Metaplastic thymoma in the middle mediastinum: a rare case report and surgical treatment analysis of a 32-year-old female patient.

Background: Metaplastic thymoma (MT), an exceedingly rare variant of primary thymic epithelial neoplasms, is distinguished by its indolent progression and unique histopathological profile. It presents a biphasic pattern characterized by solid epithelial and spindle cell components, potentially leading to diagnostic confusion with type A thymomas or the type A component of type AB thymomas. Accurate diagnosis is pivotal for optimal therapeutic strategies and prognostication.

Case description: We document an exceptional case of a 32-year-old woman, incidentally discovered to have a mediastinal nodule in the middle compartment on chest computed tomography (CT). The lesion was excised via video-assisted thoracoscopic surgery. Histological evaluation revealed a biphasic cellular architecture comprising epithelioid and spindle cells. Immunohistochemical analysis demonstrated significant positivity for CK5/6 and P40 in epithelial cells, and vimentin and epithelial membrane antigen in spindle cells, with a low proliferation index marked by Ki-67. Noteworthy, fluorescence in situ hybridization (FISH) analysis identified a YAP1::MAML2 gene fusion, with a predominant pattern suggestive of fusion gene presence, thus corroborating the diagnosis of MT.

Conclusions: This report underscores the critical role of a multifaceted diagnostic approach, including histopathological, immunohistochemical, and genetic analyses, in the identification of MT. The detection of the YAP1::MAML2 gene fusion through FISH analysis provides a robust diagnostic marker, highlighting the necessity for clinical and pathological vigilance for this rare tumor.