An Extragastrointestinal Stromal Tumor in the Omentum With Peritoneal Seeding Mimicking an Appendiceal Mucinous Cancer With Carcinomatosis

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract. These tumors tend to present most frequently in the stomach, followed by the small intestine. GISTs can also arise from the omentum, retroperitoneum, mesentery, or pleura and are termed extragastrointestinal stromal tumors (EGISTs) when they do so. EGISTs arising from the omentum are very rare. Due to the limited incidence of EGISTs in the omentum, the diagnostic criteria are not well established, and making a correct diagnosis may be difficult. In this report, we present a case of an EGIST of the omentum with peritoneal metastasis that was initially suspected to be an appendiceal mucinous carcinoma with carcinomatosis on positron emission tomography/computed tomography imaging.