Malignant proliferating trichilemmal tumors--report of three cases.

We are reporting three patients with recent history of rapid enlargement of long standing nodular scalp lesions. Histologic sections revealed areas of relatively benign proliferation of pilar sheath epithelium and foci of trichilemmal keratinization characteristic of a proliferating trichilemmal cyst. There were also extensive areas of severe cellular dysplasia and surrounding dermal invasion indicating malignant transformation.