Squamous Cell Carcinoma Arising in YAP1::MAML2 Fusion-Associated Metaplastic Thymoma: A Case Report.

Metaplastic thymoma is a rare thymic neoplasm that has generally been considered to follow a benign to indolent clinical course; however, 3 metaplastic thymomas with high-grade malignant transformation to sarcomatoid carcinoma have been reported. In recent years, both conventional metaplastic thymomas and this subset showing malignant transformation have been associated with recurrent YAP1::MAML2 fusions. We report a metaplastic thymoma showing transformation to squamous cell carcinoma, that to our knowledge is the fourth reported in the literature with transition to overtly malignant features and the first showing pure carcinomatous transformation, and in which YAP1::MAML2 fusion was demonstrated via next generation sequencing.