A 70-year-old man was referred to our hospital with exacerbation of diabetes. His blood tests showed elevated levels of serum IgG4 and HbA1c. Computed tomography of the pancreatic body demonstrated a weakly enhanced mass, 2 cm in size, with indistinct borders. Magnetic resonance cholangiopancreatography revealed a narrowing of the main pancreatic duct (MPD) at the pancreatic body, a markedly dilated upstream duct, and a slightly dilated downstream duct. Endoscopic ultrasonography demonstrated an iso-hypoechoic heterogeneous mass, protruding and spreading in the pancreatic duct. The histology of a fine needle aspiration sample demonstrated fibrous tissue containing abundant IgG4-positive plasma cells and atypical epithelial cells. The imaging findings and histology were not typical for either pancreatic ductal adenocarcinoma or type 1 autoimmune pancreatitis (AIP), but these were not completely excluded, and a distal pancreatectomy was performed. Histological examination showed an intraductal tubulopapillary epithelial proliferation, which contained cytoplasmic mucin (MUC5AC and MUC6), and severe IgG4-positive lymphoplasmacytic infiltration in the interstitium around the MPD. Next-generation sequencing using DNA extracted from the tumor revealed no mutation of K-ras, GNAS, or TP53. The entire lesion was ultimately diagnosed as AIP with an intraductal tubular and papillary epithelial hyperplasia producing gastric-type mucin. Some recent reports have described AIP development in the background of intraductal papillary mucinous neoplasms, and some have hypothesized a paraneoplastic occurrence of IgG4-related disease. The current case indicates issues in the clinical diagnosis of rare variants of AIP, and raises questions about the relationship between AIP and pancreatic epithelial lesions.


autoimmune pancreatitis, intraductal tumor, IgG4, K-ras, diagnosis