Impact of 2013 American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines on HER2 fluorescent in situ hybridization (FISH) testing in breast cancers: Experience from a national reference laboratory.
e23188 Background: Accurate HER2 testing is expected to identify patients who will benefit from HER2 directed therapy (HDT) minimizing false positive and negatives. Like many biologic processes, HER2 protein expression and gene copy numbers are a continuum, however these tests are expected to be reported dichotomously. In 2013, ASCO/CAP revised the guidelines in an attempt to decrease false negative results and reverted to FDA approved ratio of ≥ 2.0 for HER2 positivity. We reviewed HER2 FISH testing results after the implementation of 2013 ASCO/CAP guidelines to determine the effects on HER2 reporting from our national reference laboratory. Methods: HER2 FISH testing performed between 5/2015-4/2016 at ARUP Labs following current 2013 ASCO/CAP guidelines was included. HER2 to control probe ratios, mean HER2 and control probe copy numbers were used to reassign HER2 status using 2007 ASCO/CAP, and FDA guidelines for each case. Results: HER2 FISH results were available in 2,017 cases. 342 (17.0%) cases were amplified, 301 (14.9%) were equivocal, and 1374 (68.1%) were non-amplified. After additional testing with alternate probe, 93 (31.2%) of the equivocal cases were reclassified as amplified increasing amplified cases to 21.6%. All of the equivocal cases which were reinterpreted as amplified with alternate probe showed low level amplification (range: 2.0-3.6; mean: 2.3). HER2 positivity rates following 2013 ASCO/CAP guidelines, both at initial testing and after additional testing to resolve equivocal results were significantly higher compared to 2007 ASCO/CAP guidelines and FDA criteria. Conclusions: 2013 ASCO/CAP guidelines lead to higher number of HER2 FISH positive and equivocal cases. In a reference laboratory setting where alternative control probe was used to resolves equivocal FISH cases, 31.2% of patients with initial equivocal results become HER2 positive. However, it is not known if these patients benefit from targeted therapies as they would not be included in the original adjuvant or metastatic trials.