The gastrointestinal stromal tumour (GIST) is a tumour that arises from the interstitial cells of Cajal within the GI tract and represents the most common mesenchymal tumour of the abdomen. GISTs may be found at all levels of the GI tract; however the stomach is the most common location. The tumour may arise sporadically or in association with syndromes such as the Carney triad, neurofibromatosis type 1, and familial GISTs.

Grossly, GISTs are typically circumscribed fleshy masses centred within the muscularis propria of the GI tract. Microscopically, two morphologic variants of GISTs exist: the spindle cell type and epithelioid type. The present case demonstrates a spindle cell GIST of the duodenum with a slightly lobular and fascicular pattern of growth within a fibromyxoid stroma. The cells are spindled in nature with a characteristic perinuclear vacuolization and mild to moderate nuclear pleomorphism.

By immunohistochemistry, GISTs may be identified by positive staining for CD117 and/or DOG1.

GISTs may also be classified by molecular testing and commonly demonstrate alterations in KIT, PDGFRA, or SDH which ultimately have implications in targeted therapy.