There are a number of thymic conditions and the terminology often gets confused between them.
A thymoma is typically a slow growing tumor. Its cells look similar to the normal cells of the thymus and it usually does not spread beyond the thymus. People with thymoma, however, often have other diseases as well. The most common is myasthenia gravis, a disease that weakens the muscles, but others include polymyositis, lupus erythematosus, rheumatoid arthritis, thyroiditis, Sjogren’s syndrome, and hypogammaglobulinemia. 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 thymoma can recur after a long interval, it has been recommended that surveillance should be lifelong.
Thymoma is rare, accounting for about 0.2% to 1.5% of all cancers. Approximately 90% of tumors of the thymus are thymoma. The remaining 10% are thymic carcinoma, carcinoid tumors, or lymphomas. Approximately 500 people in the United States are diagnosed with thymoma each year. Thymoma usually occurs in people between the ages of 40 and 80.
In some cases, however, the thymoma can grow into the lungs and heart. These are called invasive thymomas and they are often treated using surgery, radiation, and chemotherapy to kill or shrink the tumor.
In a thymic carcinoma, the cells no longer look like normal thymic cells. These grow more quickly than thymomas and have usually spread to other parts of the body when the cancer is found. Thymic carcinoma is more difficult to treat than thymoma and have the trendency to metastasize throughout the body.
Thymic carcinomas are difficult to diagnose because they are rare, and because there are several pathologic types:
- Keratinizing squamous cell carcinoma
- Nonkeratinizing squamous cell carcinoma
- Lymphoepithelioma-like carcinoma
- Adenosquamous carcinoma
- Mucoepidermoid carcinoma
- Clear-cell carcinoma
- Papillary adenocarcinoma
- Adenocarcinoma not otherwise specified
- Basaloid carcinoma
- Sarcomatoid carcinoma
Generally, well-differentiated squamous carcinoma, low-grade mucoepidermoid carcinoma, and basaloid thymus carcinoma have a more favorable prognosis. The other types are more aggressive.
Thymic Carcinoids (Thymic Neuroendocrine Tumor)
When a pathologist looks at cancer cells, they apply certain stains, or coloring agents, before looking at the cells under a microscope. The way the cells appear when stained tell the pathologist the type of cancer. Thymomas and thymic carcinomas are from epithelial cells that line the surface of the thymus. There are also thymic carcinoid tumors that arise from glandular endocrine-hormone producing cells that are found in small quantities in the thymus. These cells can be found almost anywhere in the body, but is largest amounts in the small intestine. Other places containing endocrine cells include the appendix, rectum, and lung.
A specific cancer is named for the origin of the endocrine or epithelial cells. For example, renal cell carcinoma is a form of kidney cancer caused when cells in the lining of the renal tubule undergo cancerous changes. Squamous cell carcinoma often arises from middle portion of the epidermal skin layer, but squamous cell carcinoma can appear almost anywhere, including the mucous membranes. Cancers from mesenchymal tissues — connective tissues, blood and blood vessels, and the lymphatic system — are known as sarcomas.
The thymus consists of two lobes connected by areolar tissue. The lobes are enclosed in a fibrous capsule which dips into their substance dividing them into lobules that consist of an irregular branching framework of epithelial cells and lymphocytes. Occasionally the normal anterior mediastinal thymus may extend into the middle or posterior mediastinum.
Ectopic Thymic Carcinoma
In some very rare cases, however, thymus cells may be ectopic, which means in a location in which they are not normally found. The most likely place for ectopic thymus cells is in the neck near the thyroid gland as a result of abnormal migration during fetal development. During fetal development, the cells that make up the thymus migrate down to the mediastinum. In very rare cases, some of this tissue fails to migrate, forming in the neck