Thymic Epithelial Neoplasms

Thymic epithelial neoplasms are rare tumors that account for no more than 1.5% of all malignancies (Engels and Pfeiffer, J Cancer 105:546-551, 2003). These tumors originate from the epithelial cells of the thymic gland and can be divided into thymoma and thymic carcinoma. The presence of organotypical features, i.e., features recapitulating the normal thymic gland, such as a lobulated architecture, the presence of thymic lymphocytes (thymocytes) and perivascular spaces and the absence of malignant cytologic features is used to separate thymomas from the overtly malignant thymic carcinomas. Although our knowledge about the microscopic anatomy and function of the thymic gland has progressed significantly in the last few decades, the classification and staging of its tumors remains a contentious issue. This is partly to blame on their wide morphological variability but may also be due to the fact that many pathologists are unfamiliar with these exceptional neoplasms. Above all, the lack of a widely accepted classification schema has limited focused research into these tumors, and to date treatment strategies are still inconsistent. This chapter will discuss the clinical, pathological, immunohistochemical, and molecular characteristics of thymic epithelial neoplasms with particular focus on the contentious issues of the classification and staging of these tumors.