S2210 A Rare Retroperitoneal Mass Simulating a Complicated Diverticulitis

Abstract

Introduction: Extra-gastrointestinal stromal tumors (EGISTs) are a group of rare neoplasm that arises from cells outside the gastrointestinal (GI) tract but with similar pathologic characteristics as of gastrointestinal stromal tumors (GISTs). They account for only 5% of all GISTs involving the retroperitoneum, mesentery or omentum. EGISTs typically presents as large intra-abdominal masses with unspecific symptoms and are known to have a more aggressive behavior than its GI counterpart. Case Description/Methods: A 50-year-old male inmate with no medical history presents to the ED after having unquantified episodes of non-bloody, non-watery diarrhea and vomiting of gastric content for 5 days prior to admission. The patient also reports subjective fever and constant left quadrant abdominal pain. Upon physical exam, he was found with signs of dehydration, a distended abdomen with hyperactive bowel sounds, and a non-mobile large mass on the left lower quadrant with tenderness to light palpation. Laboratories were remarkable for neutrophilic leukocytosis, thrombocytosis, and elevated inflammatory markers. Abdomino-pelvic CT revealed a large left heterogeneously enhancing retroperitoneal mass with central necrosis measuring 18.6 cm AP x 18.4 cm transverse x 25.3 cm CC causing mass effect over the left kidney with loss of intervening fat planes, flattening of the left adrenal gland and displacement of the stomach, small and large bowel with almost complete collapse of the distal transverse and descending colon. Core-needle biopsy revealed high-risk GIST with positive immunohistochemistry for C-KIT, CD34, CD56, actin, and desmin with a very high mitotic index (45 mitosis/25 HPF). Surgery was deferred due to large size of mass and closure to adjacent structures. Neoadjuvant therapy with Imatinib was started but patient was lost to follow up upon discharge. Discussion: This case describes a rare presentation of EGIST emerging from the retroperitoneum with a positive immunohistochemistry for desmin and manifesting as a complicated diverticulitis. Although there is limited data about its origin, EGISTs and GISTs are recognized to be molecularly identical. Furthermore, because EGISTs are found outside the GI system, symptoms only appear when the tumor has progressed to an advanced stage giving it a worse prognosis. As a result, having EGIST as a differential diagnosis for abdominal mass is crucial. More research is needed to better understand the behavior, prognosis, and treatment options of these tumors.