Abstract P1 18 20: Breast cancer with HER2 FISH test result in group 2: Should HER2 test be repeated?

Introduction: Interpretation of HER2 result in breast cancer patients with monosomy of CEP17 has been controversial due to the loss of CEP17 signals and the subsequent relatively increased HER2/CEP17 ratio. HER2 FISH group 2 (HER2< 4 and HER2/CEP17 ratio ≥ 2) was historically considered HER2 positive (HER2+) irrespective of the negative HER2 overexpression and anti-HER2 therapy response. The updated 2018 ASCO/CAP guidelines incorporated immunohistochemistry (IHC) results to facilitate HER2 reporting. However both IHC and FISH can be subjective to analytical variables, which can lead to challenge in interpretation cases in this group. This study aimed to assess if repeat IHC and FISH tests could assist final HER2 interpretation for group 2 cases.Methods: We retrospectively reviewed HER2 FISH cases performed at our institution from 2012 to 2017 and identified 23 of 3554 (0.6%) breast …