AB005. Metaplastic thymoma of the mediastinum—a clinicopathological report of three cases
Case Report

AB005. Metaplastic thymoma of the mediastinum—a clinicopathological report of three cases

Po-Kuei Hsu1, Yi-Chen Yeh2, Min-Shu Hsieh3

1Department of Surgery, Taipei Veterans General Hospital, Taipei; 2Department of Pathology, Taipei Veterans General Hospital, Taipei; 3Department of Pathology, National Taiwan University Hospital, Taipei

Correspondence to: Po-Kuei Hsu, MD, PhD. Department of Surgery, Taipei Veterans General Hospital, 201 Shih Pai Rd., 112, Taipei. Email: hsupokuei@yahoo.com.tw.

Background: Metaplastic thymoma (MT) is an exceptionally rare, biphasic epithelial tumor of the thymus, officially designated by the World Health Organization in 2004. It is characterized histologically by intermixed epithelioid and spindle cell components and harbors a unique YAP1-MAML2 gene fusion, distinguishing it from other thymoma subtypes which typically carry GTF2I mutations. Fewer than 100 cases have been documented in the English literature, often presenting as incidental anterior mediastinal masses in asymptomatic middle-aged women.

Case Description: We present three cases of MT to enrich the current understanding of this rare entity. The first case involved a 47-year-old woman with non-specific symptoms and a 6.5-cm anterior mediastinal mass, managed successfully with bilateral video-assisted thoracoscopic thymectomy followed by radiotherapy, with no recurrence at 11 years. The second case was an asymptomatic 37-year-old woman with a 3.1-cm lesion detected during routine health screening. She underwent curative resection without adjuvant therapy and remains disease-free at 2 years. The third case involved a 56-year-old woman with long-standing chest tightness and a 10-cm anterior mediastinal tumor, excised via median sternotomy with no evidence of recurrence at 2 years postoperatively. Pathologically, all three tumors displayed the characteristic biphasic morphology of MT. Immunohistochemistry revealed expression of epithelial markers (CK AE1/AE3, p40, p63), and fluorescence in situ hybridization confirmed MAML2 rearrangement. RNA sequencing identified in-frame YAP1-MAML2 fusions in two cases.

Conclusions: MT typically presents as a well-circumscribed anterior mediastinal mass in asymptomatic middle-aged women and demonstrates excellent outcomes with complete surgical resection. The presence of YAP1-MAML2 fusion serves as a valuable diagnostic marker. Our cases contribute further to the limited pool of documented MTs, supporting its indolent behavior and favorable prognosis.

Keywords: Thymoma; case report; mediastinum

Acknowledgments

None.

Footnote

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://med.amegroups.com/article/view/10.21037/med-25-ab005/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patients for publication of this case report and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

doi: 10.21037/med-25-ab005
Cite this abstract as: Hsu PK, Yeh YC, Hsieh MS. AB005. Metaplastic thymoma of the mediastinum—a clinicopathological report of three cases. Mediastinum 2025;9:AB005.

Article Options

Download Citation

Share