What is cancer?

Cancer starts when cells in the body change (mutate) and grow out of control. Your body is made up of tiny building blocks called cells. Normal cells grow when your body needs them and die when your body doesn't need them any longer.

Cancer is made up of abnormal cells that grow even though your body doesn't need them. In most cancers, the abnormal cells grow to form a lump or mass called a tumor. If cancer cells are in the body long enough, they can grow into (invade) nearby areas. They can even spread to other parts of the body (metastasis).

What is thymus cancer?

Thymus cancer is a rare cancer that starts in the cells that make up the thymus. The thymus is a small organ in the front part of your chest under the breastbone (sternum) and in front of and above the heart. The thymus has two lobes.

When your body is forming and growing during infancy and childhood, the thymus makes a type of white blood cell called a T-lymphocyte (T cell). These blood cells are a key part of your immune system. They grow in the thymus and then travel to the lymph nodes. This is where they help protect your body against infections. The thymus becomes smaller in adulthood as it's replaced by fat.

The thymus is covered by epithelial cells on its outer surface. These cells are where thymus tumors start. 

Types of thymus tumors

There are two main types of thymus tumors. Both are quite rare. These both start from thymic epithelial cells but act very differently.

Thymomas

These are the more common type of tumor in the thymus. Most thymomas are slow-growing cancers. In rare cases, they can spread to other parts of the body.

There are different types of thymomas. When your healthcare provider finds a thymoma, it’s classified based on two factors: how the cells look under a microscope and whether the tumor has spread past the thymus (and, if so, how far). Thymomas are often linked to autoimmune diseases, like myasthenia gravis and Good syndrome.

Thymic carcinomas

These tumors also form from epithelial cells in the thymus, but they grow faster. They've often spread to other parts of the body by the time they are found. They can be harder to treat.

Other types

There are other types of tumors that can start in the thymus, but these are very rare. These include lymphoma, carcinoid, and germ cell tumors of the thymus.

Talk with your healthcare provider

If you have questions about thymus cancer, talk with your healthcare provider. They can help you understand more about this cancer.