Proliferating trichilemmal cyst in an organoid nevus.

An unusual tumor mass of the scalp found in an elderly woman is presented. This lesion had many features simulating a verrucous squamous cell carcinoma of the scalp. Pathologic findings eventually revealed the tumor mass to be a proliferating trichilemmal cyst. Differentiation between a benign and malignant process in these lesions can sometimes be difficult. Clinical as well as histopathologic evaluation is necessary to make an accurate diagnosis.