Primary thymic epithelial neoplasms: spectrum of differentiation and histological features

Primary thymic epithelial neoplasms have been the source of much controversy over the years because of difficulties in their histopathologic classification and prognostication of clinical behavior. Despite recent advances in our understanding of the immunopathology and molecular pathology of these neoplasms, no universally accepted system of classification is yet available for these tumors. A variety of classification schemes have been proposed in the past based on either morphological, histogenetic, or immunophenotypic considerations. For the most part, the majority of such classifications have been merely descriptive with little value for prognostication, or have proven cumbersome and difficult to apply in clinical practice. Recent studies have shown that primary thymic epithelial neoplasms, rather than representing several discrete, separate entities constitute part of a continuous spectrum of differentiation ranging from well-differentiated neoplasms to poorly-differentiated neoplasms. Based on these observations, a novel classification 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 carcinoma (corresponding to well-differentiated, moderately-differentiated, and poorly-differentiated neoplasms, respectively). The histopathologic features and diagnostic criteria for these tumors is the subject of this review.