An autopsy case of autoimmune pancreatitis after a 6-year history of steroid therapy accompanied by malignant dissemination of unknown origin.

Little is known about the long-term outcome of autoimmune pancreatitis (AIP), and whether AIP possesses malignant potential. We report herein a 68-year-old Japanese AIP patient who rapidly developed systemic malignant dissemination of unknown origin, resulting in death. The patient was diagnosed histopathologically as having AIP in 1999. After a 6-year history of 5 mg/day of prednisolone therapy, a sudden onset of abdominal pain and convulsive seizure occurred, and the patient died on the tenth hospital day owing to diffuse peritoneal disseminations and metastases in the bilateral lungs and brain. Autopsy disclosed that the primary site was renal cell carcinoma, detectable only by autopsy, originating in the left kidney. On microscopy, metastatic cells obtained from the brain, lung, and peritoneum were composed of pleomorphic malignant cells identical to those from the renal cell carcinoma. Unexpectedly, abundant IgG4-positive plasma cell infiltration, suggesting high activity of AIP in pancreatic parenchyma and around dilated bile ducts, was still observed.