Breast cancer is caused by the development of tumors in breast tissue. There are several different types of breast cancer, classified according to whether tumors begin in the lining, milk gland or connective tissue of the breast, and whether they are confined or invasive. According to the American Cancer Society, a single breast tumor may sometimes bear a mixture of these characteristics.
Non-Invasive Breast Cancers
Non-invasive, or "in situ," cancers are those in their early stages where the tumor is confined to the layer of cells where it originated. The most common type of non-invasive breast cancer is ductal carcinoma in situ, or DCIS. Because these tumors will likely be too small to feel upon physical examination, a mammogram is the best way to identify DCIS, according to the American Cancer Society (ACS). An aggressive form of DCIS called comedocarcinoma, characterized by the presence of dead or dying cancer cells, may be diagnosed through examination of a tissue sample.
The ACS notes that lobular carcinoma in situ--though not technically a cancer--is sometimes classified as a non-invasive type of breast cancer. It starts in the glands of the breast that produce milk and rarely becomes invasive.
Invasive Breast Cancers
Invasive breast cancers represent the majority of all breast cancers and are characterized by tumors that have grown beyond the original layer of cells where they originated. According to the National Breast Cancer Foundation, invasive or infiltrating ductal carcinoma is the most common type of breast cancer, accounting for approximately 78 percent of all breast malignancies. Infiltrating ductal carcinoma develops in a milk duct of the breast where it may then spread to fatty breast tissue and other parts of the body through the bloodstream and lymphatic system.
Another invasive breast cancer known as infiltrating lobular carcinoma starts in the glands that produce milk and can then metastasize throughout the body. This type of cancer can be relatively harder to detect through mammogram than infiltrating ductal carcinoma. The ACS estimates that 1 out of every 10 invasive breast cancers is an infiltrating lobular carcinoma.
Breast Carcinomas
Breast carcinomas are types of cancer that begin in the layer of cells lining the breast (called epithelial cells). Medullary carcinoma is a relatively uncommon type of invasive breast cancer--accounting for 3 to 5 percent of all breast cancers--with a better prognosis than other infiltrating types, according to the ACS. Papillary carcinomas may be either invasive or non-invasive, account for 1 to 2 percent of all breast cancers and tend to occur in older women. Equally rare, mucinous carcinoma is formed by mucous-producing cancer cells and usually has a favorable prognosis. Tubular carcinomas account for roughly 2 percent of all breast cancer diagnoses and have a 10-year survival rate of 95 percent, according to the National Breast Cancer Foundation.
Rare Types of Breast Cancer
Inflammatory breast cancer is a rare breast cancer that accounts for 1 to 5 percent of all breast cancer cases in the United States, according to the National Breast Cancer Foundation. It is an aggressive cancer in which the lymph vessels in the skin of the breast become blocked, causing the breasts to become red and swollen.
Other very rare types of breast cancer include phyllodes tumor--a cancer that develops in the connective tissue of the breast--and angiosarcoma--a cancer that starts in the cells lining blood or lymph vessels that seldom occurs in the breasts. According to the ACS, angiosarcoma usually develops as a rare complication 5 to 10 years after prior radiation treatment.
