An atypical case with recurrent episodes of pancreatitis: is eosinophilic pancreatitis a diagnosis or a mimic in the spectrum of IgG4-related diseases? A case-based review : Eosinophilic pancreatitis or IGG4-releated disease.

Eosinophilic pancreatitis (EP) is a rare disorder characterized by prominent pancreatic eosinophilic infiltration and often associated with peripheral eosinophilia and elevated IgE. IgG4-related disease (IgG4-RD) may involve the pancreas as type 1 autoimmune pancreatitis, with lymphoplasmacytic infiltration, storiform fibrosis, and increased IgG4-positive plasma cells. Both can mimic pancreatic malignancy. We report a 43-year-old man who underwent distal pancreatectomy in 2016 for suspected cancer and later experienced three episodes of acute pancreatitis. In 2024, modest serum IgG4 elevation (304 mg/dL; reference range, 3-201 mg/dL) and persistent monocytosis prompted re-evaluation of the surgical specimen. Histology showed dense, patchy eosinophilic infiltration with microabscesses and non-storiform fibrosis, while IgG4-positive plasma cells were sparse. These findings favored EP over type 1 autoimmune pancreatitis. This case highlights that EP may occur without peripheral eosinophilia, mimic malignancy and IgG4-RD, and require re-examination of archival tissue. Careful clinicopathological correlation is therefore essential to avoid diagnostic delay and misclassification.