Chemotherapy Omission in HR-positive/HER2-negative Breast Cancer With Lymph Node Negative Disease Receiving Adjuvant Endocrine Therapy and CDK4/6 Inhibitor

Inclusion Criteria:

* Females and males aged 18-70 years;
* ECOG performance status 0-1;
* Early-stage breast cancer, with postoperative pathology confirming invasive carcinoma, HR-positive and HER2-negative (In this study, HR-positive is defined as: ER-positive by IHC with \>10% tumor cells staining positive; HER2-negative defined as HER2 0-1+ by IHC or HER2++ with negative FISH result, without amplification);
* Postoperative pathological stage pT2N0M0 and:

Histological grade 3, or Histological grade 2 with Ki67 ≥ 20% or high-risk multigene assay results;

* No prior neoadjuvant therapy received;
* Good postoperative recovery, with randomization occurring no more than 8 weeks after surgery;
* Normal function of major organs, meeting the following criteria:

Hematological tests must meet: HB ≥90 g/L (no transfusion within 14 days); ANC ≥1.5×10⁹/L; PLT ≥100×10⁹/L; Biochemical tests must meet: TBIL ≤1.5×ULN (upper limit of normal); ALT and AST ≤3×ULN; serum Cr ≤1.5×ULN;

* Contraception required for male participants and women of childbearing potential during treatment;
* Participants voluntarily enroll in the study, sign informed consent, demonstrate good compliance, and cooperate with follow-up.

Exclusion Criteria:

* Bilateral breast cancer or carcinoma in situ (DCIS/LCIS);
* Metastasis at any site;
* Clinical or imaging suspicion of malignancy in the contralateral breast requiring biopsy (unless ruled out);
* Prior neoadjuvant therapy, including chemotherapy, radiotherapy, or endocrine therapy;
* Use of tamoxifen, raloxifene, or aromatase inhibitors (AIs) for breast cancer risk reduction ("chemoprevention") and/or osteoporosis treatment within the past 2 years;
* History of other malignancies within the past 5 years (except basal cell carcinoma of the skin or carcinoma in situ of the cervix), including contralateral breast cancer;
* Concurrent participation in another clinical trial;
* Severe systemic diseases and/or uncontrolled infections that preclude study participation;
* Severe cardiovascular or cerebrovascular events within 6 months before randomization (e.g., unstable angina, chronic heart failure, uncontrolled hypertension \>150/90 mmHg, myocardial infarction, or stroke);
* Known hypersensitivity to the study drugs;
* Men and women of childbearing potential unwilling to use contraception during treatment and for 8 weeks after treatment completion;
* Pregnant or lactating women;
* Positive pregnancy test before study drug administration (for women of childbearing potential);
* Psychiatric disorders, cognitive impairment, or inability to comprehend the trial protocol, adverse effects, or comply with study procedures and follow-up (requires systematic evaluation before enrollment);
* Individuals lacking personal freedom or legal capacity for independent civil conduct.