Advantages of and problems encountered with UICC version 8 TNM staging system for thymic epithelial tumors

Although the Masaoka clinico-pathological classification presented in 1981 has been accepted for more than 30 years, this staging system became difficult to adapt to more modern clinical practice in the 21st century after enormous progress in pathological classification, biological understanding, imaging diagnosis and surgical treatment for thymic epithelial tumors. International Thymic Malignancy Interest Group (ITMIG), the first international academic organization specifically related to thymic neoplasms, confirmed the necessity of a global database project to establish a new staging system in 2010, and finally compiled data from 22 countries and completed the database of 10,808 patients in 2014. ITMIG and the thymic domain of the Staging and Prognostic Factors Committee (SPFC) of the International Association for Study of Lung Cancer (IASLC) collaboratively analyzed this retrospective database, and finally proposed a new TNM-based staging system. The definition of the T category strongly reflects the level of invasion to adjacent organs, the same philosophy used for Masaoka staging. Although this new staging system was approved by Union for International Cancer Control (UICC) TNM version 8, the current global retrospective database mainly consists of surgical cases and information from unresectable cases is lacking. Prognostic significance of nodal involvement remains to be confirmed by a greater number of the cases, and survival of the patients with distant metastasis is actually unclear. Additional development along with use of the prospective database is required.