Tumor-forming autoimmune pancreatitis confused with pancreatic cancer that developed during the course of chronic pancreatitis

We report on a 67-year-old man with a 5-year history of alcoholic pancreatitis, and whose serum IgG4 values had been within normal range. He suffered abdominal pain in 2015, and a close examination revealed pancreatic stone and multiple stenosis in the tail of the pancreas, and a pancreatic fistula leading from the retroperitoneum to the mediastinum. Pancreatic fistula improved with pancreatic duct stent, but endoscopic ultrasound revealed a pancreatic tail tumor. As a result of rubbing cytology, we suspected pancreatic cancer caused by chronic pancreatitis. Therefore, we performed distal pancreatectomy with D2 lymphadenectomy. The pathological findings of resected specimens revealed infiltration of inflammatory cells, mainly lymphocytes and plasma cells, and pancreatic fibrosis. Immunohistological findings revealed some IgG4 positive cells. Finally, we diagnosed autoimmune pancreatitis with chronic pancreatitis. Autoimmune pancreatitis is known to be difficult to distinguish from pancreatic cancer and often results in chronic inflammation, but the onset of it during the course of chronic pancreatitis is very rare.