Coexistent trichilemmoma and trichoblastoma without associated nevus sebaceus.

Trichilemmoma and trichoblastoma are benign adnexal neoplasms derived from the hair follicle unit. While trichilemmomas are closely associated with the epidermis, trichoblastomas are found within the dermis and subcutaneous tissue. Both tumors have been reported to arise within nevus sebaceus of Jadassohn (NSJ). We present a 42-year-old white male with a 5 mm crusted, erythematous papule on the right occipital scalp that had been present for years. A shave biopsy was performed and read as trichilemmoma involving the biopsy base. The patient returned for follow-up 2 months later with recurrence of a crusted papule, measuring 9 mm in greatest diameter at the site of the previous biopsy. The lesion was excised for complete histologic evaluation, diagnosed as trichilemmoma with verrucoid features and associated basaloid proliferation with adnexal differentiation, again involving the biopsy base. The lesion recurred 2 months later in the form of an 8 mm multilobulated pink nodule. It was again excised and diagnosed as trichoblastoma with overlying trichilemmoma. The significance of this finding is that coexistent lesions do not necessarily necessitate a preexisting nevus sebaceous. Rather, this finding supports the notion of a common stem cell capable of differentiating toward the various portions of the hair follicle unit and adnexal structures. The idea is that any portion of the skin adnexal structure may develop out of a pluripotential germ cell and develop into a tumor.