Foregut cyst (FC) is an uncommon, benign, solitary cyst comprising four layers: ciliated pseudostratified columnar epithelium, subepithelial connective tissue, smooth muscle layer, and outer fibrous capsule. FCs are classified as bronchogenic, enterogastric, or undifferentiated. Friedreich first described the lesion in 1857 and predicted its congenital origin. We present an FC that developed in the retroperitoneum and was difficult to separate from other retroperitoneal cystic mass lesions. A 27-year-old female presented with flank pain. Abdominal computed tomography revealed a cystic lesion, measuring approximately 7.8 cm in diameter, located near the posterior stomach, superior pole of the left kidney, and pancreas. Total cystectomy was performed. Macroscopic examination revealed a 7.8×6 cm unilocular cyst enclosed by a thin capsule, which contained a viscous, yellowish fluid. No solid component was observed in the lesion. The surgeon confirmed no continuity of the cyst into the surrounding organs. Microscopy revealed the cyst to comprise four layers. In focal areas, seromucous glands were observed in the wall. There was no evidence of malignancy. With these findings, this case was diagnosed as retroperitoneal ciliated FC. FC located in the abdomen or retroperitoneum is extremely rare, and few cases have been described in literature. Intraabdominal and retroperitoneal locations have been explained by the presence of pleuroperitoneal canals in the early embryonic stage. FC should be considered in the differential diagnosis of retroperitoneal cystic masses including lymphangiomas, cystic pancreatic tumors, pseudocysts, and hematomas.

Keywords: Retroperitoneum, foregut cyst, malformation