Background: Primary retroperitoneal mucinous borderline tumors are rare. These tumors can be present in women with otherwise normal pelvic anatomy.
Case: A 22-year-old woman with abdominal pain was found to have a large midline mass superior to the uterus with ascites. In surgery, a large mass was arising from the sigmoid mesentery. The pelvis was normal on inspection. Final pathology of the mass revealed borderline tumor (tumor of low malignant potential) and focus of ovarian tissue.
Conclusion: This is a case report of primary retroperitoneal mucinous borderline tumor. The gynecologic oncologist should be aware of primary retroperitoneal tumors and the differential diagnosis.