Uncommon Etiology of Pancreatic Mass: a Case Report.

IgG4-related autoimmune pancreatitis (AIP) is a chronic inflammatory disease of the pancreas with a distinct histological feature. Its diagnosis remains challenging as some features overlap with pancreatic cancer. We present a case of IgG4-related AIP mimicking pancreatic cancer. A 70-year-old male patient presented with epigastric pain, radiating to the entire abdomen with an unquantified weight loss. Magnetic resonance cholangiopancreatography (MRCP) showed a mass with a 28 mm long axis, in the head of the pancreas with pancreatic duct dilatation. Thus, it was presumed to be a pancreatic neoplasm and pancreatic resection was undertaken without a definitive preoperative diagnosis. In terms of clinical presentation, imaging characteristics, and laboratory parameters, IgG4-related AIP can resemble pancreatic cancer. Thus, histopathological studies remain the gold standard for a definitive diagnosis that may show a diffuse lymphoplasmacytic infiltrate with storiform fibrosis. On immunohistochemistry, the majority of plasma cells are positive for IgG4 (>50 per high-power field (HPF)). In our case, the histologic diagnosis allowed us to suggest the diagnosis of IgG4-related AIP and the immunohistochemical diagnosis confirmed the diagnosis. It is critical to distinguish pancreatic cancer from IgG4-related AIP due to its completely different prognosis and therapy. Steroids are the first-line treatment that allow a reduction of risk of relapse; therefore, a misdiagnosis as a malignancy leads to inappropriate surgical interventions. In this case, a biopsy is recommended.