HER2 FISH classification of equivocal HER2 IHC breast cancers with use of the 2013 ASCO/CAP practice guideline

The status of human epidermal growth factor receptor 2 (HER2, ERBB2) determines the eligibility of breast cancer patients to receive HER2-targeted therapy. The majority of HER2 testing in the U.S. is performed using a combination of immunohistochemistry (IHC) screening followed by fluorescence in situ hybridization (FISH) for IHC equivocal cases. In 2013, the American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) updated the guideline for HER2 testing. This study evaluates the impact of the 2013 ASCO/CAP updated guideline on final HER2 FISH classification of breast cancers with an equivocal IHC result. For each case, we reported a FISH result according to the 2013 updated guideline and recorded a separated result using the 2007 guideline for investigational purpose. McNemar's test and Bowker's symmetry test were used to compare the classifications by the two guidelines. Among 172 HER2 IHC 2+ equivocal cases, use of the 2103 guideline changed classifications in 36 cases (21 %) when compared with the results expected by use of the 2007 guideline, and yielded a higher proportion of positive (28.5 vs. 23.3 %) and equivocal (16.3 vs. 4.1 %), and a lower proportion of negative (55.2 vs. 72.7 %) cases (p < 0.001). The major classification change with use of the updated guideline is from the HER2 FISH negative to equivocal in 26 cases (15 %). Our study has shown that implementation of the 2013 ASCO/CAP updated guideline has significant impact on HER2 classification for breast cancers with an equivocal HER2 IHC result and therefore increased the use of HER2-targeted therapy. Our data have also shown that reflex FISH is effective for final classification of the IHC equivocal cases and that polysomy 17 (CEP17 copy number ≥3/cell) is present in a significantly higher proportion of cases with an equivocal HER2 FISH classification.