Skin, Adenomyoepithelioma

Myoepitheliomas are exceedingly rare cutaneous tumours that present as dome-shaped, exophytic nodules on the face, neck, extremities, or trunk. They also occur in salivary glands, deep soft tissues, and other organs. Myoepitheliomas may occur at any age. Cutaneous myoepitheliomas range in size from 0.5 to 2.0 cm in diameter. The lesions are almost invariably benign; local recurrences have been recorded. Malignant cases have been designated myoepithelial carcinomas and have a very low metastatic potential.

Myoepitheliomas are circumscribed, non-encapsulated tumours situated in the dermis or subcutis. Dermal tumours may extend into the subcutis. They are composed of spindle-shaped, epithelioid, histiocytoid, and plasmacytoid (hyaline) cells. The cells usually have pale eosinophilic cytoplasm and relatively monomorphous ovoid nuclei. Mitoses are usually uncommon or absent. Some tumours have very little stroma, while others have a myxoid or collagenous hyalinized stroma. The rare adenomyoepithelioma combines glandular sweat gland elements with myoepithelial cells.

This case presents a fragmented excision of acral skin containing a subcutaneous nodular adnexal tumour. The tumour appears well-circumscribed (and at least partially encapsulated) with an expansile growth pattern and without significant infiltrative growth. The tumour also shows divergent differentiation including prominent well-formed glandular structures with surrounding variably hyperplastic myoepithelium and sheet-like areas comprised of similar cuboidal to slightly spindled myoepithelial cells. There is focal squamous and apocrine differentiation. The glandular structures show good basilar orientation of nuclei and there is no multilayering or significant atypia. Occasional mitotic figures are, however, noted (approx. 2 mitoses per 10hpf) consistent with adenomyoepithelioma.