History:

Elderly female who noted a "change" in a chest lesion. Examination disclosed a classic seborrheic keratosis on the anterior chest adjacent to a redder, scaly plaque. The differential diagnosis included irritation in a seb ker OR a collision tumor (two different tumors over-lapping). The brown area was, indeed, just a seb ker. But the red area (outlined with arrows) was a SCCA adjacent to and abutting on the seb ker.