Tumors of the Thymus

Abstract

Primary thymic epithelial neoplasms represent the most common type of tumors of the anterior mediastinum. These tumors have been a source of major controversy over the years due to their difficulties for histopathologic typing and often unpredictable biologic behavior. Unlike malignant epithelial neoplasms arising at other organs, these tumors were felt for many years to be unsuitable for histologic grading. In fact, the latest WHO schema for the classification of thymic epithelial neoplasms does not mention grading for these tumors at all. More recent observations, however, have demonstrated that thymic epithelial neoplasms form part of a continuous spectrum of lesions that may closely resemble their parent organ at the one end or be very poorly differentiated at the other extreme [1]. Based on these observations, a novel conceptual approach was recently introduced for the classification of thymic epithelial neoplasms that is based on the histologic degree of differentiation of the lesions [2]. The histologic grading of these tumors is based on the premise that these lesions can range from well-differentiated to moderately differentiated to poorly differentiated neoplasms. This is supported by the observation of tumor progression in thymoma whereby recurrences show transformation of a low-grade histologic type to that of a higher-grade histology [3]. The degree of differentiation in any given tumor will depend on the presence or absence of the organotypical features of differentiation of the thymus and on the degree of cytological atypia displayed by the tumor cells (see Table 4.1).