What is thymus cancer?
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 has different types of cells, each of which 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, 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. This document does not discuss carcinoid tumors of the thymus. 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 grown 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 grown into tissues 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 by how they look under a microscope. 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 appear to be 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 except that 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-appearing 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 consists 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 have a very abnormal appearance under the microscope. The cells may no longer even look like thymus cells. These tumors have often grown into (invaded) nearby tissues and/or metastasized (spread to distant tissues and organs) at the time they are found. This type of thymoma has the worst prognosis (outlook).
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, with type A having the best outlook, and type C having the worst. Still, for most types of thymoma, the stage (extent of growth and spread) is a better predictor of a person's outcome. Staging of thymus cancer is discussed in the section "How is thymus cancer staged?"
Last Medical Review: 11/16/2012
Last Revised: 11/16/2012