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Cancer that starts in the thymus gland is called thymus cancer. It starts when cells in the thymus gland grow out of control and crowd out normal cells. Thymus cancers are rare.

The thymus is divided into 2 halves, called lobes. It has an irregular shape. There are a lot of small bumps called lobules on its surface. The thymus has 3 main layers:

• The medulla is the inside part of the thymus.
• The cortex is the layer that surrounds the medulla.
• The capsule is the thin covering over the outside of the thymus.

The thymus reaches its maximum weight of about 1 ounce during puberty. Then it decreases in size during adulthood as it's replaced by fat tissue.

The thymus is an important part of the body’s immune system. During fetal development and childhood, the thymus is involved in the production and maturation of T lymphocytes (also known as T cells), a type of white blood cell. T lymphocytes develop in the thymus and then travel to lymph nodes (bean-sized collections of immune system cells) throughout the body. There they help the immune system protect the body from viruses, fungus, and other types of infections.

The thymus is made of different types of cells. Each kind can develop into different types of cancer:

• Epithelial cells give the thymus its structure and shape. Thymomas and thymic carcinomas, which are the main focus of the rest of this document, develop from these cells.
• Lymphocytes make up most of the rest of the thymus. Whether in the thymus or in other parts of the body, these immune system cells can develop into cancers called Hodgkin disease and non-Hodgkin lymphoma.
• Kulchitsky cells, or neuroendocrine cells, are much less common cells that normally release certain hormones. These cells can give rise to cancers called carcinoid tumors. This document does not discuss carcinoid tumors of the thymus. Much of the information in Lung Carcinoid Tumor and Gastrointestinal Carcinoid Tumors also applies to carcinoids of the thymus.

Thymomas and thymic carcinomas

Thymomas and thymic carcinomas are tumors that start from thymic epithelial cells. Not all doctors agree about the best way to describe and classify these tumors. In the past, thymomas were sometimes divided into benign (non-cancer) thymomas and malignant (cancer) thymomas, based on whether they had grown beyond the thymus into other tissues or organs. Now, most doctors think all thymomas may become cancer over time, and the best way to predict how likely they are to come back after treatment is to describe whether they have grown into tissues beyond the thymus (and if so, how far). This is done by the surgeon who notes whether or not the tumor is attached to nearby organs and by the pathologist who looks at samples from the margins (edges) of the tumor under the microscope. The system used to describe the stage (extent) of thymomas is covered in How Is Thymus Cancer Staged?

WHO classification system for thymomas

Most doctors also classify thymomas by how they look under a microscope and by tests done on the tissue samples. This is called the histologic type. The system used for this classification, which was developed by the World Health Organization (WHO), assigns letters to the different types of thymomas.

Type A: The cells in these tumors are spindle-shaped or oval epithelial cells that are fairly normal looking. This is the rarest type of thymoma, but it seems to have the best prognosis (outlook).

Type AB: This type, also known as a mixed thymoma, looks like type A but there are also areas of lymphocytes mixed in the tumor.

Type B1: This type looks a lot like the normal structure of the thymus. It has a lot of lymphocytes along with normal-looking thymus cells.

Type B2: This type also has a lot of lymphocytes, but the thymus epithelial cells are larger with abnormal nuclei (the DNA-containing part of the cell).

Type B3: This type has few lymphocytes and mostly made of thymus epithelial cells that look pretty close to normal.

Type C: This is the most dangerous form and is also known as thymic carcinoma. It contains cells that look very abnormal under the microscope. The cells may no longer even look like thymus cells. These tumors have often grown into nearby tissues and/or spread to distant tissues and organs (metastasized) at the time they are found. This type of thymoma has the worst outlook (prognosis).

Type AB and type B2 are the most common types of thymoma, and type A is the least common. As you go from A to C, the outlook for survival tends to get worse. Type A has the best outlook, and type C has the worst. Still, for most types of thymoma, the stage (extent of growth and spread) is a better predictor of a person’s outcome. 

Other cancers in the mediastinum

Other cancers and tumors can occur in the mediastinum. Cancers can start in the esophagus (esophageal cancer), in the heart (and the tissue surrounding it), in the trachea, and in the lymph nodes (lymphoma).

Rarely, cancers and tumors known as germ cell tumors can also start in the mediastinum. These come from cells like those found in the testicles and ovaries.

Sometimes the thyroid gland, which is normally in the neck, is misplaced into the mediastinum. This can become enlarged, called a goiter. A thyroid tumor or cancer can also develop in the mediastinum.

More often, cancer spreads there from other areas, especially the lungs.