Assessment of HER2 using the 2018 ASCO/CAP guideline update for invasive breast cancer: a critical look at cases classified as HER2 2+ by immunohistochemistry.
In 2018, the American Society of Clinical Oncology/College of American Pathologists revised the criteria for HER2 immunohistochemistry (IHC) equivocal (2+) classification in their updated guideline. We reviewed invasive breast cancer specimens originally classified as equivocal (2+) under the 2018 guideline that underwent HER2 fluorescence in situ hybridization (FISH) testing from August 2018 to August 2019 at our Canadian reference hospital to investigate cases with ambiguous staining patterns between the 1+ and 2+ definitions. Demographics, pathologic features, and pre-analytic conditions were recorded. The H&E and corresponding HER2 IHC slides were reviewed to confirm tumor type and grade, and classify as HER2 indeterminate, 0, 1+, 2+, or "Intermediate" (staining features between the 1+ and 2+ classifications). FISH testing was performed on 289 cases and 273 met inclusion criteria. The FISH-amplified rate was 12.1%. Upon IHC review, 44.7% (122/273) of cases were reclassified as Intermediate. These cases had incomplete staining with moderate intensity (43/122, 35.3%) and/or <10% complete weak or moderate staining (102/122, 83.6%). Intermediate cases had a significantly lower frequency of amplified FISH results than 2+ cases (p < 0.0001), with only four (3.3%) FISH positive and two (1.6%) FISH heterogeneous. Our study highlights the ambiguity in the current guideline for classifying some HER2 IHC patterns. As the rate of gene amplification in these cases was low (4.9%), we recommend adhering to the 2018 HER2 2+ criteria for reflex FISH testing. However, cases with <10% moderate complete staining and certain heterogeneous patterns warrant special consideration. Further descriptive clarification of 1+ criteria is needed.
['Taylor|Valerie J|VJ|', 'Barnes|Penny J|PJ|', 'Godwin|Sean C|SC|', 'Bethune|Gillian C|GC|http://orcid.org/0000-0001-9147-759X']
D000328:Adult D000368:Aged D000369:Aged, 80 and over D014408:Biomarkers, Tumor D001943:Breast Neoplasms D005260:Female D005784:Gene Amplification D006801:Humans D007150:Immunohistochemistry D017404:In Situ Hybridization, Fluorescence D008875:Middle Aged D060787:Neoplasm Grading D009361:Neoplasm Invasiveness D011237:Predictive Value of Tests D018719:Receptor, ErbB-2
ASCO/CAP guidelines Invasive breast cancer HER2 immunohistochemistry Equivocal HER2 testing