Learn The BasicsThere are different types of breast cancer and various ways of classifying them.
These classification methods can be based on the cell types that are involved, how cancer cells appear under a microscope, whether cancer cells test positive or negative for certain hormones and whether they make extra human epidermal growth factor-receptor 2 (HER2) protein.
Diagnosing breast cancer by cell type
Breast cancer can be classified based on the type of cells that are involved and their location.
Most breast cancers are carcinomas, meaning they start in the cells that line the ducts or lobules. The most common histologic types are:
- Ductal carcinoma, including ductal carcinoma in situ and invasive ductal carcinoma.
- Lobular carcinoma, including lobular carcinoma in situ and invasive lobular carcinoma).
In ductal carcinoma, cancer starts in cells found in the lining of a milk ducts, which carry breast milk to the nipple. This is the most common type of breast cancer.
Lobular carcinoma, on the other hand, starts in the lobules of the breast, which make breast milk.
Rarely, malignant tumors can arise from other tissues in the breast, such as breast sarcomas and primary breast lymphoma
In soft tissue sarcoma of the breast, the cancer is made up of cells of the connective tissue, which includes the muscles, fat and blood vessels. This type of breast cancer is rare and includes angiosarcoma and leiomyosarcoma.
Finally, primary breast lymphoma is also rare and originates in the breast’s lymphatic system. Most breast lymphomas are a form of non-Hodgkin lymphoma.
Breast cancer types by microscopic appearance
Sometimes, breast cancer is classified based on the way cancer cells look under the microscope. These include tubular breast cancer, mucinous breast cancer, medullary breast cancer and micropapillary breast cancer.
The appearance of cancer cells under the microscope can also determine the grade of breast cancer. According to this, grade 1 breast cancer is when cancer cells look a little different from healthy cells, whereas grade 3 breast cancer is when cancer cells look very different from healthy cells.
Breast cancer types by response to hormonal stimulation
Breast cancer can also be classified based on the hormone receptors that they express. Some types of breast cancer express estrogen receptor (ER) on their surface, while others express progesterone receptor (PR) and some do not express either hormone receptor on their surface.
A single tumor can test positive for multiple receptors at once. A tumor may be classified as ER-positive and PR-positive breast cancer, ER-positive and PR-negative breast cancer, ER-negative and PR-positive breast cancer or hormone receptor-negative breast cancer.
Hormone receptor-positive breast cancer is likely to respond to hormone therapy, while the treatment is not effective in hormone receptor-negative breast cancer.
Breast cancer types by HER2 expression
Some breast cancer cells make too much of a protein called HER2. Based on this, breast cancer can be classified into three groups: HER2-positive, HER2-low, and HER2-negative.
In HER2-positive breast cancer, the cancer cells make a lot of extra HER2 protein, while in HER2-low breast cancer, they produce small amounts of HER2, but not enough to be considered HER2-positive.
In HER2-negative breast cancer, cancer cells do not make extra HER2.
Common types of breast cancer
One of the most common types of breast cancer is ductal carcinoma in situ (DCIS), which is non-invasive and confined to the breast ducts. This type of breast cancer is sometimes also called noninvasive breast cancer or stage 0 breast cancer.
Invasive ductal carcinoma also starts in the breast ducts and is the most common type of breast cancer. This type spreads to other parts of the breast tissue.
Invasive lobular carcinoma is also quite common and spreads to the breast tissue from the lobules.
HER2-positive breast cancer is also common and invasive. Targeted therapies are usually very effective against this type.
Triple-negative breast cancer has no receptors for HER2, estrogen and progesterone. This is a type that is less common and that is more difficult to treat, as therapies that target HER2 or hormone therapy are not effective.
Less common types of breast cancer
There are also rarer types of breast cancer, which include inflammatory breast cancer, Paget’s disease of the breast, medullary breast cancer, mucinous breast cancer, tubular breast cancer, adenoid cystic carcinoma of the breast, metaplastic BC, basal type BC, papillary breast cancer and angiosarcoma of the breast.
Inflammatory breast cancer is characterized by cancer cells often blocking the small lymph vessels in the breast that are part of the lymphatic system, causing the skin of the breast to become red and inflamed.
Paget’s disease of the breast is a rare condition affecting the nipple and the areola, which may be associated with cancer in the tissue behind it.
Medullary breast cancer is a rare and invasive type of BC, which is soft, fleshy and grey, resembling the part of the brain called the medulla, from which it takes its name.
Mucinous breast cancer is made of a large amount of mucin, a component of mucus. This type of BC usually occurs in older women and grows more slowly.
In tubular breast cancer, cancer cells look like tubes under a microscope. This type usually occurs in older women and sometimes spreads to nearby lymph nodes.
Adenoid cystic carcinoma of the breast is usually diagnosed in the salivary glands but can also develop in the breast tissue.
Metaplastic breast cancer is a rare but aggressive form. It looks like a mix of different cancer types, probably because it began in very young cells that had the ability to develop into different types of mature cells.
Basal type breast cancer is characterized by particular genetic changes in the cells, leading them to make large amounts of a protein called cytokeratin 5/6.
Papillary breast cancer can refer to other types of breast cancer, including invasive papillary breast cancer, invasive micropapillary breast cancer, intracystic/encapsulated/encysted papillary cancer and papillary ductal carcinoma in situ.
Finally, angiosarcoma of the breast is a cancer that occurs in the breast and in the skin of the arms and that develops in the blood or lymphatic vessels in the breast. It is a very rare type that represents 0.1% to 0.2% of all breast cancer cases. It is a type of cancer that can grow and spread quickly to the rest of the body.