A proliferating pilar tumor (PPT) is a rare neoplasm arising from the isthmus region of the outer root sheath of the hair follicle. It is also commonly called a proliferating trichilemmal cyst or, less commonly, proliferating follicular-cystic neoplasm. It was first described by Wilson-Jones  as a proliferating epidermoid cyst in 1966. PPT was then distinguished from proliferating epidermoid cysts in 1995.  It occurs most commonly on the scalp in women older than 50 years.  Most tumors arise within a preexisting pilar cyst. Even though they usually are benign in nature, malignant transformation with local invasion and metastasis has been described. [4, 5, 6, 7, 8] A tentative stratification of PPTs into groups as benign, low-grade malignancy, and high-grade malignancy has been introduced. They may be inherited in an autosomal-dominant mode, linked to chromosome 3. See Pilar Cyst for more information.
A proliferating pilar tumor (PPT) usually arises in the setting of single or multiple pilar cysts. An asymptomatic nodule is often present for months to years before a rapid increase occurs in the size of the lesion. This is thought to signal neoplastic transformation of the cyst, with progression into a PPT. The underlying stimulus for this transformation is not known, but it has been hypothesized to be secondary to trauma, irritation, or chronic inflammation. In a review of 76 cases of PPTs seen in consultation from 1989-2000, Ye et al  proposed a distinction between benign and malignant variants. The authors also proposed histologic criteria that could predict behavior.
It is believed to be analogous to proliferating onycholemmal cyst, an entity that is even rarer in the literature as it pertains to the nail bed. 
Proliferating pilar tumor (PPT) are rare.
These tumors occur more commonly in females than in males.
PPTs are most common in older individuals aged 50-75 years, although they have been reported in individuals aged 20-30 years.
The prognosis is excellent with complete excision. Local recurrence and metastasis are extremely rare in benign cases. However, in proven malignant cases, metastasis has been suggested to occur in 30% of cases.