S6182 Fistulizing Duodenal GIST Masquerading as a Large Duodenal Diverticulum

Introduction: Gastrointestinal stromal tumors (GISTs) are rare mesenchymal tumors, representing 1%–2% of all gastrointestinal neoplasms. While often asymptomatic, GISTs may present with vague symptoms including abdominal pain, melena, hematemesis, or anorexia. We present the case of a 69-year-old man admitted for severe iron deficiency anemia, incidentally found to have a large periduodenal mass with fistulization into the duodenal bulb. Case Description/Methods: A 69-year-old man was admitted for evaluation of severe iron deficiency anemia found on outpatient labs. He denied overt gastrointestinal symptoms aside from generalized fatigue. Medical history included diabetes, hypertension and prostate cancer. On admission, vitals were stable and physical exam was unremarkable. Labs revealed a hemoglobin of 6.1 g/dL. Contrast-enhanced computed tomography abdomen/pelvis showed a 14.3 × 10.4 × 13.8 cm exophytic duodenal mass with central necrosis and air-fluid level, communicating with the duodenal lumen. The mass also caused extrinsic compression of the common bile duct, resulting in mild intrahepatic biliary dilation. An esophagogastroduodenoscopy revealed a large ulcerated lesion in the duodenal bulb, initially concerning for a large diverticulum versus a mass. Biopsies showed peptic duodenitis with ulceration and chronic inflammation. Endoscopic ultrasound-guided fine needle aspiration identified a large perigastric and periduodenal mass with fistulization into the duodenum. Histopathology confirmed a GIST with spindle cells positive for DOG-1 and C-KIT and negative for S100 and SMA. The patient was referred to a tertiary center, where surgery revealed a 14 cm duodenal mass and a 1 cm gastric lesion suggestive of metastasis. Tumor invasion into the liver and encasement of the right hepatic artery rendered it unresectable, so a palliative loop retrocolic isoperistaltic gastrojejunostomy was performed to bypass the obstruction. He was initiated on neoadjuvant imatinib to downstage the tumor with plans for potential re-evaluation for resection, and continues under close outpatient oncology follow-up. Discussion: Though rare, GISTs should be considered in the differential for GI and hepatobiliary tract masses. Fistulization, while uncommon, may occur silently and complicate the clinical picture. Greater understanding of GIST-associated fistulas is needed to guide further management.