American Society of Clinical Oncology/College of American Pathologists 2018 Focused Update of Breast Cancer HER2 FISH Testing Guidelines

Results From a National Reference Laboratory

Leo Lin, MD, PhD; Deepika Sirohi, MD; Joshua F. Coleman, MD; H. Evin Gulbahce, MD


Am J Clin Pathol. 2019;152(4):479-485. 

In This Article

Abstract and Introduction


Objectives: To review impact of the ASCO/CAP 2018 update on HER2 testing.

Methods: HER2 fluorescence in situ hybridization (FISH) test requests from primary and metastatic breast cancers between August 2018 and January 2019 were included. FISH results requiring a changed algorithm under the new guidelines (groups 2, 3, and 4) were identified and HER2:CEN17 ratios, average HER2, CEN17 signals/cell, and HER2 immunohistochemistry (IHC) results were recorded.

Results: Of the HER2 FISH cases 176/812(21.7%) fell within groups 2, 3, or 4; 0/12, 1/12, and 2/152 cases were positive (3+) by IHC, and 1/12, 2/12, and 6/152 cases were positive after targeted scoring from groups 2, 3, and 4, respectively. Following 2018 updates, 8.3%, 25%, and 5.3% of the groups 2, 3, and 4 were positive, respectively.

Conclusions: Groups 2, 3, and 4 constituted over 20% of HER2 FISH tests in a reference laboratory. The 2018 ASCO/CAP update significantly decreased the HER2 positivity rate.


Improvements in the performance and accuracy of human epidermal growth factor receptor 2 (HER2) testing and attempts to decrease both false-positive and -negative results have been the focus of revisions and updates to consensus guidelines published by the American Society of Clinical Oncology (ASCO) and the College of American Pathologists (CAP).[1,2] Determining which patients will benefit from targeted therapy and demonstrating the analytical and clinical validity of HER2 testing was also the aim of recent clinical trials and HER2 testing in breast cancer guideline updates.[3,4] Improvements in standardization of preanalytical and analytical phases of HER2 testing were often not accompanied by clinical outcome data to validate the efficacy of guidelines at the time, however.

Recently, an expert panel updated some of the recommendations of the ASCO/CAP HER2 testing in breast cancer guidelines, after consideration of outcomes of clinical trials for which fluorescence in situ hybridization (FISH) scores based on ASCO/CAP guidelines had been used for enrollment.[5–7] The 2018 ASCO/CAP focused update addresses five clinical questions raised in the years following the publication of the 2013 ASCO/CAP guidelines. Clinical questions 1 and 2 are related to definition of 2+/equivocal immunohistochemical (IHC) testing and when a HER2 test should be repeated, respectively. These questions were also previously addressed in another publication by the panel in 2015.[1,8] Clinical questions 3 to 5 addressed relatively uncommon dual-probe in situ hybridization (ISH) testing result patterns, which are reported to account for approximately 5% of all breast cancer cases tested by ISH.

The 2018 revisions significantly change the testing algorithms and interpretation in these ISH subgroups. Accordingly, an amplified result indicates a HER2: centromere 17 (CEN17) ratio of 2.0 or higher with an average copy number of 4.0 or higher, designated by the 2018 revisions as group 1. A nonamplified result indicates a HER2:CEN17 ratio less than 2.0 with an average HER2 copy number fewer than 4.0 (designated group 5). Groups 2 to 4 encompass findings previously designated as either positive or equivocal. Specifically, group 2 indicates a HER2:CEN17 ratio of 2.0 or higher with an average ERBB2 copy number of fewer than 4.0. Group 3 indicates a HER2:CEN17 less than 2.0 with an average ERBB2 copy number of 6.0 or higher. Group 4 indicates a HER2:CEN17 ratio of less than 2.0 with an average ERBB2 copy number of at least 4.0 but less than 6.0.

For groups 2 to 4, the 2018 guidelines recommend concomitant review of HER2 IHC (performed in the same laboratory, if possible). If the IHC score is 3+, then the final result is considered positive for amplification. If 0 or 1+, then the final result is negative for amplification. If the IHC score is 2+, then additional tumor nuclei are enumerated with FISH from the area of highest IHC intensity by an individual blinded to the original results. If the results remain consistent with groups 2 or 4, then the final interpretation is considered negative. When the recounted population of 2+ staining cells is consistent with group 3 (HER2:CEN17 ratio of less than 2.0 with an average ERBB2 copy number of 6.0 or higher), then the case is considered amplified.

The purpose of this study is to review the impact of these revised guidelines on dual-probe FISH testing in a reference laboratory setting.