Most types of breast cancer produce similar symptoms and are discovered the same way—finding a strange new lump during a self-exam, or a healthcare provider spotting something suspicious during a mammogram. Yet on a deeper level, different types of breast cancer can behave very differently from one another. Usually, only a breast biopsy—when a tissue sample from a breast tumor is sent to the lab—can determine what specific type of breast cancer (or subtype) you have, as well as your breast cancer stage—meaning how far the cancer has already spread. These details will help determine your best treatment options and overall prognosis.
It’s crucial to get a biopsy if, like most people, you have invasive breast cancer, meaning the cancer has already spread beyond milk ducts or glands into the surrounding breast tissue. One key piece of information contained in the pathology report from your biopsy will be your HER2 status, which indicates whether your breast tumor tissue sample came up as HER2-positive or HER2-negative. The specific test within the report that determines your HER2 status is called an ImmunoHistoChemistry (IHC test), and it reveals to what extent your cancer cells have two types of receptor—HER2 receptors and/or hormone receptors—on their surface.
High levels of HER2 receptors (an IHC test score of 3+) are what lead to a diagnosis of HER2-positive breast cancer. This type of breast cancer results from “overexpression” (elevated levels) of a protein known as HER2 (human epidermal growth factor receptor) due to a mutation in the HER2 gene. Under normal circumstances, the HER2 protein aids in cell growth, but when is is over-abundant, it can spur growth of breast cancer cells. Representing about 20 percent of all breast cancer cases, HER2-positive breast cancer tends to be aggressive and spread quickly, as well as recur years after successful treatment. That said, it also usually responds well to highly effective treatments that work by interfering with production of HER2 proteins. Known as targeted therapies, HER2 inhibitors, or anti-HER2 therapies, these treatment options often have less severe side effects than other breast cancer treatments like standard chemotherapy.
That said, most breast cancers—close to 80 percent—are HER2-negative, meaning that the tissue sample sent out for biopsy contains little to no evidence of HER2 proteins (an IHC test score of 0 to 1+). When breast cancer is diagnosed as HER2-negative, the aforementioned targeted therapies are not an effective treatment option. HER2 status isn’t always clear-cut, however. Sometimes, biopsies reveal that breast cancer is “borderline” HER2 positive (an IHC test score of 2+). In this case, your oncology provider will likely request that your tissue sample be reevaluated using a method called a FISH test (short for “fluorescence in situ hybridization”) to clarify whether your cancer is HER2-positive or HER2-negative. Other biopsies have a result of HER2-low, which means that evidence of HER proteins was detected, but at low levels. HER2-low is a newly designated type of breast cancer that’s still being studied, but research has shown that people with this diagnosis may still respond well to certain HER2-targeted therapies, specifically the drug trastuzumab deruxtecan.
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