What is the diagnosis?
a) Pericardial cyst
b) Right atrial enlargement
c) Aortic aneurysm
d) Mediastinal mass
Answer: c) Aortic aneurysm.
Other conditions do come in the differential diagnosis of a similar appearance. In this case, previous X-rays were available, to support the diagnosis: http://cardiophile.org/2010/07/serial-x-rays-of-aortic-aneurysm/