Overview

This study is a Phase III non-inferiority trial targeting a specific subset of early breast cancer patients: those with HR-positive/HER2-negative, node-negative small tumors (typically ≤2cm) but who exhibit high-risk features for recurrence (such as high histological grade, high Ki-67 index, etc.). Currently, the standard adjuvant treatment for these patients often includes chemotherapy followed by endocrine therapy, despite their small tumor size, due to the high-risk biological characteristics. However, chemotherapy can bring significant toxicity and long-term side effects.

This trial explores whether a chemotherapy-sparing approach is feasible. It compares the efficacy and safety of using the CDK4/6 inhibitor Dalpiciclib combined with endocrine therapy (AI ± OFS) directly, without chemotherapy, against the traditional approach of chemotherapy (TC regimen) followed by endocrine therapy. The primary goal is to demonstrate that the chemotherapy-free regimen is not inferior to the chemotherapy-containing regimen in terms of 5-year invasive disease-free survival (iDFS). If successful, this study could potentially de-escalate treatment for this high-risk population, sparing them from chemotherapy-related toxicities while maintaining excellent oncological outcomes.

Eligibility

Inclusion Criteria:

1. Female, aged 18 to 70 years.
2. Histologically confirmed early breast cancer with pathological stage T1N0, 0.5 cm \< tumor size ≤ 2 cm, ER-positive/HER2-negative, with ER expression ≥ 50%; and willingness to receive adjuvant dalpiciclib treatment.
3. Presence of at least one of the following high-risk features:

   Ki-67 index ≥ 50%; Grade 3 (G3) tumor; an indication for chemotherapy based on results from multigene assays, including the 21-gene recurrence score and the 70-gene signature ; Age ≤ 40 years.

4. ECOG performance status: 0 to 1.
5. Adequate organ function, defined as meeting the following criteria:

   Hematology (no blood transfusion within 14 days): Hemoglobin (HB) ≥ 90 g/L; Absolute Neutrophil Count (ANC) ≥ 1.5 × 10⁹/L; Platelet (PLT) ≥ 100 × 10⁹/L.

   Biochemistry: Total Bilirubin (TBIL) ≤ 1.5 × Upper Limit of Normal (ULN); Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST) ≤ 3 × ULN; Serum Creatinine (Cr) ≤ 1 × ULN, with creatinine clearance \> 50 mL/min (calculated using the Cockcroft-Gault formula).

6. Voluntary participation with written informed consent, good compliance, and willingness to comply with follow-up requirements.

Exclusion Criteria:

1. Patients who have received neoadjuvant therapy, including chemotherapy, targeted therapy, radiotherapy, or endocrine therapy.
2. Bilateral breast cancer.
3. History of other prior malignancies, except for curatively treated basal cell carcinoma of the skin and carcinoma in situ of the cervix.

4\) Evidence of metastasis at any site. 5) Any T4 lesion (UICC 1987), including skin involvement, tumor fixation/mass, or inflammatory breast cancer.

6\) Pregnant or lactating women, or women of childbearing potential who are unwilling to use adequate contraception.

7\) Concurrent participation in another clinical trial. 8) Severe organ dysfunction (cardiac, pulmonary, hepatic, or renal). Left Ventricular Ejection Fraction (LVEF) \< 50% (by echocardiography). History of severe cardiovascular or cerebrovascular disease within 6 months prior to randomization (e.g., unstable angina, chronic heart failure, uncontrolled hypertension \>150/90 mmHg, myocardial infarction, or cerebrovascular accident). Patients with poorly controlled diabetes mellitus or severe hypertension.

9\) Known hypersensitivity to taxanes or their excipients. 10)Severe or uncontrolled infection. 11)History of psychoactive substance abuse that cannot be discontinued, or history of mental disorders.

12)Patients deemed by the investigator to be unsuitable for participation in this study.

13)Patient refusal to receive adjuvant dalpiciclib treatment.