To understand triple-negative breast cancer, it’s important to understand receptors, which are proteins found inside and on the surface of cells. These receptor proteins are the “eyes” and “ears” of the cells, receiving messages from substances in the bloodstream and then telling the cells what to do.
- Hormone receptors inside and on the surface of healthy breast cells receive messages from the hormones estrogen and progesterone. The hormones attach to the receptors and provide instructions that help the cells continue to grow and function well. Most, but not all, breast cancer cells also have these hormone receptors. Roughly 2 out of 3 women have breast cancer that tests positive for hormone receptors. (For a more complete explanation, see the previous section on Hormone Receptor Status.)
- A smaller percentage of breast cancers — about 20-30% — have too many HER2 receptors. In normal, healthy breast cells, HER2 receptors receive signals that stimulate their growth. With too many HER2 receptors, however, breast cancer cells grow and divide too quickly. (For a more complete explanation, see the previous section on HER2 Status.)
Hormonal therapies and HER2-targeted therapies work to interfere with the effects of hormones and HER2 on breast cancer, which can help slow or even stop the growth of breast cancer cells.
About 10-20% of breast cancers test negative for both hormone receptors and HER2 in the lab, which means they are triple-negative. Since hormones are not supporting its growth, the cancer is unlikely to respond to hormonal therapies, including tamoxifen, Arimidex (chemical name: anastrozole), Aromasin (chemical name: exemestane), Femara (chemical name: letrozole), and Faslodex (chemical name: fulvestrant). Triple-negative breast cancer also is unlikely to respond to medications that target HER2, such as Herceptin (chemical name: trastuzumab) or Tykerb (chemical name: lapatinib)
The Biology of Triple Negative Breast Cancer
In addition, triple-negative breast cancer
Tends to be more aggressive than other types of breast cancer. Studies have shown that triple-negative breast cancer is more likely to spread beyond the breast and more likely to recur (come back) after treatment. These risks appear to be greatest in the first few years after treatment. For example, a study of more than 1,600 women in Canada published in 2007 found that women with triple-negative breast cancer were at higher risk of having the cancer recur outside the breast — but only for the first 3 years. Other studies have reached similar conclusions. As years go by, the risks of the triple-negative breast cancer recurring become similar to those risk levels for other types of breast cancer.
- Tends to be higher grade than other types of breast cancer. The higher the grade, the less the cancer cells resemble normal, healthy breast cells in their appearance and growth patterns. On a scale of 1 to 3, triple-negative breast cancer often is grade 3.
- Usually is a cell type called “basal-like.” “Basal-like” means that the cells resemble the basal cells that line the breast ducts. This is a new subtype of breast cancer that researchers have identified using gene analysis technology. Like other types of breast cancer, basal-like cancers can be linked to family history, or they can happen without any apparent family link. Basal-like cancers tend to be more aggressive, higher grade cancers — just like triple-negative breast cancers. It’s believed that most triple-negative breast cancers are of the basal-like cell type.