Thymoma, atypical thymoma, and thymic carcinoma. A novel conceptual approach to the classification of thymic epithelial neoplasms.

Primary thymic epithelial neoplasms have been the subject of much controversy in recent years owing to the difficulties posed by these tumors for precise histopathologic typing and prognostication. A number of classification schemes using different terminology have been proposed, none of which has satisfactorily managed to address all the problems and concerns related to these tumors. We present a proposal for a novel approach to the histologic classification of primary thymic epithelial neoplasms that is based on morphologic features of differentiation. The principle behind this classification scheme is that the determination of the cytologic degree of atypia and the identification of the organotypical features of thymic differentiation may permit accurate classification of these neoplasms into 3 simple and reproducible diagnostic categories: thymoma, atypical thymoma, and thymic carcinoma. We further reiterate the traditional concept that tumor staging, not histopathologic typing, has a more crucial role for accurate and reliable prognostication for the better differentiated forms of these tumors.