Cancerous tumours of the thymus

A cancerous tumour of the thymus can grow into, and destroy, the tissue around it. The tumour 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 called thymic epithelial tumours.

Thymoma is the most common cancerous tumour of the thymus. 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). They are usually removed easily with surgery.

Invasive thymomas have grown into organs and areas around the thymus and sometimes spread to the lining covering the lungs (called the pleura). They can be harder to remove than non-invasive thymomas.

Thymic carcinoma is a less common cancerous tumour of the thymus. It is a tumour that grows quickly and can spread to other parts of the body. The cancer cells look very different from normal thymus epithelial cells.

Thymic carcinoma may be classified as low grade or high grade. The grade 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. These include:

• squamous cell carcinoma – the most common type of thymic carcinoma
• basaloid carcinoma
• mucoepidermoid carcinoma
• lymphoepithelioma-like carcinoma
• papillary thymic adenocarcinoma
• clear cell carcinoma
• undifferentiated carcinoma
• sarcomatoid carcinoma
• carcinoma with t(15;19) translocation (called a NUT carcinoma)

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

Thymic neuroendocrine tumours (TNETs) start in the neuroendocrine cells of the thymus. Like other neuroendocrine tumours (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). They are treated like neuroendocrine tumours that start in other parts of the body.

Thymic lymphomas start in the lymphocytes of the thymus. The most common type of lymphoma that starts in the thymus is primary mediastinal large B-cell lymphoma, a type of non-Hodgkin lymphoma.