Giant Sigmoid Mesenteric Gastrointestinal Stromal Tumor Presenting with Chronic Gastrointestinal Bleeding and Acute Bowel Obstruction: A Rare Case Report

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal (GI) tract, primarilyaffecting the stomach and small intestine.However, sigmoid mesenteric GISTs with extensive pseudocystic degeneration are rare and pose significant diagnostic and management challenges.We report the case of a 45-year-old male with an 18-year history of a gradually enlarging pelvic mass that progressed to a giant per abdominal tumor, intermittent per rectal bleeding for one year, and acute bowel obstruction.The patient presented with severe anemia (hemoglobin 6.6 g/dL), which improved to 9.8 g/dL after three packed red blood cell transfusions. Contrast-enhanced computed tomography (CECT) revealed an 18 17.6 20.6 cm solid-cystic lesion arising from the sigmoid mesentery. The patient underwent exploratory laparotomy with tumor excision, sigmoid colectomy, and end colostomy (Hartmann's procedure) due to edematous bowel precluding safe primary anastomosis. Histopathology confirmed a spindle cell neoplasm with extensive hemorrhagic necrosis, and immunohistochemistry was positive for CD117 (c-KIT) and DOG1, confirming the diagnosis of GIST. Postoperatively, adjuvant imatinib (400 mg/day) was prescribed for 6 months due to high-risk features (size >10 cm, necrosis). Follow-up lower GI endoscopy with biopsy at 6 months showed no residual malignancy, and tumor markers (CA 19-9 and CEA) were within normal limits. This case highlights the diagnostic challenges of rare sigmoid mesenteric GISTs with pseudocystic degeneration and underscores the importance of a multidisciplinary approach combining surgery, immunohistochemistry, and adjuvant therapy for optimal outcomes.