Fine-needle aspiration biopsy of breast adenomyoepithelioma: a potential false positive pitfall and presence of intranuclear cytoplasmic inclusions.

Cytologic diagnosis of adenomyoepithelioma can be very challenging. We report fine needle aspiration cytology (FNAC) findings of a benign adenomyoepithelioma. The cytologic features are characterized by hypercellularity and the presence of numerous atypical dispersed cells with epithelioid morphology and intact cytoplasm. The nuclei showed stippled chromatin, irregular nuclear membrane, and prominent eosinophilic nucleoli. No necrosis or mitoses were seen. The presence of naked nuclei, and extensive intranuclear cytoplasmic inclusions were identified and raised the possibility of adenomyoepithelioma. Immunohistochemically, the atypical cells showed strong positivity for myosin heavy chain, p63, and CK5/6, while the epithelial cells reacted with estrogen receptors. This immunophenotypic pattern supports the myoepithelial origin of the atypical cell proliferation and favors the diagnosis of benign adenomyoepithelioma. However, biopsy was recommended to exclude malignancy. Histologically, the tumor showed prominent myoepithelial cells with significant atypia, intranuclear cytoplasmic inclusions, and dense cytoplasm. No evidence of malignancy was identified. In conclusion, we report a case of adenomyoepithelioma with a significant cytological atypia that may result in confusion with malignant breast tumors. The presence of intranuclear cytoplasmic inclusions, naked nuclei, and expression of myoepithelial markers should provide clues to the right diagnosis and benign nature of this lesion. Cytopathologists should be familiarized with this entity to avoid a misdiagnosis of carcinoma.