Schwannomas are benign nerve sheath tumors that rarely arise in the retroperitoneum, with the ancient cystic variant being exceptionally uncommon. We report a 55-year-old male who presented with symptoms of acute calculous cholecystitis and was incidentally diagnosed with a solid cystic left iliac mass on ultrasonography of abdomen and pelvis. On further MR imaging, a well-defined, encapsulated, predominantly cystic mass was seen in the left retroperitoneum. Surgical excision was performed and histopathology confirmed the diagnosis of ancient schwannoma. Recognition of its key imaging features aids in accurate preoperative diagnosis — on ultrasound it appears as a complex cystic hypoechoic mass with posterior acoustic enhancement, and MRI reveals heterogeneous T2 hyperintense signal with a low-signal fibrous rim. Complete surgical resection remains the treatment of choice with an excellent prognosis. Awareness of this rare entity and its multimodality imaging characteristics is essential for appropriate surgical planning and avoidance of misdiagnosis.

Keywords:Ancient Schwannoma; Retroperitoneal Tumor; Cystic Schwannoma; Nerve Sheath Tumor; Pelvic Schwannoma; Ultrasonography; Cross-Sectional Imaging; Magnetic Resonance Imaging