Definition

 Basal cell adenoma (BCA) is a rare benign neoplasm characterized by the basaloid appearance of the tumour cells and absence of the myxochondroid stromal component present in pleomorphic adenoma.

 ICD-O code 8147/0

Epidemiology

Accurate epidemiological data are hard to obtain since in the past BCA was included within non-pleomorphic tumours. The BCAs are rare, accounting for 1-3% of all salivary gland tumours. They are typically seen in adults in the 7th decade with a 2:1 female predilection {2303}, except for the membranous type that has an equal female:male distribution {668}.

Localization

The majority arise in the major glands, and the parotid is the most frequent site of occurrence (~75%), followed by the submandibular gland (~5%) {162,2881}. It is extremely rare in minor salivary glands, the upper lip being the most common site, followed by the buccal mucosa {704,2711}.

Clinical features

Most tumours are solitary, well-defined, movable nodules. They are usually firm but occasionally cystic. The membranous type (dermal analogue tumour) {153} may be multiple and co-exist with dermal cylindromas or trichoepitheliomas {1033,1582,2867}.

Macroscopy

 Most of the tumours present as small, well-circumscribed, encapsulated nodules measuring between 1-3 cm, except for the membranous type that may be multinodular or multifocal. On cut section they are solid and homogeneous or cystic, with a greyish-white to brown colour.

Histopathology

 Microscopically, BCAs are composed of basaloid cells with eosinophilic cytoplasm, indistinct cell borders and round to oval nuclei, distributed in solid, trabecular, tubular, and membranous patterns. However, tumours may present with more than one of these patterns, usually with the predominance of one. The solid type is composed of sheets or islands of variable shapes and sizes, usually with peripheral palisading of cuboidal to columnar cells. The islands are separated by strands of dense collagenous tissue. The trabecular type is characterized by narrow strands, trabeculae or cords of basaloid cells separated by cellular and vascular stroma. A rare but distinctive feature is the presence of a richly cellular stroma composed of modified myoepithelial cells {542}. Ductal lumina are often observed among the basaloid cells and these cases are considered as tubulo-trabecular type. The membranous type of BCA has thick bands of hyaline material at the periphery of basaloid cells and as intercellular coalescing droplets. In the tubular type, ductal structures are a prominent feature. All variants may demonstrate cystic change, squamous differentiation in the form of whorls or ‘eddies’, or rare cribriform patterns. Occasional tumours, particularly of the tubular type, are largely oncocytic.

Immunoprofile

Immunopositivity for keratin, myogenic markers, vimentin and p63 indicate ductal and myoepithelial differentiation {214, 553,1598,2883}. Also the palisading cells of the solid type can stain for vimentin and myogenic markers. The pattern of expression reflects the different differentiation stages of the tumour cells, varying from the solid type, the less differentiated, to the tubular type, the most differentiated.

Genetics

 Genetic aberration has been described in three cases of BCA. Two cases presented trisomy 8 and one case the 7;13 translocation and/or inv(13) {1136,2385}.

 Prognostic and predictive factors

 BCA is usually a non-recurrent tumour, except for the membranous type, that has a recurrence rate of approximately 25% {1582}. Although exceedingly rare, malignant transformation of BCA has been reported {1825}.