Benign Cutaneous Adnexal Tumors with Combined Folliculosebaceous, Apocrine, and Eccrine Differentiation

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Abstract
Benign cutaneous adnexal tumors displaying divergent differentiation are rare, with very few well-documented cases reported in the literature. We describe eight cases of benign adnexal tumors showing a variable combination of eccrine, apocrine, and folliculosebaceous differentiation. Clinically, all tumors presented as solitary, slowly enlarging dermal or subcutaneous nodules located in the head and neck and the extremities. Histologically, they were characterized by well-circumscribed, unencapsulated nodules composed of a lobular proliferation of epithelial cells displaying a spectrum of trichogenic, sebaceous, apocrine, and eccrine differentiation. The histological spectrum included lobules and trabeculae of basaloid cells with glandular and ductal elements, well-formed folliculosebaceous units, primitive follicles, and foci of tricholemmal keratinization. Immunohistochemical evaluation in four cases showed similar cytokeratin, carcinoembryonic antigen, and epithelial membrane antigen staining profiles as those reported for sweat gland adenomas; in addition, focal S-100 protein positivity and GCDFP-15 positivity could also be demonstrated, suggesting eccrine-apocrine differentiation. The tumors were most frequently confused histologically with other adnexal neoplasms, including sebaceoma, sebaceous adenoma, basal cell carcinoma, chondroid syringoma, and trichoepithelioma. The present series highlights the capability for multidirectional differentiation of pluripotential germinal cells in the dermis and serves to underscore the difficulties occasionally encountered in the classification of benign skin adnexal neoplasms.