In breast cancer, HER2 testing looks at a tumor tissue sample to see whether the cancer cells overexpress the HER2 protein. If so, the cancer may grow and spread faster and be more likely to return after treatment.
What is HER2?
The HER2 gene creates the HER2 protein, which helps control cell growth and repair. When cancer cells overexpress HER2, they grow faster and can spread. The first targeted therapy for patients with HER2-positive breast cancer tests positive was introduced in 1998. Since then, further research has improved treatments and outcomes for patients with HER2-positive tumors. Your doctor can determine your HER2 status by looking at tissue from your biopsy under a microscope. One immunohistochemistry (IHC) test highlights HER2 proteins with a chemical dye, and your cancer is scored (0 to 3+). Another test, fluorescence in situ hybridization (FISH), tests for HER2 amplification by looking for extra copies of the HER2 gene.
HER2-negative breast cancers are less likely to respond to hormone-blocking drugs, but some may benefit from treatment with HER2-targeted therapies. Your doctor will also test for estrogen and progesterone receptors in your tumor to see if it is HR-positive or HR-/HER2-, which predicts how well you might respond to hormone therapies. A study of HER2 testing in women with early-stage breast cancer found that pathology reports missing HER2 results were more common in lower-income neighborhoods and urban areas.
Still, these differences likely reflect variations in system factors such as electronic pathology reporting and cancer registry processes rather than access to HER2 testing. Improved HER2 documentation is needed for cancer registration, quality of care measurement, and program evaluation.
A HER2 test determines whether cancer is sensitive to trastuzumab (Herceptin) treatment or other drugs targeting the protein. It may be tested by immunohistochemistry (IHC) or fluorescence in situ hybridization (FISH). IHC and FISH tests are usually performed on tumor tissue removed from a biopsy. They can only be accurate if the tumor is completely removed or the biopsy sample contains a small number of non-tumor cells. Non-tumor cells include lymphocytes and normal breast tissue. HER2 staining may be less accurate in these cases and can sometimes be confused with other breast cancer cell staining types.
In addition, HER2 testing can be affected by where a biopsy is taken in the tumor. Different tumor parts can have different characteristics, affecting HER2 test results. This is known as tumor heterogeneity. Inaccurate HER2 testing can delay or restrict access to HER2-targeted therapy for patients who would benefit significantly from it. To reduce inaccuracy, the ASCO/CAP guidelines recommend retesting with an alternative assay and strict interpretation criteria for HER2 test results that are equivocal (HER2 overexpression or gene amplification but not 3+ IHC). HER2 testing is also recommended for all patients with grade 3 invasive breast cancer, even if ER and progesterone receptor (ER and PR) testing is negative.
In about 15% to 20% of breast cancers, the cells make too much of a protein called HER2. This type of cancer is called HER2-positive. It grows faster and spreads more quickly than other types of breast cancer. It also is more likely to come back (recur). Treatment that targets the HER2 protein can help prevent or delay the return of cancer. Researchers found a way to target the HER2 protein in the 1990s when they developed a drug. The drug works by blocking the cancer cells from receiving growth signals. It is taken by mouth or intravenously.
It is part of standard breast cancer treatment for people with HER2-positive disease. Besides targeted therapy, other types of breast cancer treatment include surgery to remove the tumor and reconstruction that surgically recreates the shape and function of the breast after the removal. We also offer chemotherapy, radiation, and other hormone therapies to destroy any remaining cancer cells and lower your risk of the tumor growing again.
If you have HER2-positive cancer, your doctor will ask whether you want to monitor your blood levels of HER2. You may be able to take a blood test to find out how your body is responding to treatment. HER2 testing can be done by a simple blood test or by biopsy. Blood tests can also help your doctor track the response of your tumor to chemotherapy. Breast cancers that are HER2-positive tend to grow faster, spread (metastasize), and come back (recur) more often than those that aren’t. But they also respond better to treatment that targets the HER2 protein.
A new, quick way to check if a patient’s cancer is responding to treatment involves measuring the level of HER2 in the patient’s blood rather than looking at samples from their biopsy. The researchers compared the results from a new blood test for HER2 with those of an older standard tumor marker, CA15-3. The results showed a strong correlation between the two markers, and both were better at monitoring clinical responses to therapy than CA15-3. The correlation was more potent when the patients were grouped by their tissue HER2 status.