The thymus is a gland in the center of the chest (mediastinum) that has the important function during embryonic life of producing lymphocytes for the immune system . These thymus-derived, or T lymphocytes, are a major part of our immune system (cellular immunity) for our entire lives. Although these T lymphocytes look fairly similar under the microscope to the other major class of lymphocytes, B lymphocytes, they perform specialized immunologic functions to protect us from foreign organisms.
A variety of cancers can develop in the thymus. The most common tumor is malignant thymoma, which originates in the covering, or epithelium, of the thymus gland. All other tumor types are uncommon. These include lymphomas, germ cell tumors, carcinoids, carcinoma and thymolipomas.
Types Under the microscope, there are a number of cell sub-types of thymoma, but it is difficult to relate this microscopic appearance to how a tumor behaves. Some thymomas are non-invasive and appear to behave like benign tumors; others have all of the invasive characteristics of malignancy. So whether the tumors are called malignant depends not on their appearance but on their behavior (whether they have invasive characteristics). In the absence of invasion, the presence of a clearly defined covering, or "capsule," tends to classify the tumor as
