Malignant proliferating trichilemmal tumor (MPTT) is a rare adnexal neoplasm originating from the outer root sheath of the hair follicle. It most often arises from a pre-existing proliferating trichilemmal (pilar) tumor. MPTT occurs most commonly (approximately 90%) on the scalp, although other anatomic locations have been reported. Women over the age of 40 are most often affected.

The diagnosis of MPTT is based mainly on histological features. A histological classification system has been proposed to differentiate the more common and benign proliferating trichilemmal tumor (PTT) from low-grade and high-grade MPTTs, which have potential for metastasis.

Recurrences are not uncommon. MPTT is at high risk for metastasis, with regional and distant metastatic disease in up to 25% of patients.

More than 40 cases of MPTT have been described in the literature, including 12 cases with metastatic disease and several mortalities.