Pathology of Basal Cell Adenoma of the Salivary Gland
Basal cell adenoma is a rare benign salivary gland tumour that
accounts for 1% to 3% of all salivary gland neoplasms.
(2) About 70% of these tumours occur in the parotid gland.
(3) The upper lip (10-20% cases), is the commonest site for intraoral basal cell adenoma. Palate, buccal mucosa and lower lip are rarely involved.
(4) The term Basal cell adenoma was first described by Kleinsasser and Klein in 1967.
(5) Clinically, the tumour tends to be an asymptomatic, slowly enlarging, freely mobile mass with a maximum diameter of less than 3 cm.
(6) Basal cell adenoma is characterized by the basaloid appearance of the tumour cells and the absence of the myxochondroid tissue that is usually found in pleomorphic adenomas.
(7) It is divided into 6 types of growth patterns: solid, trabecular, tubular, membranous, cribriform, and myoepithelium-derived stroma rich.
(8) The most common type is the solid basal cell adenoma, in which round or oval tumor cells show a solid proliferation and form cell nests of various sizes, the peripheral nest cells are lined with a palisading row of tumour cells
(9) In the tubular type of basal cell adenoma, bilayered tubular structures consisting predominantly of inner eosinophilic luminal cells and outer cuboidal cells are featured.
(10) In the trabecular type, the tumour cells are arranged in trabecular cords and occasionally form tubular lumens or intercellular canalicular slits within the trabeculae.
(11) Membranous basal cell adenomas are composed of peripheral cell layers arrayed in a palisade fashion, and the cell layers are surrounded by excessive hyaline basal membrane material. It is histologically identical to dermal cylindroma.
(12) Basal cell adenoma with myoepithelium-derived stroma is a rare variant characterized by spindle cell rich stroma that separates the cords and islands of basaloid cells.
(13) The vast majority of basal cell adenomas exhibit dual epithelial- myoepithelial differentiation - Epithelial markers (CK, CEA and EMA) as well as myoepithelial markers (calponin, actin, GFAP, S100) are variably demonstrated in the luminal and myoepithelial cells respectively.
(14) Membranous basal cell adenoma when associated with cutaneous adnexal tumours such as dermal cylindroma, trihoepithelioma and spiradenoma, could signify a genetic disorder affecting the multipotential duct reserve stem cell.
(15) For basal cell adenomas, the primary treatment is surgical excision by means of a superficial or total parotidectomy.
(16) Recurrence is rare except for the membranous variant, which is associated with a recurrence rate of 25% because of its multifocal nature.
(17) Basal cell adenoma, specially the membranous type, may rarely undergo malignant transformation giving rise basaloid cell carcinoma (basal cell adenocarcinoma, adenoid cystic carcinoma).
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