Metaplastic thymoma is a rare histologic variant of thymic epithelial tumors and is characterized by a biphasic growth pattern. We herein report the case of 44-year-old woman who underwent surgery for metaplastic thymoma. Computed tomography scan revealed a well-circumscribed mediastinal tumor: 56 mm in diameter with homogenous enhancement. The tumor was suspected to be a non-invasive thymoma, and thymomectomy with resection of the surrounding thymus was performed using thoracoscopy. The resected tumor measured 60 mm and was grossly well-encapsulated. The cut surface was gray to white and homogenous. Microscopically, the epithelial components took the form of an anastomosing nest to broad trabeculae intertwining with the bundle of spindle cells. Mitosis was not found and the Ki-67 index was < 1%. Cytokeratin 5/6 was strongly positive in the epithelial components composed of polygonal cells. Terminal deoxynucleotidyl transferase positive immature T cells were not observed. Based on these pathologic findings, the tumor was identified as metaplastic thymoma.
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Immunohistochemistry for YAP1 N-terminus and C-terminus highlights metaplastic thymoma and high-grade thymic epithelial tumors by different staining patterns.
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Institute of Pathology, University Medical Center Göttingen, University of Göttingen, Göttingen, Germany.
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- Metaplastic thymoma (MT) is a rare type of thymic tumor that has a specific genetic fusion (YAP1::MAML2), similar to skin tumors called poromas.
- Recent studies revealed that MT lacks YAP1 C-terminus expression, but the expression of YAP1 N-terminus in MT and other thymic epithelial tumors (TETs) needed further exploration.
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