Supplementary Material for: Metaplastic Thymoma underwent open thymectomy: A rare case report of biphasic thymic tumor

Abstract:

Introduction
Metaplastic thymoma is a rare thymic epithelial tumor characterized by a distinctive biphasic pattern of epithelial and spindle cell components. It typically follows an indolent course and is associated with a favorable prognosis, distinguishing it from other more aggressive thymic neoplasms.
Case Presentation
We report the case of a 44-year-old woman who presented with a persistent productive cough. Chest X-ray and subsequent CT imaging revealed a 6.5 × 4.5 cm anterior mediastinal mass. A CT-guided biopsy suggested metaplastic thymoma, which was confirmed postoperatively following an open thymectomy. Histopathological examination demonstrated a biphasic tumor with islands of epithelial cells (positive for pan-cytokeratin, P63, P40) and spindle cells (positive for vimentin, EMA), with a low Ki-67 index. Next-generation sequencing identified a YAP1-MAML2 rearrangement. The patient recovered uneventfully, and follow-up imaging at six months showed no recurrence.
Conclusion
This case highlights the importance of integrating clinical, radiological, and histopathological findings for the accurate diagnosis of metaplastic thymoma. Complete surgical resection remains the treatment of choice and is associated with excellent long-term outcomes.