Collision tumor of eccrine poroma, seborrheic keratosis, and a viral wart.

A 68-year-old woman presented for evaluation of a large, red-brown plaque on her left buttock with irregular borders and prominent overlying verrucous changes. The plaque had been present since childhood but over a three-year period had been enlarging with increasing nodularity and easy bleeding with trauma. Histopathologic examination demonstrated an enlarged papillated and polypoid heterogeneous lesion. In part of the specimen, there are bulbous aggregates of small squamous cells with foci of eccrine ductal differentiation. There are other areas with horn pseudocysts, hypergranulosis, and compact orthokeratosis with parakeratosis. There are scattered enlarged heavily pigmented melanocytes, some of which have long and thick dendrites. This collision tumor consisted of an eccrine poroma, a seborrheic keratosis, and a viral wart. The clinical and histopathologic features of collision tumors and poromas are reviewed.