S3029 Unexpected Diagnosis of Worsening Abdominal Distension

INTRODUCTION: Gastrointestinal stromal tumors (GISTs) constitute about 1% of primary gastrointestinal cancers. Primary omental GIST without gastrointestinal involvement (extra-gastrointestinal stromal tumor or EGIST) is extremely rare. We present a case of EGIST with peritoneal metastases and partial response to Imatinib. CASE DESCRIPTION/METHODS: A 68-year-old Caucasian male with a history of coronary artery disease and chronic obstructive lung disease presented with shortness of breath and progressively worsening abdominal distension for several months. Labs were significant for hemoglobin of 10.8 g/dL and positive fecal occult blood test. Computed tomography (CT) of the abdomen and pelvis revealed innumerable masses in the peritoneum and omentum (Figure 1), with no solid visceral abnormality. CT-guided core biopsy of an omental mass revealed cluster spindle cells diffusely positive for c-kit (CD117) (Figure 2) and CD34, with negative stains for cytokeratin 5/6, D2-40, calretinin, cytokeratin AE1/3, Melan-A, S-100, and actin, consistent with EGIST. Subsequent esophagogastroduodenoscopy and colonoscopy with biopsy did not reveal any neoplastic lesion. His symptoms dramatically improved with Imatinib. Follow-up CT scan 3 months later showed partial remission (Figure 3). DISCUSSION: Primary omental stromal tumors without GI tract involvement or attachment are exceedingly rare. Classic GISTs originate from the interstitial cells of Cajal (ICCs) or their stem cells. EGISTs share the same immunophenotype despite arising outside of GI tract, as the tumors either lose their original connection with GI tract or originate from ICC-like precursor cells dispersed during fetal development. The most common symptoms associated with omental stromal tumors are abdominal distension and pain. The tumors are often found as large masses as they tend not to cause obstruction and associated symptoms at earlier stages. CT is the preferred imaging modality for identification of the primary tumors and metastases. Endoscopy can be used to obtain additional information and biopsy. There are no established treatment guidelines for EGIST, and the guidelines developed for GIST are typically used for management of EGIST. Surgery is the primary treatment for localized disease. In our case, Imatinib was utilized due to numerous non-resectable masses, leading to partial remission. More studies are needed to better understand the pathophysiology and evaluate treatment options for EGIST.Figure 1.: Initial abdominal CT revealed extensive lymphadenopathy and multiple soft tissue masses in the peritoneum and omentum.Figure 2.: The biopsied mass was mostly made of cluster spindle cells with high cellularity and frequent mitotic activity, with diffusely positive stain for CD117/c-KIT, a key proto-oncogene activated in GISTs.Figure 3.: A repeat abdominal CT, in 3 months after the diagnosis, showed partial response to Imatinib.