How is HER2 testing performed in the evaluation of breast cancer?

Updated: Feb 04, 2021
  • Author: Pavani Chalasani, MD, MPH; Chief Editor: John V Kiluk, MD, FACS  more...
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Although several methods for HER2 testing have been developed, approximately 20% of current HER2 testing may be inaccurate; accordingly, the American Society of Clinical Oncology (ASCO) and CAP have recommended guidelines to ensure the accuracy of HER2 testing. Breast cancer specimens should initially undergo HER2 testing by a validated immunohistochemistry (IHC) assay (eg, HercepTest; Dako, Glostrup, Denmark) for HER2 protein expression. [109] (See Breast Cancer and HER2.)

The scoring method for HER2 expression is based on the cell membrane staining pattern and is as follows:

  • 3+ – Positive for HER2 protein expression; uniform intense membrane staining of more than 30% of invasive tumor cells

  • 2+ – Equivocal for HER2 protein expression; complete membrane staining that is either nonuniform or weak in intensity but has circumferential distribution in at least 10% of cells, or uniform intense membrane staining in 30% or less of tumor cells

  • 1+ – Weak or incomplete membrane staining in any tumor cells

  • 0 – Negative for HER2 protein expression; no staining

Breast cancer specimens with equivocal IHC results should undergo validation with a HER2 gene amplification method, such as fluorescence in situ hybridization (FISH). More centers are relying on FISH alone for determining HER2 status.

In general, FISH testing is thought to be more reliable than IHC, but it is more expensive. Equivocal IHC results can be seen in 15% of invasive breast cancers, whereas equivocal HER2 FISH results are seen in fewer than 3% of invasive breast cancer specimens and those that had previously been considered HER2 positive. Discordant results (IHC 3+/FISH negative or IHC < 3+/FISH positive) have been observed in approximately 4% of specimens. Currently, no data support excluding this group from treatment with trastuzumab.

Newer methodologies for establishing HER2 status, including reverse transcriptase–polymerase chain reaction (RT-PCR) and chromogenic in situ hybridization (CISH), have been developed. The HER2 CISH PharmDX Kit (Dako Denmark A/S, Glostrup, Denmark) was approved by the FDA in November 2011. The interpretation for HER2 FISH testing (ratio of HER2 to chromosome 17 centromere [HER2/CEP17] and gene copy number) is as follows:

  • Positive  HER2 amplification – HER2:CEP17 ratio > 2.2 or  HER2 gene copy > 6.0
  • Equivocal  HER2 amplification – HER2:CEP17 ratio of 1.8-2.2 or  HER2 gene copy of 4.0-6.0
  • Negative  HER2 amplification – HER2:CEP17 ratio < 1.8 or  HER2 gene copy < 4.0

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