Thymoma

by Jason Wasserman MD PhD FRCPC
September 20, 2024

A thymoma is a rare tumour that starts in the thymus, a small organ in the chest behind the breastbone. The thymus plays a key role in the immune system, particularly during childhood, by helping develop T cells, which are important for fighting infections.

What are the symptoms of a thymoma?

Some people with a thymoma may have no symptoms, and the tumour is often found by chance during imaging tests. Symptoms may include chest pain, cough, shortness of breath, or difficulty swallowing. Thymomas can also be associated with autoimmune conditions like myasthenia gravis, which causes muscle weakness.

What causes a thymoma?

The exact cause of thymoma is not known, but it is thought to develop from abnormal growth of thymic epithelial cells, which form part of the thymus’s structure. While genetic mutations have been identified in some thymomas, no single known cause exists for the tumour.

How are thymomas classified?

Thymomas are classified into several types based on the appearance of the tumour cells and the amount of lymphocytes (a type of white blood cell) present. Here is a brief overview of the different types:

• Type A: Contains mostly spindle-shaped epithelial cells with few or no lymphocytes. These are generally low grade and slow-growing tumours. Learn more about type A thymomas.
• Type AB: A mix of type A cells and areas with many lymphocytes. The behaviour of this type is similar to type A thymomas. Learn more about type AB thymomas.
• Type B1: Resembles normal thymic tissue, with many lymphocytes and fewer epithelial cells. This type tends to grow slowly. Learn more about type B1 thymomas.
• Type B2: Contains more atypical (abnormal-looking) epithelial cells and many lymphocytes. It can be more aggressive than type B1. Learn more about type B2 thymomas.
• Type B3: Composed mostly of atypical (abnormal-looking) epithelial cells with fewer lymphocytes. These tumours are more likely to invade nearby tissues and are considered higher-grade. Learn more about type B3 thymomas.

Is a thymoma benign or malignant?

Most thymomas are benign, meaning they grow slowly and do not spread to other parts of the body. However, some can behave more like cancers by invading surrounding tissues, such as the lungs or pericardium (the tissue covering the surface of the heart). For this reason, all thymomas should be removed surgically.