Coloured X-ray of a mediastinal mass (red) in the chest of a 53 year old male patient with a thymoma, a rare cancer of the thymus gland. The thymus is a gland in the upper chest that plays an important role in the immune system, especially before puberty. Thymomas are often symptomless, but as the tumour grows they may cause chest pressure or pain. Treatment is with surgery to remove the thymus gland, sometimes with radiotherapy and/or chemotherapy.