A case report of retroperitoneal schwannoma with malignant transformation and unusual size.

Introduction: Schwannomas are typically benign tumors of peripheral nerves, rarely found in the retroperitoneum and even more rarely undergoing malignant transformation into malignant peripheral nerve sheath tumors (MPNSTs). Their nonspecific clinical presentation and overlapping radiologic features with benign variants pose significant diagnostic challenges. Observation: We report the case of a 27-year-old male presenting with a progressively enlarging right-sided retroperitoneal mass associated with lower limb pain and radiculopathy. Imaging revealed a giant retroperitoneal tumor involving multiple lumbosacral nerve roots, suggestive of malignant transformation. Initial biopsy indicated a benign schwannoma, but complete surgical resection was performed due to size and compressive symptoms. Histopathological examination confirmed high-grade sarcoma arising from a preexisting schwannoma. Postoperative CT scan at 3 weeks revealed local recurrence with peritoneal involvement. Discussion: This case illustrates the diagnostic pitfalls of retroperitoneal schwannomas with malignant transformation. Large tumor size (>5 cm), rapid progression, and compressive symptoms should raise suspicion for MPNST, even when biopsy suggests benignity. Imaging may guide suspicion but remains insufficient for definitive diagnosis, particularly in ancient or cellular schwannomas. Multiple or PET/CT-guided biopsies may improve accuracy. Radical surgical excision remains the cornerstone of management, though complete resection is often limited by anatomical constraints. Adjuvant radiotherapy should be considered in high-grade, large, or incompletely resected tumors. Conclusion: Retroperitoneal schwannomas with malignant transformation are rare but aggressive. A high index of suspicion, thorough histopathological assessment, and complete resection whenever feasible are crucial to optimize outcomes.