Ancient Schwannoma of the Thumb Pulp Presenting as a Cystic, Non-enhancing Lesion on MRI: A Diagnostic Pitfall.

Schwannomas are benign peripheral nerve sheath tumors that rarely occur in the digits, with the ancient (degenerative) variant being even more uncommon. We describe the case of a 59-year-old male with a recurrent, painless thumb pulp mass. Preoperative magnetic resonance imaging (MRI) demonstrated a cystic-appearing lesion without significant postcontrast enhancement, interpreted as an epidermal inclusion cyst (EIC). Intraoperatively, the lesion was larger than expected and extended to the periosteum of the distal phalanx. Complete excision revealed a schwannoma with degenerative ("ancient") changes, confirmed by diffuse S100 and SOX10 immunoreactivity. This case provides MRI characterization of an ancient schwannoma of the thumb, demonstrating an atypical cystic, non-enhancing appearance. In this case, the lesion demonstrated atypical imaging features, leading to diagnostic confusion with an EIC. This report highlights a potential diagnostic pitfall of imaging and the importance of histopathology for definitive diagnosis, while expanding the spectrum of reported radiologic appearances of digital ancient schwannomas.