Introduction : Intraabdominal fibromatosis is a rare benign tumor that often affects the mesentery or retroperitoneum, and can infiltrate adjacent organs. However, it is rare for fibromatosis mimicking pancreatic pseudocyst with internal hemorrhage.

Methods : A 44-year-old male with a tangible abdominal mass & pain is described. He is chronic alcoholics. In initial lab, Amylase was 233 U/L and Lipase was 644 IU/L. Abdomen & pelvis CT & Abdomen MRI scan revealed a 15 cm, heterogeneous thick walled cystic mass at right retroperitoneum. Image show Loss of fat plane with right side colon and adjacent right side colon wall thickening and compression of pancreas and proximal duodenum. In Colonofiberscopy(CFS), extrinsic luminal compression within A-colon & proximal T-colon was observed.

Results : The patient received En bloc resection(PPPD with Rt. colectomy) Further pathological examination confirmed the tumor as a retroperitoneal fibromatosis that adhered to pancreas head, serosa of ascending colon and duodenum with extensive hemorrhage

Conclusions : Retroperitoneal fibromatosis can be associated with intracystic hemorrhage, and imaging will not show contrast enhancement, mimicking pancreas pseudocyst and other. Surgical resection is the recommended first line treatment. The patient will be followed for any recurrence.