A person’s treatment options and prognosis is dependent on the specific type and progression of their breast cancer. According to the American Cancer Society, the most common types are ductal carcinoma in situ, invasive ductal carcinoma, and invasive lobular carcinoma.

Determining Type

Most breast cancers are carcinomas, which start in the cells that line tissues and organs. More specifically, they are often adenocarcinomas, which start in the milk ducts or lobules of milk-producing glands. Less common forms are sarcomas, which start in the cells of connective tissue, fat, or muscle.

If the cancer is described as “in situ,” it means that it has not spread. If it is described as invasive or infiltrating, it means that the cancer has invaded the surrounding breast tissue.

Breast Cancer Grade

An important piece of information, a breast cancer’s grade determines how quickly it is likely to grow and spread. A grade is determined by checking the cancer cells under a microscope to see how much the cancer cells look like normal cells. A lower grade number typically means the cancer is slower-growing and less likely to spread. A higher grade number refers to a faster-growing cancer. The grade helps predict prognosis as well as helps determine which treatments may work best.

Most Common Types

Ductal carcinoma in situ (DCIS) is a non-invasive or pre-invasive breast cancer. Since DCIS has not spread, it is the easiest form of cancer to treat successfully.

Lobular carcinoma in situ (LCIS), while the name sounds like cancer, is not actually a cancer. In this type, cells that look like cancer cells grow in the lobules of the milk-producing glands but they do not spread through the lobular wall.

Invasive ductal carcinoma (IDC) is the most common type of breast cancer. It starts in a milk duct, spreads through the wall of the duct and invades the fatty tissue of the breast.

Invasive lobular carcinoma (ILC) starts in the lobules (milk-producing glands) and spreads into outside tissue.

There are also sub-types of invasive carcinoma, some of which may have a better or worse prognosis than standard invasive ductal carcinoma. These special types are often named after specific features that have been identified under the microscope. These sub-types include adenoid cystic carcinoma, low-grade adenosquamous carcinoma, medullary carcinoma, mucinous carcinoma, papillary carcinoma, tubular carcinoma, metaplastic carcinoma, micropapillary carcinoma, and mixed carcinoma (which has features of both ILC and IDC).

Less Common Forms

There are a few types of breast cancer that occur but are more uncommon. Inflammatory breast cancer (invasive) accounts for approximately one to three percent of all breast cancers. Another, rarer, type of breast cancer is Paget disease of the nipple, which starts in the breast ducts and spreads to the skin of the nipple and areola. There is also Phyllodes tumor, which are rare breast tumors which develop in the stroma (connective tissue) of the breast. Finally, there is angiosarcoma, which starts in the cells that line lymph vessels or blood vessels, but rarely occurs in the breasts.