Benign Cutaneous Mixed Tumor with Combined Folliculosebaceous and Eccrine Differentiation

Abstract

Cutaneous adnexal tumors manifesting divergent differentiation are a rare but recognized subset of adnexal neoplasms. We present a 42‐year‐old woman with a cystic tumor of the skin above the left eyebrow. A biopsy revealed a tumor comprising eccrine tubules lined by a single row of columnar epithelium surrounded by a peripheral small cuboidal layer and containing, adhered to the internal cuticle, mucin‐like material. In some areas, the eccrine tubules were associated with sheets of small plasmacytoid myoepithelial cells embedded in a chondroid matrix. Elsewhere, there were cysts containing central keratinous debris associated with a peripheral basaloid epithelial proliferation showing the formation of buds mimicking abortive hair follicle structures. No polarizable terminal or vellus hairs were seen. These two elements were geographically separate in the biopsy but collided in the center of the tissue element. A diagnosis of benign cutaneous mixed tumor with combined folliculosebaceous and eccrine differentiation was rendered. Such tumors present a diagnostic challenge and are frequently confused with neoplasms including the benign mixed tumor (chondroid syringoma) and a variety of eccrine and folliculosebaceous neoplasms and harmatomata. This case illustrates the potential multilineage differentiation of pluripoential germinative cells of the cutaneous adnexae.