Thymus cancers are uncommon cancers that start in the thymus. This small organ is located just behind the breast bone (sternum) in the front part of the mediastinum, the space in the chest between the lungs. The thymus sits just in front of and above the heart.

The thymus is divided into 2 halves, called lobes. It has an irregular shape and a surface that is made up of many small bumps called lobules. The thymus has 3 main layers:

• The medulla is the innermost part of the thymus.
• The cortex is the layer surrounding 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 slowly decreases in size during adulthood as it is gradually 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 contains different types of cells, each of which can develop into different types of cancer:

Epithelial cells give the thymus its structure and shape. They can give rise to thymomas and thymic carcinomas, which are the main focus of the rest of this document.

Lymphocytes make up most of the rest of the thymus. Whether in the thymus or in other parts of the body, these immunes system cells can develop into cancers called Hodgkin disease and non-Hodgkin lymphomas, which are described in other documents from the American Cancer Society.

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. Much of the information in the American Cancer Society documents Lung Carcinoid Tumor and Gastrointestinal Carcinoid Tumors also applies to carcinoids of the thymus.

Doctors can tell the different thymic cancers apart by how they look under the microscope and by the results of other lab tests done on tissue samples.

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-cancerous) thymomas and malignant (cancerous) thymomas, based on whether they had invaded beyond the thymus into other tissues or organs. The view now held by most doctors is that all thymomas are potentially cancerous, and the best way to predict how likely they are to come back after treatment is to describe whether they have invaded beyond the thymus (and if so, how far). This is done by the surgeon who notes whether or not the tumor appears 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 discussed in the section "How is thymus cancer staged?"

WHO classification system for thymomas

Most doctors also classify thymomas based on how they look under a microscope. 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 appear to be fairly normal looking. Around 5% of thymomas are of this type. The outlook for people with this kind of thymoma is quite good and almost all are cured.

Type AB: This type, also known as a mixed thymoma, looks like type A except that there are also areas of lymphocytes mixed in the tumor. About 1 out of 3 thymomas are of this type. The outlook for people with this type is also quite good, with at least 9 out of 10 people being cured.

Type B1: This type looks a lot like the normal structure of the thymus. It has a lot of lymphocytes along with normal-appearing thymus cells. About 10% to 20% are of this type. It has a very good outlook, with about 9 out of 10 people being cured.

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). About 20% to 35% are of this type. About 6 out of 10 patients with this type are cured.

Type B3: This type has few lymphocytes and mostly consists of thymus epithelial cells that look pretty close to normal. About 10% to 15% of thymomas are this type. The outlook for this type is somewhat less favorable than for B2 thymomas, with about 4 out of 10 patients cured.

Type C: This is the most dangerous form and is also known as thymic carcinoma. It contains cells that have a very abnormal appearance under the microscope. The cells may no longer even look like thymus cells. These tumors have often invaded nearby tissues and/or metastasized (spread to distant tissues and organs) at the time they are found. Around 1 out of 4 people with thymic carcinoma survive at least 10 years after being diagnosed.

Although we have listed survival statistics for these different thymus cancer types, most doctors feel that the stage (extent of growth and spread) of the thymus cancer is the best predictor of a person's outcome. Survival statistics based on the stage of the cancer are discussed in the section "How is thymus cancer staged?"

Last Medical Review: 05/18/2009
Last Revised: 05/18/2009