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[Histopathological and molecular characteristics of metaplastic thymomas: report of five cases].

Objective: To investigate the clinicopathological characteristics, immunophenotype, molecular genetic characteristics and prognosis of the metaplastic thymoma (MT). Methods: The clinicopathological and follow-up data of five MT cases were collected at Fujian Provincial Hospital from 2008 to 2019. Immunohistochemical staining and MAML2 gene detection were performed, and the relevant literature was reviewed. Results: There were 2 males and 3 females, aged 36-64 years (mean age 52 years). The tumors ranged 3.2-7.3 cm in the greatest diameter (average 5.1 cm).Microscopically, the tumor showed a biphasic pattern with epithelial cells merging gradually with the spindle cell component. The two areas transited to each other or had obvious boundary. Both components showed mild atypia. No mitosis was observed in either area, and a small number of lymphocytes were observed in the stroma. Immunohistochemical staining showed that epithelioid cells were positive for CKpan, p63 and E-cadherin. Spindle cells were positive for vimentin and EMA, while the Ki-67 index was less than 5%, and lymphocytes were negative for TdT. MAML2 gene apart signal was detected in two of the cases (2/4) that were tested by FISH. Conclusions: MT is a low-grade malignant epithelioid thymic tumor. Its diagnosis and differential diagnosis are dependent on the morphological characteristics, immunohistochemical staining and MAML2 gene detection. The primary treatment option is surgical resection, with an overall good prognosis.