Breast cancer. Light micrograph of a section through human breast tissue showing a carcinoma (large mass, centre) arising from the the milk ducts. This is known as a ductal carcinoma. Carcinomas are malignant (cancerous) tumours that arise from epithelial (cavity lining) cells. Here the cancer has caused the mammary lobules to increase in size and has spread to the surrounding tissue. This is now called an infiltrating lobular carcinoma. Breast cancer is the most common type of cancer in women, and most breast cancers are ductal.