HER2 Gene Amplification in Breast Cancer A Rogues' Gallery of Challenging Diagnostic Cases: UKNEQAS Interpretation Guidelines and Research Recommendations

International and national guidelines highlight the importance of accuracy, reproducibility, and quality control of in situ hybridization (ISH) methods for testing breast carcinomas. However, few guidelines cover the reporting of ISH cases with unusual signal patterns, including, eg, heterogeneity and loss of chromosome enumeration probe or gene signals. These cases are, in fact, relatively frequent, and there is a need for developing evidence- or consensus-based reporting guidelines to ensure consistency of treatment. Following an audit of cases from a single center (including > 1,700 cases) we show that approximately 10% of ISH results reflect unusual signal patterns. We illustrate the most common of these patterns and provide reporting guidelines for diagnosticians and recommendations for future research. Our goal is to ensure that in the future such rogues are reported in a consistent manner that, ultimately, will be supported by molecular and biochemical evidence.