Fluorescence in situ hybridization (FISH) is a test that “maps” the genetic material in a person’s cells. This test can be used to visualize specific genes or portions of genes. FISH testing is done on breast cancer tissue removed during biopsy to see if the cells have extra copies of the HER2 gene. The more copies of the HER2 gene that are present, the more HER2 receptors the cells have. These HER2 receptors receive signals that stimulate the growth of breast cancer cells.
The FISH test results will tell you that the cancer is either “positive” or “negative” (a result sometimes reported as “zero”) for HER2.
Generally, the FISH test is not as widely available as another method of HER2 testing, called ImmunoHistoChemistry, or IHC. However, FISH is considered more accurate. In many cases, a lab will do the IHC test first, ordering FISH only if the IHC results don’t clearly show whether the cells are HER2-positive or negative.
Research has shown that some HER2 test results may be wrong. This is probably because different labs have different rules for classifying positive and negative HER2 status. Each pathologist also may use slightly different criteria to decide whether the results are positive or negative. In most cases, this happens when the test results are borderline -- meaning they aren't strongly HER2-positive or HER2-negative.
In other cases, tissue from one area of a breast cancer can test HER2-positive and tissue from a different area of the cancer can test HER2-negative.
Inaccurate HER2 test results may cause women diagnosed with breast cancer to get less than the best care possible. If all or part of a breast cancer is HER2-positive but test results classify it as HER2-negative, doctors aren't likely to recommend medicines that work against HER2-positive breast cancers -- even though the woman could potentially benefit from those medicines. If a breast cancer is HER2-negative but test results classify it as HER2-positive, doctors may recommend anti-HER2 treatments -- even though the woman is unlikely to get any benefits and is exposed to the medicines' risks.
There are six medicines that work against HER2-positive breast cancer:
- Enhertu (chemical name: fam-trastuzumab-deruxtecan-nxki)
- Herceptin (chemical name: trastuzumab)
- Kadcyla (chemical name: T-DM1 or ado-trastuzumab emtansine)
- Nerlynx (chemical name: neratinib)
- Perjeta (chemical name: pertuzumab)
- Tykerb (chemical name: lapatinib)
If your HER2 test results are HER2-negative, you may want to ask your doctor about how confident he or she is in the lab that did the HER2 testing and if another HER2 test might make sense for your unique situation.
If your HER2 test results are borderline, you might want to ask your doctor if more than one pathologist reviewed the results. If the HER2 test results weren't reviewed by more than one pathologist, you may want to ask if the results can be reviewed again.
FISH is best performed on tissue that has been preserved in wax or chemicals, rather than on fresh or frozen tissue.