Microscopic examination revealed irregular epidermal hyperplasia with a central cystic invagination communicating with the surface. Arising from this cystic lesion were numerous, irregular, well-differentiated lobules of squamous epithelium with infundibular-type keratinization. No notable atypia was identified. The diagnosis of multifocal proliferating epidermoid cysts was made. The patient was treated by excision of the lesions.
Proliferating epidermoid cyst (PEC) is a rare neoplasm arising from the epithelium of the hair follicle. The term was first coined by Jones1 in 1966 to describe what we now call “proliferating trichilemmal cyst” (PTC). It was not until 1995 that the 2 types of epithelial tumors were distinguished on clinical and histopathological grounds by Sau et al,2 and their report to our knowledge remains the only reported series on PEC to date.