Immature trichoepithelioma: report of six cases.

We report 6 cases of an immature variant of trichoepithelioma which histologically appears to show differentiation toward the primitive hair germ. The lesions presented in mature adults (mean age 44 years). Four occurred in men and 2 in women. Four lesions occurred on extremities, an unusual location for trichoepitheliomas. Histologically, the lesions were characterized by well-circumscribed, but unencapsulated, dermal collections of small tumor lobules composed of basaloid cells with invaginations resembling primitive dermal hair papillae. There was no adenoidal growth pattern or horn cyst formation. The separation of the immature lesions from those of classical trichoepithelioma and basal cell carcinoma can be made if multiple morphological features are considered; no one particular finding is diagnostic. The major differential features between the immature trichoepithelioma and basal cell carcinoma are circumscription, tumor lobule uniformity, occasional immature hair germs, and lack of retraction artifact of the tumor lobules from the stroma. The differential features between the immature and classical trichoepithelioma are less conspicuous. The immature form typically exhibits no horn cysts, displays fewer primitive hair structures, and lacks the adenoidal growth patterns of the tumor lobules which are usually present in the classical trichoepitheliomas.