Thymoma is a rare kind of tumour that develops in the thymus gland, which is a small organ located in the upper part of the chest just behind the breastbone. The thymus plays an important role in the early years of life because it supports the development of T-cells, which help the immune system recognise and fight infections. As a person grows older, the thymus gradually becomes smaller and less active, but it still remains present in the body, and in some cases tumours can arise from its epithelial cells.
