Abstract P6-05-04: Quantification of Intra-Tumoral Heterogeneity for HER2 Gene Amplification by FISH with Proposed New (“High-ICR”) Reporting System

Abstract

Background: Current recommendations for reporting Her2 amplification by FISH have several problems, including imperfect concordance between different reporting methods, presence of an ‘equivocal’ category that is clinically problematic, and lack of accommodation for the common presence of intratumoral heterogeneity (ITH). Using a large dataset, we sought to better define ITH and develop a FISH reporting system that is more useful for patient management. Materials and Methods: We analyzed HER2 and CEP17 counts in 32,116 individual tumor cells from 1329 consecutive breast cancer cases. Based on the individual tumor cell ratios (ICRs) and the overall average case ratios, cases were divided into three groups to explore the significance of ITH. These groups were as follows: 1) Uniformly non-amplified cases defined as < 5% of individual cells with a HER2:CEP17 ratio > 2.2 and an overall average case ratio < 1.8 (n=763). 2) Uniformly amplified cases defined as > 90% of individual cells with a HER2:CEP17 ratio > 2.2 and the overall average case ratio > 2.2 (n=137). 3) Non-uniformly amplified defined as between 5-90% of cells with ratio > 2.2. (n=429). The Her2:CEP17 ratios for individual cells were then compared between groups to identify the ICRs characteristic of uniformly amplified cancers. Results: ITH for HER2 gene amplification is very common and is present in non-amplified, equivocal and amplified cases by traditional reporting criteria. Cells with Her2:CEP17 ratios >= 4 are characteristic of uniformly amplified cases, are essentially absent in uniformly non-amplified cases and represent only 5.3% of cells in non-uniformly amplified cases (as defined above). Lower ratios are not characteristic of uniformly amplified cancers. Using an individual cell ratio >= 4 to define significantly amplified (“High- ICR”) cells, the following diagnostic categories are proposed: uniformly amplified (defined as having >90% High-ICR cells), uniformly non-amplified (<10% High-ICR cells and heterogeneous with X% amplified cells (10-90% of High-ICR cells). The distribution of cases using this proposed reporting system versus the classic CAP/ASCO categories is shown in the below table. Proposed ITH reporting system vs classic CAP/ASCO categories Discussion: Intratumoral heterogeneity for HER2 gene amplification is common in amplified, non-amplified and equivocal cases by current reporting guidelines but not all heterogeneity is likely to be clinically significant. The High-ICR scoring system categorizes cases based on the number of cells with significant amplification, eliminates the problematic equivocal category. and directly reports the fraction of cells that are presumed to be most amenable to treatment. We propose that the High-ICR reporting system be further validated and adopted for clinical use. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P6-05-04.