Breast cancer is a malignant tumour that starts in the cells of the breast. Malignant means that it can spread, or metastasize, to other parts of the body.
Cells in the breast sometimes change and no longer grow or behave normally. These changes may lead to benign breast conditions such as atypical hyperplasia and cysts. They can also lead to benign tumours such as intraductal papillomas. Benign conditions and tumours are not cancerous. But in some cases, changes to breast cells can cause breast cancer.
Most often, breast cancer starts in cells that line the ducts, which are the tubes that carry milk from the glands to the nipple. This type of breast cancer is called ductal carcinoma. Cancer can also start in the cells of the lobules, which are the groups of glands that make milk. This type of cancer is called lobular carcinoma. Both ductal carcinoma and lobular carcinoma can be in situ, which means that the cancer is still where it started and has not grown into surrounding tissues. They can also be invasive, which means they have grown into surrounding tissues.
Less common types of breast cancer can also develop. These include inflammatory breast cancer, Paget disease of the nipple and triple negative and basal-like breast cancers. Rare types of breast cancer include non-Hodgkin lymphoma and sarcoma.