Reactive fibroinflammatory pseudotumor of the pancreas: report of three cases.

Reactive fibroinflammatory pseudotumor located in the pancreas is a rare condition. However, we experienced three cases of reactive fibroinflammatory pseudotumor of the pancreas over the previous 2 years. Immunohistochemical staining of immunoglobulin G4 (IgG4) confirmed that Cases 1 and 3 involved IgG4-related disease and that Case 2 did not. In Cases 1 and 3, the masses were formed through autoimmune reaction and were diagnosed as autoimmune pancreatitis (AIP). In Case 2, because only a few IgG4-positive cells were found, it was difficult to diagnose AIP or IgG4-related disease. Reactive fibroinflammatory pseudotumors can develop without the involvement of any autoimmune mechanisms. There seems to be partial overlap between reactive fibroinflammatory pseudotumors and AIP.