Epithelioid Myoepithelioma of the Parotid Gland: A Histopathological and Immunohistochemical Study
The diagnosis and classification of salivary gland tumours is complicated by the wide variety of histological types that exist. Many authors attribute this complexity to the myoepithelial component of these tumours. The objective of this study is to evaluate the histological and immunohistochemical properties of a parotid gland myoepithelioma, in order to further our understanding of the differential diagnosis of salivary gland tumours which contain myoepitheliocytes. Histological specimens were analyzed using haematoxylin and eosin (H&E), periodic acid Schiff (PAS), Cason, Alcian blue, toluidine blue, a-SMA, p63 and ki67. The tumour examined was completely encapsulated, with solid cellular regions delimitated by a stroma. The stroma consisted of wide acidophilic and PAS-positive hyaline septae with areas of metachromasia. The tumour cells contained clear cytoplasm and round nuclei with lax chromatin, although some had more elongated nuclei and occasional dense chromatin. Neither cellular atypia nor mitotic figures were observed. Immunostaining was positive for a-SMA and p63, while it was negative for ki67. The histological characteristics of the tumour analyzed were consistent with a benign myoepithelioma, a rare tumour which represents less than 1% of salivary gland neoplasias. Immunostaining confirmed the morphological diagnosis of myoepithelioma. The absence of cytological changes and mitosis and its encapsulation differentiate it from its malignant counterpart. In comparison to pleomorphic adenoma, the myoepithelioma does not demonstrate ductal differentiation or chondromyxoid stroma. Importantly, the epithelial-myoepithelial carcinoma does develop tubular structures not seen in myoepithelioma. p63, which may act as an oncogene, is expressed within the nuclei of myoepitheliocytes of normal salivary glands. Its expression is retained in tumour myoepitheliocytes and thus it may play a role in oncogenesis.
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