A proliferating pilar tumor is a rare neoplasm arising from the isthmus region of the outer root sheath of the hair follicle.
Most tumors arise within a preexisting pilar cyst.
Usually benign in nature, malignant transformation with local invasion and metastasis has been described. A proliferating pilar tumor usually arises in the setting of single or multiple pilar cysts. Often an asymptomatic nodule is present for months to years before a rapid increase in the size of the lesion occurs. This is thought to signal neoplastic transformation of the cyst, leading to the proliferating pilar tumor. The stimulus for this transformation is not known but may be trauma, irritation, or inflammation.