Answer: Low risk GIST

Histology: Localized within the muscularis propria of the duodenum is a fairly well circumscribed nodule measuring 3.1 cm. The lesion consists of fascicles of spindle cells with eosinophilic cytoplasm. The nuclei appear ovoid and blunt ended with areas of palisading. Mitotic activity is virtually absent. The lesion lacks pleomorphism and lacks necrosis. The prominent feature in this case is the presence of acellular collagen bundles interspersed between the spindle cells.

Discussion: This lesion has the classic histological appearance of a GIST. In the duodenum, the presence of a GIST that is greater than 2 cm and less than 5 cm without mitotic activity is considered to be low risk for malignant behavior. The relatively uncommon finding in this case is the prominent acellular collagen bundles which are known as skenoid fibers. These are relatively uncommon in GIST yet when they occur they tend to do so in duodenal GIST. It has been proposed that the presence of skenoid fibers is associated with a good prognosis in GIST which in this case goes along with the lesion’s small size and lack of mitotic activity. Another lesion which may have similar appearing collagen bundles is angiomyolipoma. Although they are not as frequently referred to as skenoid fibers, occasional articles in the literature describe angiomyolipomas with skenoid fibers. In this case, stains for CD117 were performed which were diffusely and strongly positive and stains for desmin were negative consistent with a diagnosis of GIST.