Background: To elucidate the correlation of autoimmune pancreatitis (AIP) and mass-forming pancreatitis, in which a localized mass is formed in the pancreas.
Methods: Nine cases of mass-forming pancreatitis were divided into two groups, one consisting of cases that met the histological diagnostic criteria for AIP by the Japan Pancreas Society (JPS) and the other consisting of cases which did not. Histological findings, immunohistological findings, and pancreatograms were compared between these groups.
Results: Six cases met the histological criteria of JPS (group A) and the other three did not (group B). In the mass-forming portion in group A, the wall of the pancreatic duct showed marked thickening. However, the epithelium was well preserved, and dilatation of the branch ducts or protein plugs were rarely observed. All cases showed marked obliterative phlebitis. The IgG4 labeling index (LI) was 25% or more in all but one case. In the portion other than the mass, the lobular structure was well preserved and the IgG4 LI was less than 10%. The pancreatogram showed localized stenosis or obstruction at the site of the mass associated with a normal-appearing main pancreatic duct in the remaining portion. In group B, histological findings typical of chronic pancreatitis with dilated branch ducts and protein plugs were observed. Obliterative phlebitis was not confirmed. The IgG4 LI in the mass-forming portion was low (2.3%-11.1%).
Conclusions: There exists a subgroup of AIP showing localized mass formation and stenosis or obstruction of the main pancreatic duct with prominent obliterative phlebitis associated with a normal ductal segment.