Types of Breast Cancer

There are several ways to categorize different kinds of breast cancers. Each helps to give a better understanding of how a specific breast cancer behaves and what treatments are most likely to be effective. Over the last 30 years, there has been a shift in how cancer doctors and researchers think about and categorize breast cancers. In the 1980s, breast cancers were primarily described in terms of tumor size and extent of the disease. By the 2000s, the tumor’s receptor status and molecular subtype became more meaningful ways to group breast cancers and recommend treatment. There is ongoing interest in understanding different subtypes and new ways to distinguish and categorize breast cancer. Currently, the common ways that breast cancers are categorized include:

The breast is a complex organ with specialized tissue and structure that includes milk-producing glands called lobules, and a system of ducts that transport milk to the nipple. Breast cancer can begin in any of these different areas of the breast.

Most breast cancers originate in the milk ducts: 80% of all breast cancers are invasive ductal carcinoma (IDC), also sometimes called infiltrating ductal carcinoma. Invasive lobular carcinoma (ILC), sometimes called infiltrating lobular carcinoma, is the second most common type of breast cancer, representing about 10% of all breast cancers.

There are a number of much less common forms that each account for a small percentage of breast cancers. These may start in the cells of the chest muscle, fat, or connective tissue, and include inflammatory breast cancer, Paget’s disease, sarcomas, phyllodes tumors, and angiosarcomas. Each of these rare forms is distinct and should be treated by a specialist in this subtype of breast cancer.

Doctors evaluate the size of the tumor and extent of the disease and use four stages to indicate how far it has progressed. Stage 1 indicates the cancer cells are confined to an area of less than two centimeters, and the lymph nodes are not involved. Stage 2 indicates some evidence that the cancer has begun to grow larger than two centimeters or has spread to the lymph nodes, but it is still considered relatively early-stage. Stage 3 indicates there is evidence that the cancer has begun to invade the surrounding tissues near the breast. Stage 4, or metastatic breast cancer, indicates that the cancer has spread to other areas of the body.

Cancer grade is a description of how the cancer cells look under a microscope, and a measure of how quickly the cells might grow or spread. Breast cancer is graded on a pathology report scale from 1 to 3, with a lower grade indicating less aggressive cancer. Pathologists examine the cancer cells to determine how different they are in appearance and growth from normal, healthy cells. Although higher grade cancers are considered more aggressive, they may be very responsive to chemotherapy and radiation treatments that target fast-dividing cells.

Note: Historically, the term Stage 0 has been used to describe carcinoma in situ. When breast cancer cells are found only in the ducts or lobules of the breast, and have not traveled into the tissues around them, the cancer is called “in situ.” If the cells are in the breast ducts, the cancer is called DCIS, or ductal carcinoma in situ; if they are in the lobules, the cancer is called LCIS, or lobular carcinoma in situ. In the past, DCIS and LCIS have been considered to be non-invasive cancers. More recently, because these atypical cells have not spread outside of the ducts or lobules into the surrounding breast tissue, some leading doctors and researchers argue that this is not in fact a true cancer and the term carcinoma should be dropped. Instead, in situ lesions (DCIS or LCIS) are better understood as a risk factor for possibly developing invasive breast cancer.

The majority of breast cancers are hormone positive, which means they grow in response to hormones. Up to 80% of all breast cancers are estrogen receptor positive, often called simply ER-positive or ER+, which means the cancer cells grow in response to the hormone estrogen. About 65% of these are also progesterone receptor positive, or PR-positive (PR+), and grow in response to another hormone, progester- one. These cancers are usually treated with hormone therapies in addition to surgery and radiation.

In about 20% of breast cancers, the cells overexpress, or make too many copies of, a protein known as HER2. In the mid 1990s the development of the drug trastuzumab (brand name Herceptin) represented a major breakthrough in the treatment of these aggressive, fast-growing cancers. Since then several newer targeted treatments have been developed to also target HER2+ breast cancers.

Just 10 to 20% of breast cancers are known as “triple negative” because they don’t have estrogen receptors or progesterone receptors and don’t overexpress the HER2 protein. In some cases, triple negative breast cancers are associated with inherited BRCA mutations. They are typically treated with chemotherapy, and there are no targeted treatments for these cancers because they are neither hormone positive or HER2 positive. But recently (2019), the FDA approved the first immunotherapy drug for triple negative breast cancer.

Molecular subtyping is mostly used in research settings as another tool to help guide researchers in studying new treatments. If you attend research conferences, you may hear about Luminal A, Luminal B, basal-like, HER2-enriched and normal-like breast cancers. The molecular subtype is related to HER2 and estrogen positivity, but helps researchers describe more deeply how the cancer might behave.