Breast cancer is the most commonly diagnosed malignancy in women globally and in China, classified by histology (invasive ductal carcinoma, 70-80% of cases; invasive lobular carcinoma, 10-15%) and molecular subtypes: Luminal A, Luminal B, HER2-positive, and triple-negative breast cancer (TNBC).

• Etiology: Key risk factors include prolonged estrogen exposure (early menarche, late menopause, nulliparity, late first birth, no breastfeeding), germline BRCA1/2 mutations, family history, obesity, excessive alcohol intake, and hormone replacement therapy. Over 70% of Chinese women have dense breast tissue, which reduces the sensitivity of mammography for early detection.
• Clinical Manifestations: The most common presentation is a painless, palpable breast lump. Additional signs include nipple discharge, retraction, skin dimpling/peau d'orange change, axillary lymphadenopathy, and breast pain. Advanced disease presents with bone pain, dyspnea, weight loss, and neurological deficits from distant metastases.
• Diagnosis: Breast ultrasound is the first-line screening and diagnostic modality in China, optimized for dense breast tissue. Mammography, breast MRI, and image-guided core needle biopsy (the gold standard for histopathological diagnosis) are used for further evaluation. Mandatory testing includes hormone receptor (ER/PR), HER2, Ki-67, and germline BRCA testing for high-risk patients.

• Early-Stage Breast Cancer (Stage I-II): Surgical resection is the curative foundation, with breast-conserving surgery (BCS) plus whole-breast radiotherapy, or total mastectomy with sentinel lymph node biopsy (SLNB) to avoid unnecessary axillary lymph node dissection (ALND). Adjuvant therapy is stratified by subtype: endocrine therapy for Luminal disease, anti-HER2 targeted therapy plus chemotherapy for HER2-positive disease, and chemotherapy with or without immunotherapy for high-risk TNBC.
• Locally Advanced Breast Cancer (Stage III): Neoadjuvant systemic therapy (chemotherapy, targeted therapy, endocrine therapy, or immunotherapy) followed by surgery and adjuvant therapy is the standard of care to improve resectability and breast conservation rates.
• Advanced/Metastatic Breast Cancer (Stage IV): Systemic therapy is the core of management. For Luminal disease, endocrine therapy combined with CDK4/6 inhibitors is first-line. For HER2-positive disease, dual anti-HER2 targeted therapy with chemotherapy is standard. For TNBC, chemotherapy combined with ICIs (for PD-L1-positive disease) or antibody-drug conjugates (ADCs) is first-line. Local radiotherapy and surgery are used for palliative symptom control.