Discovered on gastrointestinal stromal tumor 1, a keystone in the diagnosis of extraintestinal gastrointestinal stromal tumors.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract (GIT) but have a low incidence. Arising from the interstitial cells of Cajal, GISTs occur at different sites in the GIT with stomach being the most common. They can rarely be seen at sites outside the GIT such as omentum, retroperitoneum and are called as extraintestinal GISTs (EGIST). They have a spindle or epithelioid cell morphology and show positivity by immunohistochemistry (IHC) for CD117. Our aim was to study the clinicopathological and immunohistochemical profile of our cases of EGISTs. A cross-sectional study of EGISTs received from 2010 to 2015 was done. IHC with CD117 and discovered on GIST1 (DOG1) was performed and tumors were scored based on the percentage of cells that stained positive. Thirteen abdominal non-GIST spindle cell tumors were included in the study as controls. Seven cases of EGIST were included (four-omental, three-retroperitoneal). All cases stained positive for CD117 and DOG1. One case of epithelioid EGIST scored 4 + with DOG1 and 2 + with CD117. Another case with mixed morphology scored 2 + with DOG1 and 4 + with CD117. All controls were negative for both markers. EGISTs are one of the rare differentials for spindle cell lesions outside the GIT. Although both markers stain positive, DOG1 showed higher score with epithelioid GISTs.

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