• Overexpressed in ~15% of breast tumors (J Clin Oncol 2023;41:3867)
• Overexpressed in ~30% of ductal carcinoma in situ (DCIS) but prognostic importance is unclear (BMC Cancer 2015;15:468, Med Oncol 2022;40:16)
• Prognostic biomarker, associated with higher histologic grade, advanced stage, brain and visceral organ metastases
• Overexpression can also be seen in nonbreast cancers (Mod Pathol 2007;20:192)
• Immunohistochemistry (IHC) detects evidence of protein overexpression via evaluation of the membranous staining in the tumor cells

American Society of Clinical Oncology (ASCO) / College of American Pathologists (CAP) recommendations for HER2 testing and result interpretation

• Click here for 2013 update
• Click here for 2018 focused update
• Click here for the 2023 update
• HER2 testing must be performed on every primary invasive breast carcinomas and also metastatic / recurrent sites
• Cold ischemic time and formalin fixation time should be provided in the report

• HER2 classification in breast cancer is determined by ASCO / CAP guideline criteria (J Clin Oncol 2023;41:3867)
• Gastric / gastroesophageal junction cancers are assessed using different HER2 IHC scoring criteria
• In breast cancer, only membranous reactivity on the invasive tumor cells is counted towards scoring, expression in the in situ component is ignored
• Nonspecific cytoplasmic reactivity is ignored

HER2 IHC scoring

• Positive
  • Score 3+: tumor displays complete, intense circumferential membranous staining in > 10% of tumor cells (readily appreciated using a low power objective and observed within a homogenous and contiguous invasive cell population)
• Equivocal
  • Score 2+: weak to moderate complete membrane staining observed in > 10% of invasive tumor cells
  • Other less common patterns also considered equivocal
    • Circumferential membrane staining that is intense but within ≤ 10% of tumor cells
    • IHC staining that is moderate to intense but incomplete (basolateral or lateral) often seen in micropapillary breast cancer
• Negative
  • Score 1+: incomplete faint membrane staining and within > 10% of invasive tumor cells
  • Score 0: no staining observed or incomplete faint / barely perceptible membrane staining within ≤ 10% of invasive tumor cells
• Indeterminate
  • 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

In situ hybridization (ISH) scoring

• Positive
  • Dual probe HER2/CEP17 ratio ≥ 2.0 with average HER2 copy number ≥ 4.0
• Negative
  • Dual probe HER2/CEP17 ratio < 2.0 with an average HER2 copy number < 4.0 signals/cell
• Additional workup required
  • If a case has a HER2/CEP17 ratio ≥ 2.0 but the average HER2 signals/cell is < 4.0 (ISH group 2)
  • If a case has an average of ≥ 6.0 HER2 signals/cell with a HER2/CEP17 ratio of < 2.0 (ISH group 3)
  • If the case has an average HER2 signals/tumor cell of ≥ 4.0 and < 6.0 HER2 signals/cell and HER2/CEP17 ratio is < 2.0 (ISH group 4)
• Additional workup steps
  • IHC testing for HER2 should be performed using sections from the same tissue sample used for ISH
    • If the IHC result is 3+, diagnosis is HER2 positive
    • If the IHC result is 2+, recount ISH by having an additional observer, blinded to previous ISH results, count at least 20 cells that include the area of invasion with IHC 2+ staining
      • If reviewing the count by the additional observer changes the result into another ISH category, the result should be adjudicated per internal procedures to define the final category
      • If the count remains an average of < 4.0 HER2 signals/cell and HER2/CEP17 ratio is ≥ 2.0, the diagnosis is HER2 negative with a comment
      • If the HER2/CEP17 ratio remains < 2.0 with ≥ 6.0 HER2 signals/cell, the diagnosis is HER2 positive
      • If the count remains an average of ≥ 4.0 and < 6.0 HER2 signals/cell with HER2/CEP17 ratio < 2.0, the diagnosis is HER2 negative with a comment
    • If the IHC result is 0 / 1+, diagnosis is HER2 negative with comment
• Patient is considered HER2 positive if
  • HER2 IHC score 3+ or
  • IHC score 2+ and HER2 ISH positive or
  • HER2 ISH positive with any IHC result
• Patient is considered HER2 negative if
  • HER2 IHC score 0 or 1+ or
  • IHC score 2+ and HER2 ISH negative
• If initial HER2 testing by immunohistochemistry results are equivocal, 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
• Breast cancers that are IHC score 1+ or 2+ and ISH nonamplified often informally referred to as having HER2 low status (this terminology is not accepted as a result category by current ASCO / CAP guidelines, however, can be referred to in a comment within the report) (J Clin Oncol 2023;41:3867)

Contributed by Gary Tozbikian, M.D., 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 breast carcinoma, no special type (NST) with a positive lymph node. HER2 by IHC was performed and resulted in an equivocal score (2+) on the core biopsy of the breast. What is the next step in management?

1. Perform HER2 ISH on the breast core
2. Report the HER2 status of the patient as indeterminate
3. Start anti-HER2 treatment with neoadjuvant chemotherapy
4. Surgical resection followed by radiation therapy

A. Perform HER2 ISH on the breast core. The ASCO / CAP guidelines recommend reflex testing by ISH if the initial HER2 test by immunohistochemistry results in an equivocal score of 2+, which should be performed on the same specimen or a new test should be ordered on an alternative specimen using either IHC or ISH (in this case, the lymph node). Answer B is incorrect because an indeterminate result is given when technical issues preclude interpretation of the test result. Answer D is incorrect because HER2 status should be determined before next steps in care. Answer C is incorrect because anti-HER2 treatment should only begin if the patient is classified as HER2 positive.

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Reference: HER2 (c-erbB2) breast