The types of breast cancer can refer to a few different things. Doctors can classify breast cancer based on whether or not it is invasive (meaning it has spread to surrounding breast tissue), whether or not it begins in the milk ducts, whether or not it is carcinoma (meaning it begins in the lining of the tissues lining internal organs), and more.
Another way to categorize breast cancer is by receptor status. “There are different subtypes of breast cancer based on what receptors are expressed on the surface of the breast cancer cell. There are hormone receptors, estrogen and progesterone, and then there’s a protein called HER2,” says Amy Tiersten, MD, oncologist at Dubin Breast Center at Mount Sinai Hospital in New York City.
Receptors on breast cancer cells help fuel the growth of cancer tumors. This is important, because if doctors know what’s helping cancer grow, they can use that information to choose a treatment to halt that growth.
“Breast cancers that express either the estrogen receptor or the progesterone receptor on the surface of the cell are considered hormone receptor-positive breast cancers,” says Dr. Tiersten. “Those tend to be more slow-growing and less aggressive types of breast cancer.”
Whether the receptor is estrogen or progesterone, doctors can use anti-estrogen medication (which is called hormone therapy) to treat the cancer and lower the risk of breast cancer recurrence.
HER2-positive breast cancer means the cancer cells contain a receptor for the HER2 protein (sometimes called HER2/neu). This is the case for about 25 percent of all breast cancers.
HER2 stands for human epidermal growth factor receptor 2, and it’s a growth-promoting protein on the outside of all breast cells. However, someone with HER2-positive breast cancer has higher than normal levels of the protein on the cancer cells.
HER2-positive breast cancers “are cancers that used to be considered very aggressive, but nowadays, it’s considered a good thing to have HER2-positive breast cancer because we have incredibly effective, targeted antibodies to that HER2 protein,” says Dr. Tiersten.
Triple-negative breast cancers contain none of these receptors, and they account for 15 to 20 percent of breast cancer diagnoses. Because they lack the known receptors for breast cancer, neither hormone therapy nor targeted therapy is effective against triple-negative breast cancer.
“Triple-negative breast cancers are inherently more aggressive,” says Dr. Tiersten. “At the present time, all we really have for triple-negative breast cancer is your standard traditional chemotherapy.” That said, chemotherapy tends to work more effectively in triple-negative breast cancer than for other types of breast cancer.
Triple-positive breast cancer means the breast cancer cells contain all three receptors. They respond to all of the treatments that target receptors.
“The entire prognosis and treatment plan is completely based on the subtype of breast cancer,” says Dr. Tiersten. “Years ago, we thought of breast cancer as one disease, but biology has moved so far forward that we really tailor the treatment to the specific type of breast cancer.”