Clear cell hidradenoma of the breast is rare. A 55-year-old woman demonstrated a left breast tumor during follow-up examination of the right breast. Focal asymmetric density was shown on mammogram, and ultrasonography showed an intracystic tumor. Since the diagnosis was not clear on aspiration cytology, excisional biopsy was performed. The lesion was an intracystic tumor macroscopically. Histological examination demonstrated characteristic histological features of clear cell hidradenoma, such as proliferation of uniform epithelial cells, clear or slightly eosinophilic cytoplasm, and cuboidal cell-lined ductal structures. Immunohistochemically, these proliferating epithelial cells were negative for myoepithelial markers, such as alpha-smooth muscle actin, CD10 and anti-muscle actin, but positive for p63. These features were consistent with clear cell hidradenoma.
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