The OMSLNB trial adopts a prospective, single-arm, non-inferiority, phase-II, open-label study design. Patients with unilateral invasive breast cancer （tumor ≤3cm） and assessed as cN0 will be eligible for inclusion, enrolled patients are required to complete 2 or more imaging tests including axillary ultrasound assessed as axillary lymph node negative, and other tests including MRI, PET-CT, [18F]-FDG PET-MRI, eligible patients will avoid axillary surgery but will undergo breast surgery, which is not limited to breast-conserving surgery (BCS). So as to decrease the edema of upper arm, and finally improve the quality of life of the patients.

Inclusion Criteria:

1. Female patients aged 18-70 years;
2. Pathologically confirmed invasive breast cancer (regardless of pathological type) with a tumor diameter ≤ 3 cm, and planning to undergo breast surgery;
3. Negative axillary lymph nodes assessed by physical examination and imaging (2 or more of the following tests including ultrasound, breast MRI, breast PET-CT, breast PET-MRI, breast PET);
4. All patients are required to undergo immunohistochemical staining for Estrogen Receptor (ER), Progesterone Receptor (PR), human epidermal growth factor receptor 2 (HER2), Ki-67 proliferation index, and further fluorescence in situ hybridization (FISH) should be performed in HER2 2+ cases;
5. Good compliance, normal comprehension and ability to receive treatment and follow-up as required;
6. ECOG score 0-1;
7. Patients volunteered for this study and signed the informed consent form.

Exclusion Criteria:

1. Bilateral/lactating/pregnant breast cancer;
2. Previous history of malignant tumor or neoplasm;
3. Clinical or imaging confirmation of distant metastasis;
4. History of previous surgery on the affected axilla; or history of surgery affecting the function of the upper extremity;
5. Prior history of radiotherapy to the breast or chest;
6. Positive pathological margins after breast-conserving surgery or mastectomy;
7. Severe coagulation disorder, or a serious systemic disease, or an uncontrollable infection;
8. Aspartate aminortransferse (AST) and Alanine aminotransferase (ALT) ≥ 1.5 times the upper limit of normal, alkaline phosphatase ≥ 2.5 times the upper limit of normal, total bilirubin ≥ 1.5 times the upper limit of normal, serum creatinine ≥ 1.5 times the upper limit of normal; left ventricular injection fraction (LVEF) < 50% on cardiac ultrasound;
9. Inability to complete the full course of follow-up adjuvant therapy as prescribed by the doctor for various reasons;
10. No personal freedom and independent civil capacity;
11. Presence of mental disorders, addictions, etc;
12. Not eligible for enrollmentas as judged by the investigator.