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...