Impact of the updated 2018 ASCO/CAP guidelines on HER2 FISH testing in invasive breast cancer: a retrospective study of HER2 fish results of 2233 cases.

OBJECTIVES: Human epidermal growth factor receptor 2 (HER2, ERBB2) is a valuable prognostic and predictive biomarker in breast cancer. Accurate assessment of HER2 status is essential in selecting the patients with invasive breast cancer who will likely response to HER2-targeted therapies. Some major modifications in the diagnostic recommendation for fluorescence in situ hybridization (FISH) have been made in the updated 2018 American Society of Clinical Oncology (ASCO)/College of American Pathologist (CAP) guideline. According to the revised guideline, concomitant IHC assays are required to arrive at the most accurate HER2 status designation after HER2 FISH equivocal results; however, little is known about its influence on the clinical practice of pathologist. The purpose of this study was to evaluate the impact of the revised 2018 ASCO/CAP guidelines on the HER2 status designation. METHODS: We retrospectively reviewed the HER2 FISH testing results from 2233 cases of invasive breast cancer between January 2014 and December 2017. Concomitant immunohistochemistry (IHC) were performed on the same tissue blocks that were used for the FISH testing. RESULTS: Compared to the 2013 guidelines, the HER2 status in 183 (8.2%) cases were re-defined when reassessed by the 2018 guidelines. Among these 183 cases, 175 equivocal cases according to the 2013 guideline were re-defined as HER2 negative (n = 173) or HER2 positive (n = 2). Eight previously classified as HER2 positive cases were converted to negative in the 2018 scheme, all of which were with HER2 IHC scores of 1+ or 2+. The number of cases in the negative category was 1705 according to the 2018 guidelines as opposed to 1524 by the 2013 guidelines. CONCLUSIONS: The updated 2018 ASCO/CAP guidelines eliminated the FISH equivocal category, which can be attributed to reflex HER2 IHC, and partly ease the dilemma for clinical practice. Reflex IHC for FISH equivocal cases is of prime importance; furthermore, HER2 FISH results were converted from positivity to negativity based on the concomitant IHC results in a small percentage of cases. In all, implementation of the 2018 ASCO/CAP guidelines provides much clearer instructions and recommendations for the HER2 status designation, and thus reduces the risk of misdiagnosis.

Impact of repeat HER2 testing after initial equivocal HER2 FISH results using 2013 ASCO/CAP guidelines.

PURPOSE: The updated 2013 American Society of Clinical Oncology/College of American Pathologists guideline recommendations for human epidermal growth factor receptor 2 (HER2) testing have made some major changes in HER2 fluorescence in situ hybridization (FISH) interpretation criteria with additional FISH equivocal cases. Repeat HER2 testing is recommended after initial HER2 FISH equivocal results; however, little is known about its impact on final HER2 status. The aim of this study is to investigate whether reflex test clarifies HER2 status, and to characterize clinicopathological features of the newly defined HER2 equivocal group. METHODS: A total of 886 consecutive cases of primary invasive breast cancer conducted with dual-probe HER2 FISH testing between November 2013 and December 2015 were reviewed. HER2 immunohistochemistry (IHC) and FISH testing were performed on a different tissue block or a new specimen after initial HER2 FISH equivocal results. RESULTS: Compared to 2007 guideline, 85 (9.6%) cases changed their category by using 2013 guideline. The major change of the 85 cases is that 57 (6.4%) cases in HER2 FISH-negative category changed to equivocal, and the equivocal category cases increased from 36 to 67. HER2 FISH equivocal was significantly associated with HER2 IHC equivocal (2+) and chromosome 17 polysomy (P < 0.01). Repeat testing by IHC and FISH clarified HER2 status in 33 and 42% of HER2 equivocal cases, respectively. Overall 32 (48%) initial HER2 equivocal cases stayed HER2 equivocal after repeat FISH and or IHC testing. These tumors were ER/PR+, with high KI-67 index. CONCLUSION: New guidelines classify more HER2 FISH equivocal cases. Repeat HER2 testing clarifies HER2 status in about 50% of initial HER2 FISH equivocal cases. In addition, HER2 equivocal cases merit further study as there is limited information about prognosis and optimal treatment strategy for this population.