Gigantic retroperitoneal pararenal ancient schwannoma masquerading as a mesenchymal malignancy: A diagnostic conundrum

Ancient schwannoma is a rare benign variant of schwannoma with marked degenerative changes, often mimicking malignancies, particularly when retroperitoneal and pararenal. A 34-year-old woman presented with a rapidly growing 15 cm retroperitoneal pararenal mass. Imaging suggested an aggressive malignancy. Surgical resection and histopathology revealed a well-encapsulated tumor with degenerative changes, nuclear atypia, and strong S100 positivity, confirming ancient schwannoma. Complete excision achieved negative margins, and follow-up showed no recurrence. Diagnostic challenges arise from its rarity and atypical features. Histopathology is crucial for differentiation from malignancies. Ancient schwannomas should be considered for large retroperitoneal masses to ensure accurate diagnosis and management.