Clinical meaning
Breast cancer develops when cells in the breast undergo malignant transformation through accumulated genetic mutations that disable normal cell cycle regulation. The breast is composed of 15-20 lobes arranged in a circular pattern around the nipple, each containing lobules (milk-producing glands) connected by ducts that converge at the nipple. The majority of breast cancers arise from the epithelial cells lining these ducts (ductal carcinoma, approximately 70-80% of cases) or from the lobules (lobular carcinoma, approximately 10-15%). At the cellular level, cancer development follows a multi-step progression model. Ductal carcinoma in situ (DCIS) represents a pre-invasive stage where malignant cells proliferate within the duct but have not penetrated the basement membrane. Once tumor cells breach the basement membrane and invade the surrounding stroma, the cancer is classified as invasive (infiltrating) ductal carcinoma. The invasion process requires the tumor cells to secrete matrix metalloproteinases (MMPs) that degrade the extracellular matrix, allowing cellular migration and eventually metastasis through lymphatic and hematogenous spread. Breast cancer cells are classified by receptor status, which is critical for treatment selection. Estrogen receptor-positive (ER+) tumors have receptors that bind estrogen, which stimulates cell proliferation; approximately 70% of breast cancers are ER+. Progesterone receptor-positive (PR+) tumors similarly respond to progesterone stimulation. HER2-positive tumors overexpress the human epidermal growth factor receptor 2 protein, which drives aggressive cell growth through the RAS-MAPK and PI3K-AKT signaling pathways; approximately 15-20% of breast cancers are HER2+. Triple-negative breast cancer (TNBC) lacks all three receptors (ER-, PR-, HER2-), limiting treatment options to chemotherapy alone and carrying a generally poorer prognosis. The BRCA1 and BRCA2 genes are tumor suppressor genes involved in DNA double-strand break repair through homologous recombination. Mutations in these genes significantly increase lifetime breast cancer risk: BRCA1 mutation carriers have a 55-72% lifetime risk, and BRCA2 carriers have a 45-69% lifetime risk. TNM staging classifies tumors by size (T), lymph node involvement (N), and distant metastasis (M), with higher stages indicating more advanced disease and generally poorer prognosis. Sentinel lymph node biopsy identifies the first lymph node(s) to receive drainage from the tumor site, allowing assessment of regional spread while minimizing the surgical morbidity associated with full axillary lymph node dissection.