Intra-Tumoral Heterogeneity for HER2 Gene Amplification Is Common Using the CAP Recommended Criteria and Does Not Correlate with High Protein Expression: Time for a New Look at How To Report Heterogeneity.
Kimberly H Allison, Suzanne M Dintzis, Rodney A Schmidt. University of Washington Medical Center, Seattle
Background: The College of American Pathologists (CAP) recently published recommendations for reporting intra-tumoral heterogeneity for HER2 gene amplification in breast cancers. These guidelines recommend reporting cases with between 5-50% of cells with HER2:CEP17 ratios > 2.2 as “heterogeneous for HER2 gene amplification.” We examined the implications of applying these recommendations to clinical practice and reviewed the HER2 protein expression by immunohistochemistry (IHC) in the cases with FISH heterogenous results to determine if these criteria are likely to identify additional cases with protein over-expression.
Design: We collected HER2 and CEP17 counts in 32,116 tumor cells from 1329 consecutive breast cancer cases analyzed by FISH at UWMC. Data from Excel counting sheets were imported to a SQL Server database for analysis and statistics. Cases were categorized as CAP heterogenous if between 5-50% of cells counted had HER2:CEP17 ratios > 2.2. These results were then compared with the original FISH CAP/ASCO ratio criteria. Concurrent HER2 IHC results for CAP heterogeneous cases were also pulled from the database for review.
Results: 313 of 1,329 cases (23%) met the proposed CAP criteria for heterogeneous HER2 gene amplification by HER2:CEP17 ratio. Of these cases, 80% were considered non-amplified by the traditional criteria. The below table shows the classification of all cases by both criteria. Of the 284 heterogeneous cases available for IHC review, only 1% were positive for protein over-expression by IHC and the majority were either IHC equivocal (51%) or IHC negative (48%) for HER2 protein over-expression.
|Original Classification by Ratio||Classification by CAP Heterogeneity Recommendation|
|Amplified (>50% cells > 2.2)||Heterogeneous (5-50% > 2.2)||Not Amplified (<5% > 2.2)|