Radiologic-pathologic discrepancy in a plexiform schwannoma of the ulnar nerve mimicking a malignant soft-tissue tumor: A case report.

Schwannomas are common benign peripheral nerve sheath tumors; however, plexiform variants may exhibit atypical imaging features and mimic malignant soft-tissue tumors. We report a 24-year-old woman presenting with a slowly enlarging mass in the proximal forearm associated with ulnar nerve paresthesia. Ultrasound showed a heterogeneous hypervascular lesion with irregular margins. MRI demonstrated a 33 × 24 × 49 mm mass with T2 hyperintensity, avid enhancement, and a pseudo-infiltrative appearance involving adjacent flexor muscles, raising suspicion for malignancy. Classical MRI signs of benign peripheral nerve sheath tumors were not clearly identified. Ultrasound-guided biopsy suggested a benign spindle-cell tumor compatible with perineurioma. Following fascicle-sparing microsurgical excision, definitive histopathology revealed a plexiform schwannoma, characterized by multinodular architecture, Antoni A and B areas, Verocay bodies, diffuse S100 positivity, and negative EMA staining. At 3-month follow-up, no recurrence was observed and functional outcome was favorable. This case highlights a significant radiologic-pathologic discrepancy and underscores the limitations of imaging and core biopsy in heterogeneous peripheral nerve sheath tumors, emphasizing the importance of integrated diagnostic correlation.