HER2 gene amplification / protein overexpression

• Present in approximately 15 - 20% of breast tumors
• Associated with comedocarcinoma and aggressive invasive tumors
• Usually appears first in ADH or DCIS (Mod Pathol 2002;15:116)
• Also seen in non breast cancers (Mod Pathol 2007;20:192)
• Anti-HER2 therapy (trastuzumab / Herceptin®) plus chemotherapy reduces recurrence, metastases and mortality in HER2 gene amplified breast cancer patients (Int Semin Surg Oncol 2008;5:9, Acta Oncol 2008;47:1564); Lapatinib (Tykerb®) has a similar effect (Biologics 2009;3:289)
• Anti-HER2 therapy may improve survival in metastatic disease (Am J Clin Oncol 2008;31:250, N Engl J Med 2007;357:1496) but is associated with cardiac toxicity (BMC Cancer 2007;7:153)
• To detect, the most commonly used in situ hybridization (ISH) is a dual probe fluorescent ISH (FISH) using fluorochrome labeled probes for (a) the HER2 locus on the long arm of chromosome 17 and (b) a site near the centromere of chromosome 17 (CEN17 or CEP17)
• In situ hybridization detects HER2 gene amplification as evaluated by counting at least 20 tumor cells and estimating the HER2 copy number and the HER2/CEP17 ratio
• Amplification can also be detected with chromogenic ISH (CISH), (Mod Pathol 2002;15:657, Mod Pathol 2005;18:1015, Mod Pathol 2006;19:481, Breast Cancer Res 2007;9:R68) and silver enhanced ISH (SISH) (Am J Clin Pathol 2009;132:514)
• CISH and SISH use a peroxidase enzyme labeled probe with chromogenic detection, allowing results to be visualized with standard brightfield microscopy, so histologic features and HER2 status can be evaluated in parallel; signals do not decay over time, unlike FISH (Am J Clin Pathol 2009;132:539)
• Chromogenic in situ hybridization (CISH) is the only FDA approved single probe ISH test for HER2
• Quantitative reverse transcription polymerase chain reaction can also be used (Am J Clin Pathol 2008;129:563)
• Immunohistochemistry (IHC) detects evidence of protein overexpression via evaluation of the membranous staining in the tumor cells
• Testing must be performed in accredited laboratories

ASCO / CAP recommendations for HER2 testing and result interpretation

• Click here for 2013 update
• Click here for 2018 focused update
• HER2 testing must be performed on every primary invasive carcinoma and on a metastatic site (if stage IV)

Equivocal Results
• If initial HER2 testing by immunohistochemistry results in equivocal value, reflex testing should be performed on the same specimen using the alternative test OR perform testing on a new specimen, if available, using the same or alternative test

Heterogeneity
• Approximately 20 - 30% cases could be classified as heterogeneous
• The HER2/CEP17 ratio of each cohesive clone with amplification should be included in the report
• Cases in which > 10% of cells are amplified (in a minimum of 20 non overlapping cells in the amplified and non amplified areas) should be regarded as HER2+, while cases in which 1 - 10% tumor cells show amplification should be regarded as HER2 negative
• Tumors with scattered amplified cells should be categorized based on the overall score for the cells counted (minimum of 60 cells) (Mod Pathol 2014;27:4)

Indeterminate Results
• HER2 may be reported as indeterminate if technical problems preclude the reporting of a positive, negative or equivocal test result
• Reasons include improper specimen handling, crush artifact, edge artifacts and analytical testing failures
• In these cases, the reasoning for an indeterminate result should be reported and additional tissue should be acquired for testing

Contributed by Emily S. Reisenbichler, M.D. and Semir Vranić, M.D., Ph.D.

Contributed by Emily S. Reisenbichler, M.D.


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A woman with a right breast mass and axillary lymphadenopathy was found to have invasive mammary carcinoma with a positive lymph node. HER2 by IHC was performed with equivocal (2+) result on the core biopsy of the breast. What is the next step in management?


1. Report the result as equivocal and begin treatment
2. Report the result as negative
3. Repeat HER2 IHC on the breast core
4. Perform HER2 FISH on the breast core
5. Report the result as positive

D. The ASCO / CAP guidelines recommendations if initial HER2 testing by immunohistochemistry results in equivocal values, reflex testing by FISH should be performed on the same specimen or an alternative specimen (in this case the lymph node).

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Reference: HER2 (c-erbB2) breast