The thymus is a small organ in the front part of the chest under the breastbone. It runs from the lower neck to above the level of the heart. During development in the womb and in childhood, the thymus makes a type of white blood cell called a T-lymphocyte. T-lymphocytes are important to the immune system. They grow in the thymus and then travel to the lymph nodes where they help protect the body against infections and cancer.
The thymus is lined by epithelial cells on its outer surface. These cells are where thymus tumors start.
Different types of thymus tumors can be identified by the way they look under a microscope. There are two main types of thymic tumors, both of them very rare:
Thymomas. These are the most common type of tumor in the thymus. They begin in a type of cell called thymic epithelial cells. When a thymoma is found, the doctor usually looks at whether it has spread beyond the thymus and, if so, how far. All thymomas are potentially cancerous. They often appear together with an immune or endocrine disease.
Thymic carcinomas. These tumors also develop from epithelial cells in the thymus, but this type of cancer is more aggressive. It can spread to nearby tissues and sometimes to other parts of the body.
Other rare thymus tumors include lymphomas, as well as sarcomas and carcinoids of the thymus.