Abstract P5-20-08: HER2 in-Situ hybridization positive breast cancers with HER2/CEP17 ratio ≥2.0 and average HER2 copy number <4.0 are frequently discordant with HER2 immunohistochemistry results: Implications for potential modification of testing algorithm

Background: The current ASCO/CAP guidelines for the determination of HER2 status indicate that dual-probe in-situ hybridization (ISH) samples demonstrating a HER2/CEP17 ratio ≥ 2.0 and average HER2 copy number Methods: All invasive mammary carcinomas demonstrating a HER2/CEP17 ratio ≥2.0 and average HER2 copy number Results: A total of 96 (1.4%) of 6,976 consecutive FISH cases met criteria for inclusion into the study. The majority of cases tested negative (67%) or equivocal (29%) by IHC. HER2 protein overexpression was observed in 4% of cases. The results of HER2 IHC testing are summarized below. HER2 IHC results for HER2 ISH positive tumors showing ratio≥2.0 and average HER2 copy number Conclusions: HER2 ISH positive breast cancers demonstrating a HER2/CEP17 ratio ≥2.0 and average HER2 copy number Citation Format: Livasy C, Johnson N, Domfeh A. HER2 in-Situ hybridization positive breast cancers with HER2/CEP17 ratio ≥2.0 and average HER2 copy number