NCI: The most common type of invasive breast cancer. It starts in the cells that line the milk ducts in the breast, grows outside the ducts, and often spreads to the lymph nodes.,MSH: An invasive (infiltrating) CARCINOMA of the mammary ductal system (MAMMARY GLANDS) in the human BREAST.,NCI: The most common type of invasive breast carcinoma, accounting for approximately 70% of breast carcinomas. The gross appearance is usually typical with an irregular stellate outline. Microscopically, randomly arranged epithelial elements are seen. When large sheets of malignant cells are present, necrosis may be seen. With adequate tissue sampling, in situ carcinoma can be demonstrated in association with the infiltrating carcinoma. The in situ component is nearly always ductal but occasionally may be lobular or both.