A case of malignant gastrointestinal stromal tumor (GIST) of the omentum

Background: Malignant gastrointestinal tumor (GIST) arising in the omentum is very rare, and its cytological findings have not been reported. Case: A 73-year-old man reported right upper abdominal pain. Ultrasonography, compurted tomography (CT), and magnetic resonance imaging (MRI) showed a huge mass in the left upper abdomen and 2 cystic tumors in the liver. Chemotherapy was conducted with 5-FU and CDDP, and the patient died 9 months later. At autopsy, the tumor, 22.3×16.0×7.0cm in size, was found to arisen in the omentum. Cytology revealed solitary spindle and polygonal cells with rich cytoplasm. The nucleus was round and oval, with nuclear anisokaryosis and irregularity marked, and chromatin pattern finely granular. Multinucleated cells with a giant nucleus and intracytoplasmic inclusions were also observed. Histologically, tumor cells had rich cytoplasm, proliferating in sheet or fascicular patterns with hypervascular stroma. Immunohistochemically, tumor cells were positive for CD 34, CD 117, and vimentin. Conclusion: Diagnosis of GIST by cytology alone is difficult, but it is possible to diagnose malignancy from cytological findings such as cytological atypia, cellularity, necrotic background, and tumor size.