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HER2 Testing in Breast Cancer

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7 Figure 1. Evaluation of HER2 Protein Expression by IHC Assay of the Invasive Component of a Breast Cancer Specimen HER2 Testing (invasive component) by Validated IHC Assay IHC 3+ positive Batch controls and on slide controls show appropriate staining Circumferential membrane staining that is complete, intense and within >10% of tumor cells a Weak to moderate complete membrane staining observed in >10% of tumor cells Incomplete membrane stainingthat is faint/barely perceptible and within >10% of tumor cells No staining is observed or Membrane staining that is incomplete and is faint/ barely perceptible and within ≤10% of tumor cells IHC 2+ equivocal IHC 1+ negative IHC 0 negative Must order reflex test (same specimen using ISH) or order a new test (new specimen if available, using IHC or ISH) (Strong Recommendation; EB-H) NOTE. e final reported results assume that there is no apparent histopathologic discordance observed by the pathologist. Unusual staining patterns of HER2 by IHC can be encountered that are not covered by these definitions. In practice, these patterns are rare and if encountered should be considered IHC2+ equivocal. As one example, some specific subtypesof breast cancers can show IHC staining that is moderate to intense but incomplete (basolateral or lateral) and can be found to be HER2 amplified. Another example is circumferential membrane IHC staining that is intense but within ≤10% of tumor cells (heterogeneous but very limited in extent). Such cases can be considered 2+ equivocal but additional samples may reveal different percentages of HER2 positive staining. a Readily appreciated using a low power objective and observed within a homogeneous and contiguous invasive cell population. Optimal Algorithm

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