Retroperitoneal cysts are uncommon diseases, and benign nonneoplastic Müllerian cysts are extremely rare among the known cases. We report a case of a 35-year-old woman with a retroperitoneal Müllerian cyst with the tubal type of epithelium. The patient presented with a large (20 cm in diameter), palpable abdominal mass. This multilocular cystic mass was resected from the retroperitoneum between the descending colon and the left renal fascia. Histologically, it was lined by monolayered low-cuboidal to columnar cells without atypia that resembled tubal epithelium, including cilia. Loose fibrous tissue and incomplete smooth muscle bundles were identified beneath the epithelium of the lining. Immunohistochemical tests showed that the lining cells were strongly positive for cytokeratins (CKs) (polyclonal, 7, 18, CAM 5.2, AE1/AE3), epithelial membrane antigen, cancer antigen 125, progesterone receptor, and estrogen receptor. The lining cells were also occasionally weakly positive for CK5/6. They tested negative for CK20, carcinoembryonic antigen, calretinin, and CD 10.
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