Every case of metastatic breast cancer is different. When deciding on what treatment approach to use, a healthcare team will gather as much information as possible about the cancer. One piece of information that they will test for is HER2 status.
HER2 refers to a protein called “human epidermal growth factor receptor 2.” The body uses this protein to grow and repair cells that make up breast tissue. However, some cancer cells produce excess amounts of HER2, which they use to multiply and spread to other areas of the body. Roughly 20 percent of cases of breast cancer are HER2-positive, meaning they are producing excess amounts of HER2.
There are two commonly used tests that can help determine if a cancer is HER2-positive.
IHC test
The more commonly used test is called the IHC test (ImmunoHistoChemistry test). It is usually the first test used to determine HER2 status. To perform this test, a healthcare provider will collect a tissue sample from a breast tumor with a biopsy. The sample is mixed with an antibody solution that binds to HER2 proteins. The solution is rinsed away, and the sample is examined under a microscope to see how many antibodies attached to proteins. Results are given on a scale of 0 to 3:
- A score of 0 to 1 means HER2-negative
- A score of 2 is HER2 borderline
- A score of 3+ is HER-positive
FISH test
There is also a test called the FISH test (Fluorescence In Situ Hybridization test). This test is generally considered to be more accurate than the IHC test, but costs more and results take longer. It also requires specialized equipment and involves a complex lab process, and as a result it may not be available everywhere. It is often ordered as a follow up to an IHC test with a borderline result. A biopsy is performed to take a sample of a breast tumor. The sample is used to create a visual map of the genetic material inside the cancer cells. This visual map is used to identify copies of the HER2 gene—the gene that provides instructions for making HER2 proteins. Based on the number of copies of the HER2 gene, results are determined to be positive or negative.
Targeted therapies for HER2-positive cancer
There are a number of cancer drugs that can target HER2-positive breast cancer cells. These cancer drugs are called targeted therapies or HER2-targeted therapies. These are often used in combination with chemotherapy. They can also be used in combination with endocrine therapy drugs, which may be used if a cancer is also hormone-receptor positive. People with MBC that is HER2-positive may benefit from treatment with these therapies, and they should discuss these treatment options with their healthcare providers.
Work with your healthcare team
It is important to understand that the results of HER2 testing are only one piece of information, and that testing for HER2 has its limitations—different tumor sites can have different HER2 statuses, tests results can be borderline, and HER2 status can change over time or as a result of treatment. Your healthcare providers are your best source for information about your cancer and your treatment options.
