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.

ABSTRACT

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 HER2CEP17 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 (HER2CEP17 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.