Primary Retroperitoneal Mature Cystic Teratoma in an Adult Diagnosed Using Percutaneous Needle Biopsy: A Case Report.

Retroperitoneal teratomas are rare in adults and often pose diagnostic challenges when evaluated using imaging alone. We report the case of a 35-year-old man with a 10-cm retroperitoneal mass located adjacent to the upper pole of the right kidney, which was incidentally detected during a routine health checkup. CT revealed a fat-containing lesion, initially suggesting a renal angiomyolipoma. However, MRI demonstrated heterogeneous components with internal septations and calcifications, raising suspicion for other retroperitoneal tumors, including a liposarcoma. Because imaging findings were inconclusive, an ultrasound-guided percutaneous core needle biopsy was performed. Histopathological examination revealed keratinizing squamous epithelium, skin appendages, adipose tissue, and smooth muscle tissue, findings consistent with a mature cystic teratoma. The tumor was surgically resected en bloc with the right kidney due to strong adhesion to the renal upper pole. Histopathological analysis confirmed a mature cystic teratoma without malignant components. This report highlights the clinical utility and novelty of percutaneous biopsy in establishing a preoperative diagnosis and facilitating appropriate surgical planning for complex retroperitoneal masses.