Abstract
A 75-year-old man was referred to our hospital with a suspected pancreatic cyst. Imaging tests revealed a 3-cm cystic lesion located ventrally in the duodenal bulbus, which was suspected to be a duplication cyst with its muscularis propria contiguous to that of the duodenum. One year later, the cyst grew to 6 cm due to intracystic hemorrhaging; therefore, surgery was performed. Histopathology revealed a heterotopic pancreas (HP) in the duodenal wall. The diagnosis was a large, non-malignant retention cyst. Inflammation due to impaired outflow of pancreatic juice from the HP was identified as the cause of cyst enlargement.