**1. Introduction**

Malignancies of the thymus are rare accounting for <1% of all adult neoplasms. Thymoma and thymus carcinoma account for 20% of tumors in the anterior mediastinum and are therefore the most commonly found malignancy in that specific location.

There are several groups of thymic tumors to distinguish thymic epithelial tumors, germ cell tumors, lymphoid and hematological malignancies, and mesenchymal tumors. It is known that they derive from or differentiate toward thymic cellular components, but the etiology of thymic neoplasms is largely unknown [1].

This chapter is about the epithelial tumors thymoma and thymic carcinoma (which include the neuroendocrine tumors). These tumors are histologically characterized by thymic stroma and lymphocytes. Thymomas usually exhibit a slow growth pattern, but they do have malignant potential and show a propensity for local invasion and intrathoracic recurrence. Extrathoracic metastases of thymomas are quite rare [2]. Thymic carcinomas are more aggressive, with invasion in the surrounding mediastinal structures. Extrathoracic metastases occur in <7% of patients [3]. Although thymomas and thymic carcinomas arise mostly in the anterior

mediastinum, they can also be found in other mediastinal compartments, as well as in the neck, lung, pleura, and thyroid, due to ectopic thymic tissue [1, 4, 5].

Approximately 1000 new cases are diagnosed per year in Europe [6]. Patients are usually between 20 and 70 years of age, with a peak in 30–40 years for those with myasthenia gravis and 60–70 years for those without myasthenia gravis. Incidence is similar in men and women [3, 7]. There are no known risk factors [3, 7], although clusters of thymoma are described in patients with multiple endocrine neoplasia type 1 (MEN1) syndrome [8] and an association between the Epstein– Barr virus and myasthenia gravis in thymoma patients has been suggested [9].
