except surgery), including chemotherapy, targeted therapy, radiotherapy, or endocrine therapy;
* Prior adjuvant chemotherapy;
* Use of CDK4/6 inhibitors in the adjuvant setting;
* History of other malignancies (except cured basal cell carcinoma or cervical carcinoma in situ);
* Metastasis at any site;
* Pregnancy, lactation, or women of childbearing potential unable to use effective contraception;
* Concurrent participation in other clinical trials;
* Severe cardiac, pulmonary, hepatic, or renal dysfunction; LVEF \<50% (by echocardiography); severe cardio-cerebrovascular diseases within 6 months (e.g., unstable angina, chronic heart failure, uncontrolled hypertension \>150/90 mmHg, myocardial infarction, or stroke); poorly controlled diabetes; severe hypertension;
* Severe or uncontrolled infections;
* History of drug abuse or psychiatric disorders;
* Patients deemed unsuitable for the study by the investigator.