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, or intraduct, carcinoma. Carcinomas are malignant (cancerous) tumours that arise from epithelial (cavity lining) cells. Breast cancer is the most common type of cancer in women, and most breast cancers are ductal. The cancer can invade surrounding tissue and spread to other parts of the body (metastasis).