Early diagnosis of small intestinal gastrointestinal stromal tumours (GISTs) is difficult. These tumours often present with peritonitis and intraperitoneal bleeding due to rupture. We experienced a case of a tumor that grows to 12 cm asymptomatic.
A 46-year-old man presented with sudden abdominal pain. Computed tomography revealed free air in the abdominal cavity, a 12-cm sized dilatation in the small intestine, and pooled residues. He was diagnosed with perforation of the gastrointestinal tract, and emergency surgery was performed. Inside the abdominal cavity, approximately 500 mL of bloody ascites was observed. A diverticulum-like mass measuring approximately 12-cm long was observed on the jejunum approximately 30 cm from the Treitz ligament. There was a large hematoma inside the mass, and perforation was recognised at the neck of the diverticulum. Partial resection of the jejunum and intraperitoneal drainage were performed. Immunohistochemistry revealed that the tumour was positive for KIT and CD34, and GIST was diagnosed. The patient was discharged on postoperative day 10 without significant complications. The patient did not relapse while taking imatinib as an adjuvant chemotherapy.
Gastrointestinal stromal tumours are the most common mesenchymal tumours of the gastrointestinal tract and may undergo increased diverticulum-like growth, as seen in this case. Local resection and proper chemotherapy increase long-term survival, suppress tumour growth, and reduce the risk of relapse.
Diagnosing GIST can be difficult due to the absence of clinical symptoms. It is necessary to ensure local resection and careful long-term follow-up.

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