Development of a malignant proliferating trichilemmal cyst in a patient with multiple trichilemmal cysts.

A case of multiple trichilemmal cysts is presented. After trauma, a large tumor developed on one of the cysts. The histologic features of this tumor were similar to those of proliferating trichilemmal cyst, but some of the tumor cells showed definite nuclear atypicality. The malignant potential of this tumor was confirmed by later development of metastases to the regional lymph nodes.