Definition

 Myoepithelioma is a benign salivary gland tumour composed almost exclusively of sheets, islands or cords of cells with myoepithelial differentiation that may exhibit spindle, plasmacytoid, epithelioid or clear cytoplasmic features.

 ICD-O code 8982/0

 Synonyms

 Myoepithelial adenoma, benign myoepithelial tumour.

Epidemiology

Myoepitheliomas account for 1.5% of all tumours in the major and minor salivary glands and represent 2.2% and 5.7%, respectively of all benign major and minor salivary gland tumours {668}. Both sexes are affected with equal frequency {41,128,546,668,1647,2282,2367}. Most tumours occur in adults, but rare examples have been recorded in children {1527}. The age of patients with myoepithelioma ranges from 9-85, with an average of 44 years and the peak age of occurrence in the third decade {668}.

Localization

 Myoepitheliomas develop preferentially in the parotid gland (40%) {668}. Minor salivary glands follow in frequency, especially in hard and soft palates {546,668, 2282,2367}. Other minor salivary gland sites can also be affected {41,1647}.

Clinical features

Myoepitheliomas usually present as slow growing painless masses {41,2282, 2367}.

Macroscopy

 Myoepitheliomas are well-circumscribed, solid tumours that usually measure less than 3 cm in diameter {41,541,2367}. Myoepitheliomas have a solid, tan or yellow- tan, glistening cut surface {668}.

Histopathology

 A variety of cell morphologies has been recognized, including spindle, plasmacytoid or hyaline, epithelioid, and clear {546}. Most are composed of a single cell type but combinations may occur. Spindle cells are arranged in interlacing fascicles with stroma-like appearance {1579}. Plasmacytoid cells are polygonal cells with eccentric nuclei and dense, nongranular or hyaline, abundant eosinophilic cytoplasm. Plasmacytoid cells are found more often in tumours arising in the minor salivary glands than in the parotid gland. These hyaline cells may simulate neoplastic plasma cells, skeletal muscle or “rhabdoid” cells {1575}. Epithelioid cells are arranged in nests or cords of round to polygonal cells, with centrally located nuclei and a variable amount of eosinophilic cytoplasm. The surrounding stroma may be either collagenous or mucoid. Some myoepitheliomas are composed predominantly of clear polygonal cells with abundant and optically clear cytoplasm, containing large amounts of glycogen but devoid of mucin or fat. These tumours may show intercellular microcystic spaces. In other myoepitheliomas, occasional duct-like structures and intercellular microcystic spaces may be present. An unusual reticular variant of myoepithelioma characterized by netlike arrangements of interconnected cell cords, extending through a loose, vascularized stroma, has been reported {546}. Immunoprofile The cells of myoepithelioma are usually positive for cytokeratins, especially for CK7 and 14. The reactivity of the spindle cells is variable for α-smooth muscle actin, muscle specific actin (MSA), calponin, S-100, GFAP and smooth muscle myosin heavy chain. There is considerable variation of tumour expression of MSA. The spindle cells react strongly for MSA, the epithelioid cells react sporadically, and the plasmacytoid and clear cells are often nonreactive {805}.

Electron microscopy

Ultrastructural studies confirmed the epithelial and myoepithelial differentiation of myoepithelioma {538,541}.

Differential diagnoses

Distinction from pleomorphic adenoma is based on the relative lack of ducts and the absence of myxochondroid or chondroid areas. Myoepitheliomas with clear cells, or mixed epithelioid and clear cells have to be separated from other salivary gland tumours with clear cells, such as: mucoepidermoid carcinoma, acinic cell carcinoma, epithelial-myoepithelial carcinoma, oncocytoma and clear cell carcinoma. All these tumours lack the characteristic immunoprofile of the myoepithelial cells. In contrast to carcinomas, myoepitheliomas have a non-infiltrative, well-circumscribed periphery . Predominantly spindle cell myoepitheliomas must be distinguished from benign and malignant mesenchymal tumours.

Genetics

Cytogenetic studies have demonstrated structural alterations of chromosomes 1, 9, 12, and 13: t(1;12)(q25;q12), del(9) (q22.1q22.3), del(13)(q12q22) in a paro - tid myoepithelioma {654}. Mutations of TP53 have been observed in 3 of 12 (25%) myoepitheliomas {2734}.

Prognosis and predictive factors

 According to well-documented series myoepitheliomas are less prone to recur than pleomorphic adenomas {2282}. However, higher recurrence rates have been reported by others {41,646}. Recurrence is correlated with positive margins at the first excision {646}. The recommended treatment is complete surgical excision. Benign myoepitheliomas can undergo malignant transformation, especially in long standing tumours or in tumours with multiple recurrences {41}.