Desmoid-type fibromatosis mimicking cystic retroperitoneal mass: case report and literature review.

We report an extremely rare case of retroperitoneal DF with spontaneous cystic change. DF can manifest as a mainly cystic mass with a thick wall, as in our case, which makes the correct diagnosis difficult. DF should be included in the preoperative differential diagnosis of a cystic retroperitoneal mass, regardless of its rarity.