Retroperitoneal Fibrosis with Pancreatic, Colonic Involvement Mimicking Malignancy: A Case Report.

Introduction: IgG4-related retroperitoneal fibrosis (IgG4-related RPF) is a rare systemic fibro-inflammatory disorder that may present as a mass-forming lesion, mimicking malignancy. Gastrointestinal involvement is uncommon, and acute large bowel obstruction as the initial presentation is exceedingly rare. Case Presentation: A 59-year-old male presented with acute abdominal pain, distension, and obstructive symptoms. Colonoscopy identified a stenosing lesion at the left colic flexure, histologically confirmed as a polypoid adenoma. Contrast-enhanced computed tomography (CT) revealed a mass infiltrating the left colic flexure, peripancreatic fat, and pancreatic tail, with significant colonic dilatation. Emergency surgery was performed due to bowel obstruction. En bloc extended left hemicolectomy, distal pancreatectomy, and wedge gastrectomy with loop ileostomy were carried out. Histopathology confirmed IgG4-related RPF involving pericolic and peripancreatic fat, sparing the pancreatic parenchyma. Serum IgG4 was within normal limits. The patient had an uneventful recovery. Conclusion: This case illustrates a rare presentation of IgG4-related RPF manifesting as acute bowel obstruction. Despite imaging and clinical features suggestive of malignancy, histopathological confirmation revealed benign pathology with an inflammatory fibrosing process. Awareness of such atypical manifestations is crucial to prevent unnecessary radical oncological resections.