Thymic epithelial malignancies are rare tumors. There have been many obstacles to progress in these diseases. Among these has been the lack of an official, consistent stage classification system put forth by the American Joint Committee on Cancer (AJCC) and the Union for International Cancer Control (UICC) —the bodies responsible for defining stage classification throughout the world. At least 15 different stage classification systems have been proposed and used.¹ x 1 Filosso, P, Ruffini, E, Lausi, P, Lucchi, M, Oliaro, A, and Detterbeck, F. Historical perspectives: the evolution of the thymic epithelial tumors staging system. Lung Cancer. 2014; 83: 126–132 Abstract | Full Text | Full Text PDF | PubMed | Scopus (27) | Google Scholar See all References These have been largely empirically derived, based on data from small numbers of patients. Perhaps the most widely used have been the Masaoka classification (derived from data on 91 patients),² x 2 Masaoka, A, Monden, Y, Nakahara, K, and Tanioka, T. Follow-up study of thymomas with special reference to their clinical stages. Cancer. 1981; 48: 2485–2492 Crossref | PubMed | Scopus (1191) | Google Scholar See all References and the Koga modification of this (based on 76 patients).³ x 3 Koga, K, Matsuno, Y, Noguchi, M et al. A review of 79 thymomas: modification of staging system and reappraisal of conventional division into invasive and non-invasive thymoma. Pathol Int. 1994; 44: 359–367 Crossref | PubMed | Scopus (211) | Google Scholar See all References Even among centers using one of these classification systems, often the definitions have been interpreted differently because of vague wording, thus hampering the ability to collaborate effectively.⁴ x 4 Detterbeck, FC, Moran, C, Huang, J et al. Which way is up? Policies and procedures for surgeons and pathologists regarding resection specimens of thymic malignancy. J Thorac Oncol. 2011; 6: S1730–S1738 Abstract | Full Text | Full Text PDF | PubMed | Scopus (63) | Google Scholar See all References