Mucinous nonneoplastic cyst of the pancreas penetrates the colon causing infection: a case report

BACKGROUND: Mucinous nonneoplastic cyst of the pancreas is a rare disease defined as a cystic lesion lined with mucinous epithelium, supported by hypocellular stroma and not communicating with the pancreatic ducts. Mucinous nonneoplastic cyst of the pancreas has no malignant potential and does not require surgical resection or surveillance. However, its preoperative differentiation from other cystic lesions of the pancreas is difficult because of several overlapping clinical, radiological, and biochemical features. We report a rare case of large mucinous nonneoplastic cyst of the pancreas in which surgery was required due to infection and the possibility of malignancy. CASE PRESENTATION: A 75-year-old Japanese man was found to have a pancreatic cyst in 2006 while undergoing postoperative evaluation for colon cancer. In 2015, the cyst ruptured, and it was treated conservatively. In 2017, he fell down on a road with a fever of 40 °C and was transported emergently to a nearby hospital. Enhanced computed tomography revealed a cystic lesion in the body of the pancreas measuring 119 mm × 100 mm and an adjacent left renal cyst measuring 63 mm in diameter. The wall of the pancreatic cyst was thickened. Magnetic resonance imaging demonstrated a liquid surface in the pancreatic cyst. Pancreatic cyst infection was diagnosed as the source of infection. However, identification of the organism was difficult. Furthermore, due to the increase in the size and wall thickness of the cyst, it was unclear whether the cystic mass was neoplastic with malignant potential. For these reasons, the patient underwent distal pancreatectomy and splenectomy with deroofing of the left renal cyst. Intraoperatively, the pancreatic cyst adhered to the descending colon, and partial resection of the colon was added. Pathologic analysis of the resected cyst demonstrated a simple cyst lined by mucinous epithelium. There was no underlying stromal condensation or epithelial dysplasia, and communication with the native pancreatic ducts was not observed. Based on the operative and histological findings, a final diagnosis of mucinous nonneoplastic cyst of the pancreas with colonic communication was made. The colonic fistula was presumed to be the source of infection. CONCLUSION: Mucinous nonneoplastic cyst of the pancreas is generally benign and requires little follow-up, but large cysts may penetrate other organs and cause severe complications.