Trichilemmomal Horn Arising from a Nevus Sebaceus and Pilar Cyst: A Case Report and Review of the Literature

Abstract

Nevus sebaceus can give rise to multiple neoplasms, some of which have the potential to become malignant. We describe the occurrence of a previously unreported combination of a cutaneous horn proliferating from a trichilemmoma overlying a multilocular pilar cyst and nevus sebaceus. An elderly woman with a several years’ history of multiple scalp subcutaneous nodules and an enlarging cutaneous horn presented to our clinic. The nodule underlying the large exophytic horn was tender and occasionally bled. The lesion was removed by excisional biopsy due to concern for malignancy. Histologically, a verrucous trichilemmoma was identified overlying irritated pilar cysts and a nevus sebaceus. Phosphatase and tensin homolog (PTEN) mutations are frequently found in trichilemmomas and HRAS mutations in nevus sebaceus. Mammalian target of rapamycin (mTOR) is a part of the final common pathway for HRAS and PTEN raising the therapeutic question if rapamycin can be used to treat these neoplasms.