Answer: Gastrointestinal stromal tumor

Histology: The lesion is composed of sheets of spindle cells arranged in a storiform pattern. It is densely cellular and focally shows areas of hyalinization. The cells show mild atypia and prominent mitotic figures. A C-kit immunostain shows diffuse and strong labeling in the tumor cells.

Discussion: GIST in the omentum usually represents a metastatic tumor from an intestinal or a gastric primary. Although Kit positivity is a major defining feature of GIST it is no longer considered an absolute requirement. Although not specific for GIST, Kit positivity is usually strong and global. Other tumors that can be kit positive are seminoma, small cell carcinoma, granulocytic sarcoma, mastocytoma, and to a lesser extent, metastatic melanoma and clear cell sarcoma, and few sarcomas and carcinomas. In most cases, the differential diagnosis is resolved based on morphology or performance of few additional immunostains. GIST also express CD34, nestin, smooth muscle markers but are generally negative for desmin. Keratin 18 and to a lesser degree keratin 8 are occasionally expressed. GIST represents a continuum in biologic potential and the behavior of the lesion is strongly correlated with the size of the tumor and mitotic activity (/50 HPF). These parameters should be applied differently in gastric and intestinal GIST since small intestinal GISTs display a more aggressive behavior.

Reference(s):
– “Gastrointestinal Stromal Tumors”: Review on morphology, molecular pathology, prognosis and differential diagnosis. Markku Miettinen and Jerzy Lasota. Arch Pathol Lab Med.2006;130:1466-78