Endometrial stromal sarcoma of the retroperitoneum.

Endometrial stromal sarcoma (ESS) is an uncommon neoplasm whose occurrence outside the uterus is extremely rare in the absence of metastasis or extension of a primary uterine neoplasm. When ESS occurs in such locations it is often associated with the uterine adnexa or serosal surface of various organs. Although rare, ESS is usually considered in the differential diagnosis of spindle cell neoplasms in the female patient. We report a case of ESS arising in the retroperitoneum and discuss the morphologic and immunohistochemical features in the context of the differential diagnosis of a retroperitoneal low-grade spindle cell neoplasm occurring in the female patient.