Unique Cytopathological Findings of a Mucinous Myoepithelioma Showing a Mucous Globule and a "Sieve-Like" Structure.

Myoepitheliomas are rare salivary gland-type tumors. The tumors are divided into four histological subtypes (spindle cell, plasmacytoid, epithelioid, and clear cell) and two variants (reticular and mucinous). A myoepithelioma of the mucinous variant, also referred to as mucinous or secretory myoepithelioma, is a novel variant of myoepithelioma characterized by the presence of extracellular mucin. To date, only six benign mucinous myoepitheliomas have been reported. We herein report an 84-year-old man with a four-month history of swelling in the left bucca. Computed tomography revealed a well-demarcated elliptical mass measuring 30 mm in diameter. Fine-needle aspiration (FNA) smears showed an acellular spherical matrix surrounded by basaloid cells with scant cytoplasm resembling mucous globules, in addition to clusters of spindle cells. The mass was initially diagnosed as a pleomorphic adenoma based on the presence of a mucous globule-like structure and cytological variation. The surgically resected tumor showed two different histological components: one was composed of cells arranged in thin cords with a mucoid stroma showing a "sieve-like" structure and the other component was spindle cells. Alcian blue staining confirmed extracellular mucin in both tumor components. The tumor was suspected of being a mucinous myoepithelioma. We encountered a case of a mucinous myoepithelioma with two unique features, namely its cytological features of mucous globules composed of monotonous basaloid cells in the FNA smear and its histological feature of a "sieve-like" structure. The presence of mucous globules in FNA smears might require the inclusion of the mucinous myoepithelioma in the differential diagnosis.

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