Abstract P3-10-21: Audit of the Accuracy of Immunohistochemical (IHC) Testing of HER2 Status of Breast Cancer in the United Kingdom: An Interim Analysis

Abstract Background: The analysis of the level and distribution of HER2 protein expressed by cancer cells (HER2 status) is of great clinical value in the management of breast cancer patients both for the determination of the prognosis of disease and for identification of those patients who are eligible for anti-HER2 therapy. Accurate assessment of the HER2 status is essential for identifying patients which will benefit from HER2 targeted therapy. HER2 status in the UK is established using a two tier strategy with IHC as the initial test and subsequent reflex of equivocal results to in situ hybridization (ISH). IHC staining of the HER2 protein is graded as 0- 3+ dependent upon the intensity of staining, cellular localisation and the percentage of cells positive in accordance with CAP/ASCO and UK guidelines. HER2 3+ cases are considered as positive, with HER2 2+ cases (equivocal) retested by ISH to ascertain the gene amplification status. Cases that are scored as 0 and 1+ by IHC have no additional testing and are classed as negative. The literature indicates that a subset of these IHC negative cases show HER2 gene amplification by FISH (range 1.1-10.7%). The aim of this audit is to evaluate the discordance rate of HER2 IHC negative, FISH positive breast cancer in the UK, with a secondary objective to resolve if this is related to the choice of antibody used. Materials and methods: This audit selected a total of 1000 sequential cases reported as HER2 negative on IHC, from three UK reference centres receiving cases from 29 different hospitals. The cases were given a unique identifying number and annonymised. Each of the three centres used a different IHC method for frontline HER2 testing with centre one using HercepTest™ (DAKO), centre two Pathway 4B5 (Roche), and centre three, Oracle (Leica Microsystems). HER2 gene amplification status was determined using dual colour FISH analysis, PathVysion (ABBOTT) fluorescence ISH (FISH) in a single centre to provide standardised methodology and assessment. HER2 was classed as amplified when the HER2/cep 17 ratio was two or greater in accordance with UK guidelines. All cases which showed discordance between IHC and FISH were retested with each of the HER2 IHC platforms to discover whether these are truly discordant results or if the discrepancy is a consequence of the choice of antibody. Results: An interim analysis of 170 cases shows an overall IHC negative/FISH positive discordance rate of 2.37%. The discordance rate per antibody was 1.7% for HercepTest, 3.3% with 4B5 and 2.5% with Oracle. Of the FISH positive cases the HER2/cep 17 ratios ranged from 2.2 - 6.17. The overall discordance rate between IHC negative and FISH positive is in keeping with the literature and is comparable between the three IHC platforms. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P3-10-21.