Immunohistochemistry (IHC) is routinely performed during pathology practice for various breast lesions. Hormone receptor and HER2 analysis for primary breast carcinoma and cytokeratin staining for sentinel lymph nodes analysis are widely conducted. In addition to those markers, there are several situations in which certain IHC staining is valuable as an ancillary tool. This manuscript will present three useful examples of IHC for making differential diagnosis between benign and malignant lesions. Case 1 is an intraductal papilloma with solid epithelial proliferation, for which diagnosis was resolved by myoepithelial markers and high-molecular-weight cytokeratins (HMWCKs). Case 2 is a noninvasive ductal carcinoma with solid and papillary morphology. Many cases with such morphology mimic benign papillomas, but expression of neuroendocrine markers may lead to the correct diagnosis. Case 3 is a benign complex sclerosing lesion, with recognition of a pseudoinvasive process by myoepithelial markers. Although IHC results were excellent in these cases, they are effective only for limited situations. It is important to use IHC with caution, and re-evaluation of histological findings on hematoxylin and eosin stain and clinicopathological correlation of each case is essential.
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