Changes in HER2 Amplification Status for Breast Cancer Patients After Immunohistochemistry Directed In Situ Hybridization

The 2018 ASCO/CAP guidelines for HER2 testing for breast cancer implemented the addition of immunohistochemistry (IHC) directed in situ hybridization (ISH) recount to resolve equivocal results. The implementation of an additional 2+ IHC-directed ISH recount adds additional complexity to the testing workflow for an unclear impact on HER2 results. A retrospective review of all equivocal ISH cases (groups 2, 3, and 4) that underwent 2+ IHC-directed ISH, since the 2018 guidelines, which were finalized as either amplified or not amplified, was performed. HER2 group number and final HER2 amplification status frequently changed after IHC guided ISH assessment, which was due to significant changes in HER2/CEP17 ratio and average HER2 signal number per cell. Equivocal groups 2, 3, and 4 samples with a result of HER2 amplified after 2+ IHC-directed ISH counts were closer to the threshold for amplification on the original ISH count, yet their counts also increased significantly after IHC-directed count in comparison to those samples, which were not amplified. Groups 2 and 4 ISH counts significantly increased after IHC directed ISH for HER2/CEP17 ratio and HER2 signal number per cell. This study represents the most extensive examination of efforts to resolve equivocal HER2 ISH results, highlighting a significant shift in therapeutic options after IHC-guided ISH for a subset of breast cancer patients.