Clear cell carcinoma of the skin: a tricholemmal carcinoma?

We report the light and electron microscopic findings of an unusual, locally aggressive cutaneous clear cell carcinoma involving extensively the left cheek and upper lip of a 61-year-old woman. The patient remained free of recurrence 5 years after wide local excision. Microscopically, the tumor was a poorly circumscribed and deeply invasive clear cell neoplasm characterized by large lobules and convoluted trabeculae of polygonal and columnar cells with palisading of the peripheral cells rimmed by thick basement membranes, and horn microcysts with tricholemmal keratinization. The tumor infiltrated the muscle and the salivary glands. Only slight cytologic atypia and a few mitotic figures were present. No definite ductal differentiation was found. Immunoperoxidase staining for carcinoembryonic and epithelial membrane antigens was negative. Ultrastructurally, the tumor cells were joined by desmosomes, and surrounded by thick basal lamina. The cytoplasm contained large pools of glycogen. Peculiar whorls of tonofilaments and aggregates of needle-shaped and club-shaped structures were observed in a small keratinizing focus. The tumor showed features resembling the outer root sheath of the hair follicle microscopically. Furthermore, the lack of duct-like structures and immunoreactivity of carcinoembryonic antigen tend to favor tricholemmal over sweat gland differentiation in this tumor which possibly represents a rare example of tricholemmal carcinoma.