Surgical Resection of a Rare Primary Retroperitoneal Mucinous Borderline Tumor of Müllerian Origin: A Case Report

•Primary retroperitoneal mucinous tumors (PRMTs) are a rare group of cystic neoplasms consisting of three subtypes.•PRMTs are histologically similar to ovarian mucinous tumors but lack true ovarian tissue.•PRMTs should be considered in the differential diagnosis when encountering retroperitoneal cystic lesions.•During surgical resection tumor disruption should be avoided.•Surgical resection alone provides durable disease control for mucinous borderline tumors of low malignant potential