Getting a film with a posterior mediastinal mass in the exam is one of the many exam set-pieces that can be prepared for.
On this frontal chest radiograph, there is a soft-tissue density mass adjacent to the right upper mediastinum with thickening of the right paravertebral stripe. The cardiac contour and hilar vessels can be seen through its inferior border, placing it in the posterior mediastinum.
This is an isolated posterior mediastinal mass and a lateral chest radiograph would confirm its location in the posterior mediastinum. The differential diagnosis here is large and broad*. My normal practice would be to review any previous films if they were available and to get more clinical information that would help to narrow the differential [pause].
This mass could be a paraspinal mass caused by a peripheral nerve sheath tumors—such as schwannoma or neurofibroma that can present as a localized smooth paravertebral mass. Other paravertebral masses such as paraspinal abscess, metastases, or hematoma are possibilities. The mass could represent a descending aortic aneurysm or focal mass or dilatation of the esophagus. Duplication cysts and extramedullary hematopoiesis also present as posterior mediastinal masses.
Unless this is proven to be longstanding and unchanged, further investigation is warranted. Cross-sectional examination in the form of a CT would be helpful to determine the exact location of the mass and whether there is any contrast enhancement. If there were diagnostic uncertainty following the examination, it would also help to attempt to obtain tissue for a histological diagnosis.
* in some cases, the examiner may have given a little vignette of history at the beginning of the case - this will be key to limiting the differential given.