Breast cancer arises from the lining of the milk ducts or from the lobules that produce milk. The early state of the disease is called ductal carcinoma in situ (or lobular carcinoma in situ) and is a pre-cancerous condition curable by local excision. Two sub-types of breast cancer are commonly recognised; ductal breast cancer which is the commonest form (75% of all cases) arising from the ductal cells and lobular cancer (10% of all cases) arising from the milk duct lobules. Rarer breast cancers include medullary cancer, colloid cancer, papillary cancer, mucinous cancer and cribiform cancer.
In addition to the description of the type of breast cancer, the pathologist can also provide a classification of aggressiveness (grade 1 to 3), the presence of hormone receptors and other molecular markers such as the oncogene HER2.