Case Report: IgG4-related autoimmune pancreatitis presenting as an infiltrative cystic-solid pancreatic mass: lessons from a diagnostic pitfall.

IgG4-related autoimmune pancreatitis (AIP) can resemble pancreatic ductal adenocarcinoma (PDAC), but it typically presents as a solid mass rather than a cystic-solid lesion. We report a rare case of AIP in a middle-aged man with a long-standing pancreatic tail mass that gradually enlarged and developed a cystic-solid configuration. Photon-counting CT showed a non-enhancing cystic component and a progressively enhancing solid portion with apparent invasion of adjacent organs, while FDG-PET/CT demonstrated marked metabolic activity, all strongly suggestive of malignancy. Serum IgG4 levels were normal and no extrapancreatic IgG4-related involvement was present. The patient declined biopsy and underwent radical surgery; histopathology revealed dense fibrosis, pancreatic atrophy, and lymphoplasmacytic infiltration consistent with IgG4-related AIP, with no evidence of cancer. This case highlights that atypical cystic-solid AIP can closely mimic invasive PDAC even with advanced imaging techniques, underscoring the importance of recognizing such rare presentations to avoid unnecessary radical resection.