Primary retroperitoneal mucinous cystic tumour of borderline malignancy mimicking kidney duplicate: cases report and literature review

BACKGROUND: Primary retroperitoneal mucinous cystic tumours with borderline malignancy (PRMC-BM) are rare and difficult to diagnose preoperatively. We are the first to report two cases of PRMC-BM which mimic a duplex kidney and evaluate the outcomes of different surgical procedures. CASE PRESENTATION: We describe two cases of retroperitoneal cystic tumours. Both were diagnosed with duplex kidney with hydronephrosis on computed tomography scan. The first patient underwent robot-assisted laparoscopic surgery and was found to have a retroperitoneal cystic tumour. The other patient underwent an ultrasound-guided puncture before surgery and was diagnosed with retroperitoneal lymphangioma. Retroperitoneal cystectomy was performed using an open transperitoneal procedure. The final pathologic diagnosis in both cases implies PRMC-BM. The open surgical approach was associated with a shorter operation time, less intraoperative blood loss, and protected cyst wall integrity by comparing the different surgical approaches. During follow-up, the patient in the first case had tumour recurrence six months post-surgery, and the other patient was healthy without recurrence or metastasis 12 months post-surgery. CONCLUSIONS: Primary retroperitoneal mucinous cystic tumours with borderline malignancy can be enclosed within the kidney and misdiagnosed as other cystic diseases of the urinary system. Thus, an open surgical approach may be more suitable for this type of tumour.