An Unusual Cause of Abdominal Pain

Question: A 48-year-old woman without significant past medical history developed an episode of severe abdominal pain that lasted approximately 2 hours with associated nausea and occasional emesis. Laboratory investigations were notable only for microcytic anemia. Subsequent computed tomography (CT) of the abdomen and pelvis with contrast is shown (Figure A). She was referred to gastroenterology and underwent extended esophagogastroduodenoscopy, which revealed a 4 cm mass in the fourth portion of the duodenum (Figure B). This mass was subsequently biopsied, and she ultimately underwent segmental resection with duodeno-jejunostomy. Gross surgical specimen and histopathology were as shown in Figures C and D. What is the diagnosis?