The American Society of Clinical Oncology (ASCO) and College of American Pathologists (CAP) recently convened an Update Committee to conduct a systematic literature review and update recommendations for optimal HER2 testing. In particular, the Committee identified criteria and areas requiring clarification to improve the accuracy of HER2 testing by immunohistochemistry (IHC) or in situ hybridization (ISH). The guideline was reviewed and approved by both organizations and has been published jointly by invitation and consent in both the Journal of Clinical Oncology¹ and the Archives of Pathology & Laboratory Medicine.²

In brief, the Update Committee recommends that HER2 status be determined in all patients with invasive breast cancer on the basis of one or more HER2 test results (negative, equivocal, or positive). Testing criteria define HER2 status as positive when, in examining an area of tumor that amounts to > 10% of contiguous and homogeneous tumor cells, there is evidence of protein overexpression (by IHC) or gene amplification (HER2 copy number or HER2/CEP17 ratio by ISH based on
counting at least 20 cells within the area).

If results are equivocal (on revised criteria) on IHC or ISH, reflex testing should be performed using the alternative assay. Repeat testing should be considered if results appear discordant with other histopathologic findings. Testing laboratories should demonstrate high concordance with a validated HER2 test. Testing must be performed in a laboratory accredited by CAP or another accrediting entity. The Committee urges providers and health systems to cooperate in following the updated guidelines to ensure the highest quality testing.

The following summarizes key recommendations for oncologists and pathologists.

Recommendations for Oncologists

HER2 testing should be requested for every primary invasive breast cancer (and for metastatic sites for stage IV disease) to guide the decision regarding HER2-targeted therapy. This includes patients who previously tested HER2-negative in a primary tumor who present with disease recurrence with clinical behavior suggestive of HER2-positive or triple-negative disease.

HER2-targeted therapy should be recommended if the HER2 test result is positive, if there is no apparent histopathologic discordance with HER2 testing, and if clinically appropriate. In the case of any outcomes of HER2 testing, additional testing should be discussed if the pathologist or oncologist observes an apparent histopathologic discordance after testing.

The decision regarding HER2-targeted should be delayed if the initial HER2 test is equivocal. Reflex testing should be performed on the same specimen using the alternative test or on an alternative specimen.

HER2-targeted therapy must not be recommended if the HER2 test result is negative and if there is no apparent histopathologic discordance with HER2 testing.

The decision on HER2-targeted therapy should be delayed if HER2 status cannot be confirmed as positive or negative after separate HER2 tests. The oncologist should confer with the pathologist regarding the need for additional HER2 testing on the same or another tumor specimen.

If HER2 testing is ultimately deemed to be equivocal, even after reflex testing with an alternative assay, HER2-targeted therapy may be considered. The feasibility of testing another tumor specimen to definitively establish HER2 status should also be considered. The clinical decision to consider HER2-targeted therapy should be individualized on the basis of patient status (comorbidities, prognosis, etc) and patient preferences after discussing available clinical evidence.

Recommendations for Pathologists

At least one tumor sample from all patients with breast cancer (early-stage or metastatic disease) should be tested for either HER2 protein expression (IHC assay) or HER2 gene expression (ISH assay) using a validated HER2 test.

In the United States, the ASCO/CAP Guidelines recommend use of an assay that has received FDA approval, although a CLIA-certified laboratory may use a laboratory-developed test; the analytic performance of the laboratory-developed test must be prospectively validated in the same clinical laboratory that will perform it and the test must have documented analytic validity. Bright-field ISH assays must be initially validated by comparing them with an FDA-approved fluorescence in situ hybridization (FISH) assay.

The HER2 test result must be reported as positive if it is: (a) IHC 3+ positive; or (b) ISH positive using either a single-probe ISH or dual-probe ISH. Recommendations regarding positive, equivocal, and negative results all assume that there is no apparent histopathologic discordance observed by the pathologist.

The HER2 test result must be reported as equivocal and a reflex test ordered on the same specimen (unless there are concerns about the specimen) using the alternative test if the result is: (a) IHC 2+ equivocal; or (b) ISH equivocal using single-probe ISH or dual-probe ISH. Some rare breast cancers (eg, gland-forming tumors, micropapillary carcinomas) show IHC 1+ staining that is intense but incomplete (basolateral or U shaped) and are found to be HER2 amplified. The pathologist should consider reporting these specimens as equivocal and request reflex testing using the alternative test.

The HER2 test result must be reported as negative if a single test (or all tests) performed in a tumor specimen show: (a) IHC 1+ negative or IHC 0 negative results; or (b) ISH-negative results using single-probe ISH or dual-probe ISH.

The HER2 test result must be reported as indeterminate if technical issues prevent one or both of IHC and ISH from being reported as positive, negative, or equivocal. Another specimen should be requested for testing and a comment should be included in the pathology report documenting intended action.

Bright-field ISH should be interpreted based on a comparison between patterns in normal breast and tumor cells, because artifactual patterns may occur that are difficult to interpret. If the tumor cell pattern is neither normal nor clearly amplified, the test should be submitted for expert opinion.

Any specimen used for HER2 testing should undergo the fixation process quickly (time to fixative within 1 hour) and be fixed in 10% neutral buffered formalin for 6 to 72 hours; routine processing and staining or probing should be performed according to standardized analytically validated protocols.

The testing laboratory should conform to standards of CAP accreditation or an equivalent accreditation authority, including initial test validation, ongoing internal quality assurance, ongoing external proficiency testing, and routine periodic performance monitoring.

If an apparent histopathologic discordance is observed in any testing situation, the pathologist should consider ordering additional HER2 testing and conferring with the oncologist and should document the decision-making process and results in the pathology report. The pathologist may also pursue additional HER2 testing without conferring with the oncologist.

Although categories of HER2 status on IHC or ISH not covered by the guidelines can be created, they are uncommon in practice and should be considered IHC equivocal or ISH equivocal if encountered.

Disclosure: For full disclosures of the guideline authors, visit jco.ascopubs.org or www.arachivesofpathology.org.

References

1. Wolff AC, Hammond MEH, Hicks DG, et al: Recommendations for human epidermal growth factor receptor 2 testing in breast cancer: American Society of Clinical Oncology/College of American Pathologists Clinical Practice Guideline Update. J Clin Oncol 31:3997-4013, 2013.

2. Wolff AC, Hammond MEH, Hicks DG, et al: Recommendations for human epidermal growth factor receptor 2 testing in breast cancer: American Society of Clinical Oncology/College of American Pathologists Clinical Practice Guideline Update. Arch Pathol Lab Med. October 7, 2013 (early release online).