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

Abstract

Abstract 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 <4.0 must be reported as positive. Patients with tumors showing these rare features were included in the first generation adjuvant trastuzumab clinical trials. In the absence of data showing lack of efficacy of HER2-targeted in this small subset of patients, these cases continue to be classified as positive without requiring additional work-up. Given that the HER2/CEP17 ratio is skewed in these cases by complex segmental deletions in chromosome 17, additional work-up may improve the final HER2 classification of these tumors. The objective of this study is to assess the potential impact of integrating HER2 IHC into the testing algorithm for these tumors by correlating the HER2 IHC results with ISH positive breast cancers demonstrating HER2/CEP17 ratio ≥2.0 and average HER2 copy number <4.0. Methods: All invasive mammary carcinomas demonstrating a HER2/CEP17 ratio ≥2.0 and average HER2 copy number <4.0 were identified from our database for years 2009-2017. All ISH testing was performed using the PathVysion HER2 DNA probe kit (Abbott Molecular). During the study interval, HER2 FISH was the institutional primary testing methodology to determine HER2 status. HER2 IHC (Ventana 4B5) was performed on the same sample for all identified cases and scored using 2013 ASCO/CAP scoring guidelines. 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 <4.0Cases (n)IHC Score 0IHC score 1+IHC score 2+IHC score 3+9612 (13%)52 (54%)28 (29%)4 (4%) Conclusions: HER2 ISH positive breast cancers demonstrating a HER2/CEP17 ratio ≥2.0 and average HER2 copy number <4.0 are rare (1.4% of cases in this study). HER2 IHC results for these tumors are negative in majority of cases. The observed HER2 protein overexpression rate was only 4% in this study. Integration of HER2 IHC into the testing algorithm for tumors demonstrating these uncommon ISH findings would result in reclassification of HER2 status to negative for most tumors. 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 <4.0 are frequently discordant with HER2 immunohistochemistry results: Implications for potential modification of testing algorithm [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P5-20-08.