Thymic Epithelial Neoplasms

Abstract

Thymic epithelial neoplasms are the most common primary tumors of the anterior mediastinum. The terminology and classification of these tumors have been a topic of debate for many years and have continued to evolve as we have gained a better understanding of their pathogenesis and biologic behavior. Thymic epithelial neoplasms comprise a spectrum of lesions that can range from very low-grade, well-differentiated tumors to high-grade, poorly differentiated neoplasms, with a wide range of morphologies in-between. The most widely used classification is that proposed by the World Health Organization (WHO), which consists of several categories and utilizes mostly a combination of numbers and letters to designate the various types; however, a more simplified classification scheme has been proposed by Suster and Moran that stratifies these tumors into three categories based on their degrees of differentiation and clinical behavior (Table 2.1). A confusing situation also exists with the persistent use of the terms “thymoma” and “thymic carcinoma” to refer to the ends of the morphologic spectrum in these tumors, implying that one is malignant and the other benign, when in reality all primary thymic epithelial neoplasms have the potential for aggressive or malignant behavior irrespective of their histologic appearance. For the time being, we will utilize the term “thymic carcinoma” in this chapter following the standard and traditional nomenclature utilized by the majority of investigators and reserve the term “thymoma” for those tumors that fall mainly in the low-grade and intermediate range of the morphologic spectrum.