TEST ACRONYM

FULL TERMINOLOGY

Immunohistochemistry staining

DEFINITION

In HER2-positive tumors, there is an excess amount of the HER2 protein on the cell surface. This is referred to as HER2 overexpression. IHC assays measure the amount of HER2 protein expressed on the surface of tumor cells.

WHEN TEST WOULD BE PERFORMED

The IHC test is done by a pathologist in a laboratory on a sample of a tumor removed during a biopsy, a lumpectomy or a mastectomy. This is typically one of two tests (see FISH) that may be used to determine HER2 status. An IHC score of 0 or 1+ is HER2- negative. A score of 2+ is weakly HER2-positive and a score of 3+ is strongly HER2-positive. It is typically recommended that tumors with and IHC score of 2+, also be submitted for FISH testing.

INFORMATION VALUE

It is important to accurately determine the HER2 status of each patient's breast cancer to make sure that the patients most likely to be helped (those with HER2-positive cancer) are offered the Herceptin, and those unlikely to be helped (those with HER2-negative cancer) can avoid potential side effects.

CAUTION

Evaluation of the sample stain often requires the pathologist to subjectively judge the degree of color (of the HER2 protein on the cell).