Primary thymic epithelial neoplasms: Spectrum of differentiation and histological features

Primary thymic epithelial neoplasms have been the source of much controvers y over the years because of difficulties in their histopathologic classific ation and prognostication of clinical behavior. Despite recent advances in our understanding of the immunopathology and molecular pathology of these n eoplasms, no universally accepted system of classification is yet available for these tumors. A variety of classification schemes have been proposed i n the past based on either morphological, histogenetic, or immunophenotypic considerations. For the most part, the majority of such classifications ha ve been merely descriptive with little value for prognostication, or have p roven cumbersome and difficult to apply in clinical practice. Recent studie s have shown that primary thymic epithelial neoplasms, rather than represen ting several discrete, separate entities constitute part of a continuous sp ectrum of differentiation ranging from well-differentiated neoplasms to poo rly-differentiate neoplasms. Based on these observations, a novel classific ation of primary thymic epithelial neoplasms has been proposed that divides these tumors into three easily reproducible diagnostic categories based on their degrees of differentiation: thymoma, atypical thymoma, and thymic ca rcinoma (corresponding to well-differentiated, moderately-differentiated, a nd poorly-differentiate neoplasms, respectively). The histopathologic featu res and diagnostic criteria for these tumors is the subject of this review. Copyright (C) 1999 by W.B. Saunders Company.