Question: A 53-year-old man was evaluated for nausea, progressive weakness, and loss of energy. He had a history of pituitary adenoma resected approximately 8 months prior, Barrett's esophagus without dysplasia and most relevantly, a history of a 20-cm (Figure A, computed tomography scan of the abdomen) stage IIIb gastric high-grade, predominantly epithelioid, gastrointestinal stromal tumor (GIST; Figure B, note the epithelioid appearing GIST within the black arrows, the oxyntic mucosa within the blue arrows, and the muscularis separating the 2 areas) extirpated by partial gastrectomy and Billroth II approximately 18 months prior. Family history was remarkable for thyroid cancer in a maternal uncle. He denied any history of alcohol use, tobacco use, or illicit drug use. The patient was later noted to have a 3-cm left upper quadrant abdominal mass on surveillance imaging 12 months later while being treated with adjuvant tyrosine kinase inhibitors (imatinib followed by sorafenib). Subsequent laboratory values and physical examination were unremarkable. Repeat imaging demonstrated an enlarging 6.7 Â 7.0-cm mass within the abdominal mesentery anterior to the left kidney (Figure C, repeat computed tomography scan of the abdomen). He was referred for laparoscopic partial small bowel resection and excision of a 10-cm mesenteric mass at the level of the efferent jejunal limb, a 3-cm nodule at the level of the proximal ileum, and 6 smaller nodules within the small bowel mesentery. What is the diagnosis? See the Gastroenterology web site (www.gastrojournal.org) for more information on submitting your favorite image to Clinical Challenges and Images in GI.