Clinicopathologic Features of Breast Carcinoma With 2018 American Society of Clinical Oncology/College of American Pathologists Fluorescence In Situ Hybridization Group 3 (Human Epidermal Growth Factor Receptor 2 Chromosome 17 Centromere Ratio <2.0 and Average Human Epidermal Growth Factor Receptor 2 Copy Number ≥6.0) Breast Cancers

Context.— The American Society of Clinical Oncology/College of American Pathologists 2018 update of the human epidermal growth factor receptor 2 (HER2) testing guideline includes a fluorescence in situ hybridization (FISH) group with a HER2 to chromosome 17 centromere (CEP17) ratio less than 2.0 and HER2 copy number 6.0 or greater (group 3), which requires integrated review of HER2 immunohistochemistry (IHC). Objective.— To assess the clinicopathologic features of group 3 patients and determine features associated with HER2-positive status after workup. Design.— Cases submitted for HER2 FISH between January 2019 and June 2022 were identified, and relevant clinicopathologic information was obtained. Results.— One hundred forty-two HER2 FISH cases (1.6%) were group 3. In 52 cases (36.6%) IHC was negative (0/1+), in 3 (2.8%) IHC was positive (3+), and in 86 (60.6%) IHC was 2+. Annotated IHC 2+ slides were recounted by a second reviewer in targeted areas, where 16 of 86 (18.6%) had a HER2:CEP17 ratio less than 2.0 and a HER2 copy number of 4.0 or greater to less than 6.0 (HER2 negative). After combined IHC/FISH review, 74 of 142 (52.1%) were classified as HER2 positive. HER2 copy number/cell was higher in HER2-positive compared with HER2-negative cases after the workup. The extent and intensity of staining in IHC 2+ cases did not correlate with the level of gene amplification. Twenty percent of HER2-positive patients achieved pathologic complete response. Conclusions.— About half of group 3 cases were classified as HER2 positive after additional workup. Pathologic complete response rates in HER2-positive cases were lower than expected for group 1 (HER2:CEP17 ratio ≥2.0; HER2 copy number ≥4.0) patients. IHC targeted FISH recounts may be redundant and may potentially lead to classification of some patients as HER2 negative, resulting in withholding of targeted therapy.