A148 DESMOID FIBROMATOSIS DIAGNOSED IN A 27-YEAR-OLD MALE AFTER BEING MISTAKEN FOR A GASTRIC GIST

Abstract

Abstract Background The gastrointestinal tract can be affected by both epithelial and non-epithelial tumors. When considering non-epithelial tumors, gastrointestinal stromal tumors (GISTs) are the most common with an incidence of 7–15 cases per million per year. Fibroblastic desmoid tumors, originating from mesenchymal cells, are rarer with an incidence of 2–4 cases per million per year. Despite being distinct lesions, these tumors may appear similar on imaging when they involve the stomach wall or bowel. As a result, they may be confused with one another when initially diagnosed. Aims This report aims to present a case where a desmoid tumor was mistaken for a gastric GIST despite extensive investigation prior to laparotomy. Methods Retrospective review of one patient. Results A 27-year-old gentleman presented with acute left-sided abdominal pain and postprandial fullness that began 2 days prior. A CT-abdomen/pelvis demonstrated a large exophytic mass arising from the lesser sac of the stomach, in keeping with an aggressive gastrointestinal stromal tumor (GIST). He was admitted and was further investigated with an endoscopic ultrasound, which demonstrated an exophytic hemorrhagic mass arising from the muscularis propria of the gastric wall. Again, this was most consistent with a GIST and urgent surgical resection was recommended. An exploratory laparotomy was organized the following day. There were no signs of a GIST intraoperatively, but rather a soft tissue tumor of the left mesentery of the transverse colon. This mass was resected, and a biopsy of the peritoneum was collected. Pathology identified a low-grade spindle cell tumor, CD117/CD34 negative with patchy cytoplasmic and nuclear β-catenin staining, in keeping with desmoid fibromatosis. Conclusions This case illustrates how GISTs and desmoid tumors are often mistaken for one another when associated with the stomach wall or bowel. Despite morphologic features of both tumors outlined in the literature, pathology, with focus on specific staining patterns, is necessary in many cases to distinguish the two. This distinction is paramount for appropriate prognostication and management, including timely investigation for associated diseases such as Familial Adenomatous Polyposis in patients with desmoid tumors. Funding Agencies None