NCI: Cancer that forms in tissues of the breast, usually the ducts (tubes that carry milk to the nipple) and lobules (glands that make milk). It occurs in both men and women, although male breast cancer is rare.,NCI: A carcinoma arising from the breast, most commonly the terminal ductal-lobular unit. It is the most common malignant tumor in females. Risk factors include country of birth, family history, menstrual and reproductive history, fibrocystic disease and epithelial hyperplasia, exogenous estrogens, contraceptive agents, and ionizing radiation. The vast majority of breast carcinomas are adenocarcinomas (ductal or lobular). Breast carcinoma spreads by direct invasion, by the lymphatic route, and by the blood vessel route. The most common site of lymph node involvement is the axilla.