The goal of this clinical trial is to evaluate the safety and feasibility of methylene blue single tracer-based extensive sentinel lymph node biopsy (ESLNB) and provide evidence for simplifying surgical procedures on the basis of ensuring axillary safety, as well as clarify the incidence of postoperative upper limb lymphedema in female patients with axillary node-positive breast cancer who achieve clinical axillary node negativity after neoadjuvant therapy. The main questions it aims to answer are:

• What is the false negative rate of methylene blue single tracer-based extensive sentinel lymph node biopsy in the above-mentioned breast cancer patients?
• What are the false negative rate and detection rate of methylene blue single tracer-based conventional sentinel lymph node biopsy
• What is the incidence of upper limb lymphedema within 2 years after surgery in these patients?

Participants will:

• Undergo strict screening to confirm eligibility for the trial and sign an informed consent form
• Receive a complete standard neoadjuvant therapy for 6 or 8 cycles as required
• Undergo personalized breast surgery combined with methylene blue single tracer-based ESLNB plus axillary lymph node dissection
• Accept pathological evaluation of the number of lymph nodes and metastatic lymph nodes in each resected part after surgery
• Receive adjuvant therapies such as targeted therapy, endocrine therapy or local radiotherapy in accordance with clinical guidelines
• Complete regular follow-up for 2 years after surgery (once every 6 months), including physical sign checks, surgical site evaluations, and upper limb circumference measurements to assess lymphedema

Participants will:

• Receive a full course of standard neoadjuvant therapy for 6 or 8 cycles as clinically indicated
• Undergo preoperative screening assessments to confirm clinical axillary node negativity after neoadjuvant therapy
• Have 0.5ml of methylene blue injected into the areola before surgery and then receive personalized breast surgery combined with extensive sentinel lymph node biopsy plus axillary lymph node dissection
• Undergo pathological testing to count the total number of resected lymph nodes and metastatic lymph nodes in different axillary regions
• Receive adjuvant treatments like targeted therapy, endocrine therapy or local radiotherapy following clinical practice guidelines
• Attend clinic follow-up visits every 6 months for 2 years after surgery, including physical exams, surgical site checks and upper limb circumference measurements
• Allow their clinical, surgical, pathological and follow-up data to be collected and entered into a dedicated research database Report any physical discomfort or adverse events that occur during the study period to the research medical team in a timely manner

Inclusion Criteria:

• Female treatment-naive breast cancer patients aged 18 to 70 years old.
• Histopathologically confirmed invasive breast cancer, either early-stage (T1-2N0-1M0, Stage Ⅰ-Ⅱ) or locally advanced (T3N0-1M0/T1-3N1M0, Stage ⅢA) as defined by the latest ASCO/CAP guidelines; initial clinical axillary stage cN0-1 with pathologically confirmed axillary lymph node positivity via needle biopsy.
• Voluntarily accept a complete course of standard neoadjuvant therapy for 6 or 8 cycles, and achieve clinical axillary node negativity (cN0) in axillary lymph node re-evaluation by ultrasound after completion of neoadjuvant therapy.
• Willing to undergo neoadjuvant therapy plus methylene blue single tracer-guided extensive sentinel lymph node biopsy combined with axillary lymph node dissection.
• Voluntarily participate in the study and sign the informed consent form.

Exclusion Criteria:

• Patients over 70 years old.
• Newly diagnosed metastatic breast cancer (Stage Ⅳ).
• Gestational breast cancer patients.
• Patients with abnormal function of vital organs (heart, lung, liver, kidney) or poorly controlled diabetes, who are unable to tolerate surgery.
• Other conditions that the investigator deems unsuitable for study participation.