Primary Extragastrointestinal Stromal Tumor (EGIST) of the Greater Omentum

ABSTRACT

Gastrointestinal stromal tumors (GISTs) were recently defined as spindle cell, epithelioid, or occasionally, pleomorphic mesenchymal tumors of the gastrointestinal tract that express the CD117 (proto-oncogene c-kit protein, stem cell factor receptor), as detected using immunohistochemistry. And they show a new tendency to include the CD117-positive mesenchymal spindle cell or epithelioid neoplasms primary in the omentum and mesentery, and is so termed extragastrointestinal stromal tumors (EGISTs). Omental EGISTs are very rare and similar to their gastrointestinal counterpart. We present a case of primary EGIST of the greater omentum in a 58-year-old man. The resected tumor mass measured 20X15X5 cm and weighed 1,150 g. The cut surface displayed a central cystic change and partial mural nodules. Microscopically, most parts of the tumor were composed of round or polygonal cells, with many of them containing perinuclear vacuoles. The mitotic count was less than one per 50 high-power-fields. Immunohistochemically, the tumor cells were diffusely positive for CD117 and vimentin, and focally for smooth muscle actin and CD34. Ultrastructurally, partially smooth muscle differentiation was confirmed in this case.