Tricholemmal carcinoma. A clinicopathologic study of 13 cases.

We describe 13 cases of tricholemmal carcinoma, a rarely recognized cutaneous adnexal neoplasm. The patients were nine men and four women. In general, the tumors presented as slow-growing epidermal papules, indurated plaques, or nodules showing predilection for sun-exposed, hair-bearing skin. The lesions were most frequently misdiagnosed clinically as basal cell carcinoma. Histologically, they showed a variegation of growth patterns including solid, lobular, and trabecular; they were characterized by a proliferation of epithelial cells with features of outer root sheath differentiation, including abundant glycogen-rich, clear cytoplasm, foci of pilar-type keratinization, and peripheral palisading of cells with subnuclear vacuolization. Because of their variable growth pattern, overt cytologic atypia, abundant clear cytoplasm, occasional pagetoid intraepidermal spread, and brisk mitotic activity, these tumors may pose difficulties for diagnosis and be confused with other malignant skin tumors with clear cell changes. Despite the seemingly malignant cytological appearance of these lesions, clinical follow-up in 10 cases showed no recurrence or metastasis over a period of 2-8 years. Thus, conservative surgical excision with clear margins appears to be the treatment of choice for these neoplasms.