Lipoma-like tumor mass probably arising in the retroperitoneal heterotopic pancreas: a previously undescribed lesion.

Heterotopic pancreatic tissue is found in several locations of the body. However, to the best of our knowledge, there have been no reports on heterotopic pancreas in the retroperitoneum. A case of retroperitoneal lipoma-like large tumor mass probably arising in the heterotopic pancreas is reported. A 45-year-old Japanese woman was admitted to hospital because of back pain. Imaging modalities showed an abnormal mass in the retroperitoneum separate from the surrounding organs, including the pancreas and kidney. Histologically, the mass consisted of mature adipose tissue and scattered ductal and acinar elements. Although there was no islet tissue in this fatty mass, the epithelial elements suggested heterotopic pancreatic tissue in origin. The present case is very unusual; however, heterotopic pancreas should be considered in the differential diagnosis of retroperitoneal tumors.