Probably the most common cause of mediastinal widening on a chest x-ray are technical factors such as patient rotation.

Causes of actual mediastinal widening include:

• vascular problems: thoracic aortic aneurysm
• lymphoma
• retrosternal goitre
• teratoma
• tumours of the thymus

A lateral view may help to determine whether the mass is anterior or posterior but nowadays a CT would be performed if there was any doubt about the cause.