Thymic Epithelial Tumors

There are a number of thymic tumors and the terminology often gets confused between them.

Thymomas

A thymoma is typically a slow growing tumor. Despite its usual favorable outcome, thymomas are considered malignant tumors as on occasion they can metastasize and/or recur and in rare cases lead to death of the patient. People with thymoma may develop different SYMPTOMS. Thymoma has been associated with an increased risk for second malignancies, which appears to be unrelated to thymectomy, radiation therapy, or a clinical history of myasthenia gravis. Because of this increased risk for second malignancies and the fact that thymomas can recur or metastasize even after a long interval, it has been recommended that surveillance should be lifelong.

Thymomas are rare, accounting for about 0.2% to 1.5% of all cancers. However, they are the most common solitary lesion in the anterior mediastinum (the space inside the chest right behind the sternum/front chest plate) representing approximately 20% of all mediastinal lesions. Other mediastinal lesions include benign cysts, lymphomas, thymic carcinomas, germ cell tumors, and neuroendocrine tumors such as carcinoid tumors, among others. In some very rare cases thymic tumors might arise outside of the anterior mediastinum, for instance in the neck. It is a result of abnormal migration of a thymus cells from the neck down to the anterior mediastinum during fetal development.

Metastases from tumors of origin elsewhere in the body are not uncommon in that location. Thymoma usually occurs in people between the ages of 40 and 80.

Thymomas are subtyped based on their cellular composition into types A, AB, B1, B2, and B3. Other, more rare subtypes such as micronodular thymoma with lymphoid stroma are also recognized.

Thymomas are staged according to the Tumor Nodal Metastasis (TNM) STAGING. Staging is an important prognostic marker, similar to complete resection of the thymoma. While many thymomas have a complete capsule around them, some can grow (invade) into the lungs, heart, or other organs and vessels. These invasive tumors are assigned a higher stage and are often not only treated by resection but also  with radiation and/or chemotherapy (TREATMENT).

Thymic Carcinomas

In a thymic carcinoma, the cells no longer look like normal thymic cells. Thymic carcinomas grow in general more quickly than thymomas and sometimes have spread to other parts of the body when the cancer is found. Thymic carcinomas have a higher tendency to metastasize throughout the body and to recur than thymomas.

Thymic carcinomas are more rare than thymomas. They are classified according to their histomorphologic appearance with squamous cell carcinoma being the most common subtype. Some thymic carcinoma subtypes such as well differentiated squamous cell carcinoma and mucoepidermoid carcinoma have a more favorable prognosis than others such as NUT carcinoma or lymphoepithelioma-like carcinoma which have a worse outcome.

Thymic Neuroendocrine Neoplasms

Thymic neuroendocrine neoplasms are even less common than thymomas and thymic carcinomas. The most common neuroendocrine neoplasms in the thymus are carcinoid tumors. These tumors are comprised of cells that express neuroendocrine proteins such as synaptophysin and chromogranin.  These tumors are also malignant.