Cancerous tumours of the thymus

A cancerous tumour of the thymus can grow into and destroy nearby tissue. It can also spread (metastasize) to other parts of the body. Cancerous tumours are also called malignant tumours.

Cancerous tumours of the thymus are rare. Most cancerous tumours of the thymus start in the epithelial cells of the thymus. They are usually found in the front (anterior) mediastinum.

Thymoma is the most common cancerous tumour of the thymus and the front mediastinum. It tends to grow slowly and doesn’t usually spread to areas outside of the thymus. The cancer cells look a lot like normal epithelial cells of the thymus.

When describing thymomas, doctors may use the words non-invasive and invasive.

• Non-invasive thymomas have not broken through the covering of the thymus (capsule), but they can stick to nearby organs. They are usually easily removed with surgery.
• Invasive thymomas have grown into organs and areas around the thymus and sometimes spread to the pleura. They can be harder to remove than non-invasive thymomas.

Thymic carcinoma is a less common cancerous tumour of the thymus. It tends to be an aggressive tumour that grows quickly and usually has spread to other parts of the body when it is diagnosed. The cancer cells look very different from normal thymus epithelial cells.

Doctors may classify thymic carcinoma as low grade or high grade. This depends on how different the cancer cells look and behave compared to normal cells. Low-grade thymic carcinoma often has a better prognosis than high-grade thymic carcinoma.

There are different types of thymic carcinoma. They are named based on how the cancer cells look under a microscope. Some types of thymic carcinoma are squamous cell, mucoepidermoid, undifferentiated and small cell carcinomas.

Tumours can start in different types of cells in the thymus and mediastinum. The following cancerous tumours are very rare.

Thymic neuroendocrine tumours (NETs) start in the neuroendocrine cells of the thymus. Like other NETs, thymic NETs can be made up of cells that look and act much like normal cells (well-differentiated). Or thymic NETs can have very abnormal cells and be aggressive tumours that grow very quickly (poorly differentiated). Find out more about neuroendocrine tumours (NETs).

Germ cell tumours start in the reproductive cells that make eggs in women or sperm in men (germ cells). Sometimes germ cell tumours are found outside the ovaries or testicles (extragonadal) such as in the mediastinum. Cancerous germ cell tumours in the mediastinum happen most often in men.

Thymic lymphomas start in the lymphocytes of the thymus. For example, primary mediastinal large B-cell lymphoma can start in the thymus. Find out more about the 2 main types of lymphoma – non-Hodgkin lymphoma and Hodgkin lymphoma.