Gastrointestinal stromal tumors (GISTs) albeit rare, are the most common mesenchymal neoplasms of our gastrointestinal (GI) tract. GISTs present with nonspecific symptoms and are found incidentally on endoscopy or imaging. A significant portion of GIST diagnoses expresses KIT/CD117 and DOG-1 tissue markers which are pathognomonic for GIST. More recently, Ki-67 was found to be a significant prognostic marker for determining the risk of recurrence. We present a patient with a mesenchymal mass in the small intestine with pathognomonic features of GIST and expression of Ki-67, an important immunocytochemical marker of proliferation.The patient was a 71-year-old male with a history of hyperlipidemia and hypertension. He presented to the emergency department complaining of bloody diarrhea for two days, with associated nausea, vomiting, and abdominal cramping. Initial blood pressure on presentation was 77/52 mm Hg. Computed tomography (CT) of the abdomen and pelvis revealed a large solid mass with cystic components. The mass was not visualized with esophagogastroduodenoscopy or colonoscopy, and surgical intervention was warranted. A 14 cm x 11.5 cm x 10 cm tumor was found in the ileum. The tumor was excised with small bowel segmental resection and the specimen was sent for pathological evaluation. Immunohistochemical analysis confirmed the diagnosis of GIST with diffuse CD117/c-Kit protein expression. The tumor was high grade with a high mitotic rate at 30 mitoses/50 high-power fields (HPF) and had spindle cell morphology. Of note, 10% of the tumor cells were positive for Ki-67.GISTs have a high risk of recurrence and a more favorable prognosis with advancements in management. Prior to imatinib therapy in the early 2000s, GISTs prognosis was very poor, as they are resistant to most conventional chemotherapeutic agents and radiation. While the prognosis is fair, surgical resection and imatinib therapy have improved outcomes and risk of recurrence. Prognosis and risk of recurrence can be determined by assessing the mitotic rate, tumor size, and recently, expression of Ki-67. Ki-67 provides a reliable and reproducible approach to assess the prognosis of GIST.

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