Dedifferentiated gastrointestinal stromal tumor arising de novo from the small intestine.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract and usually display monotonous cytologic features and immunoactivity for CD117. Anaplastic GIST, with pleomorphic cells and loss of CD117, until recently have only been reported in patients with chronic imatinib mesylate treatment. Dedifferentiated GISTs arising de novo is a newly identified entity that may prove to be difficult to diagnose. We present the case of a 52-year-old female found to have a dedifferentiated GIST without prior imatinib mesylate therapy. This case is the first reported dedifferentiated GIST arising de novo from the small bowel, and at 30cm in greatest diameter, the largest reported to date. Additionally, we demonstrate for the first time the loss of DOG1 in the anaplastic component of the tumor. De novo dedifferentiated GIST is a rare and diagnostically challenging tumor that may be mischaracterized unless considered in the differential diagnosis.