S6464 A Case of a Massive Gastrointestinal Stromal Tumor

Introduction: Gastrointestinal Stromal Tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract. They account for approximately 0.1%-3% of all gastrointestinal tumors. GISTs most commonly occur in the stomach (50%-60%), followed by the small intestine (30%-40%). The average size of a gastric GIST at the time of diagnosis is typically around 5-6 cm, with most tumors discovered incidentally during imaging or endoscopy for unrelated issues. Larger tumors, particularly those exceeding 10 cm, are considered rare and are associated with a higher risk of malignancy and metastasis. Case Description/Methods: A 63-year-old man presented to the GI clinic with a 3-month history of progressive abdominal distension and 30-lbs weight loss. A large intra-abdominal mass was seen on physical examination. Laboratory investigations revealed mild anemia. A computed tomography (CT) abdomen revealed a 25 cm × 20 cm × 15 cm large, well-defined, heterogeneous mass in the left upper abdomen originating from the stomach and exerting a mass effect on the left hemidiaphragm and spleen. There was displacement of the portal vasculature to the right. No evidence of distant metastasis was noted. Upper gastrointestinal endoscopy revealed a large deep ulceration in the gastric fundus/body with necrotic tissue, suggestive of a mass invading the lumen. Endoscopic endosonography confirmed the large heterogenous peri-gastric mass with solid and cystic components seen. Biopsy of the gastric mass revealed a mixed GIST with spindle and epitheloid cells present. Tumor cells were positive for CD34, CD117. Ki67 index was 30%. Mitotic index was 18/10 high power field. Patient was referred to oncology and surgery. He was subsequently started on neoadjuvant imatinib. Discussion: This case is notable for the exceptional size of the gastric GIST, measuring 25 cm, which is remarkably larger than the average gastric GIST size of 5-6 cm. While the majority of GISTs are diagnosed at smaller sizes, this case highlights the rarity of such large tumors and underscores the importance of early detection. Larger GISTs have a higher potential for malignancy and metastasis. The management of such tumors typically involves surgical resection, with or without adjuvant therapy, depending on risk factors for recurrence and metastasis.