Pancreatic Ductal Adenocarcinoma Encapsulated by a Tumor-Forming Type 1 Autoimmune Pancreatitis Located at the Pancreatic Tail: A Case Report.

Introduction: Autoimmune pancreatitis (AIP) is recognized as a disease with a good prognosis that responds well to steroids, but the complication of pancreatic ductal adenocarcinoma (PDAC) in AIP is a rare condition. We report a case of PDAC encapsulated by tumor-forming type 1 AIP. Case Presentation: The patient, a 65-year-old female, was found to have high CA19-9 levels and a pancreatic mass with a diameter of 30 mm on abdominal ultrasonography. Contrast-enhanced computed tomography revealed a 40-mm mass in the tail of the pancreas that had a 27-mm oligemic mass inside it. From these work-up examinations, this tumor was diagnosed as PDAC, with evidence of colonic invasion. As curative resection for PDAC, a distal pancreatectomy with splenectomy and combined colon resection were performed. Histopathological examination showed invasive PDAC surrounded by IgG4-positive plasma cell infiltration. Based on these findings, a diagnosis was made of PDAC located in the pancreatic tail capsulized by type 1 AIP. The postoperative course was uneventful, and the patient was discharged on postoperative day 15. She underwent postoperative adjuvant chemotherapy with S-1 for 6 months, and no recurrence was noted for 2 years after operation. Conclusion: Currently, there are two hypothetical mechanisms of PDAC induction by AIP: (1) carcinogenic stimulation due to chronic inflammation and (2) paraneoplastic syndrome caused by AIP. Further study of the relationship between AIP and pancreatic cancer is needed, and follow-up should be conducted while keeping in mind the possibility of complications.