Annals of diagnostic pathology
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Intratumoral heterogeneity can lead to uncertainty in breast carcinoma HER2 testing, both with respect to pathology reporting and clinical significance. The standard practice is to perform breast biomarker testing on a single representative section of tumor; however, concern over heterogeneity often leads to testing on additional tissue blocks. Our objective was to assess the diagnostic yield of testing multiple blocks of a single invasive breast carcinoma. ⋯ In the vast majority of cases, even those with heterogeneity, testing of a single block is sufficient for an accurate HER2 determination. High-grade tumors with equivocal HER2 status and observable heterogeneity are more likely to yield a different result on testing of additional blocks.