Pax8 expression in thymic epithelial neoplasms: an immunohistochemical analysis.

Pax8 is a transcription factor associated with the embryonic development of the thyroid gland, kidney, and the Müllerian system and plays a role in the tumorigenesis of these organs. Pax8 has been shown to be expressed immunohistochemically in a high percentage of tumors of thyroid, renal, and Müllerian origin in both primary and metastatic sites. The diagnostic utility of Pax8 protein expression in thymic epithelial neoplasms has not been comprehensively studied. This study examines the immunohistochemical expression of Pax8 in a series of thymic epithelial neoplasms, including 31 thymic carcinomas, 30 World Health Organization (WHO) type A thymomas, and 30 WHO type B thymomas (B1, B2, B3). Positive immunoreactivity for Pax8 was noted in 77% of thymic carcinomas, 100% of WHO type A thymomas, and 93% of WHO type B thymomas. Consistent but weaker staining for Pax8 was also identified in the epithelial cells of remnant thymic tissue located in the periphery of the tumors. This study expands the spectrum of tumors expressing Pax8 to include thymic epithelial neoplasms. The consistent expression of this marker in these tumors is a valuable diagnostic tool that can be of use in the differential diagnosis of anterior mediastinal neoplasms. As Pax8 expression has been demonstrated to be limited to organs the development of which depends on this transcription factor, the possibility that the organogenesis of the thymic gland also depends on this factor should be explored.