Collision tumor of the mediastinum: a rare entity.

A collision tumor is a rare entity consisting of 2 different tumors that, because of proximity, merge together. The diagnosis of a collision tumor requires that the 2 tumors be histologically distinct. This phenomenon has been described mostly at the esophagogastric junction, where a squamous cell carcinoma of esophageal origin collides with a gastric adenocarcinoma. The present case is of 2 histologically distinct tumors with aggressive features occurring at the mediastinum.