What feature make you believe that the mediastinal mass is not confined in the anterior mediastinum?
It extends further above the clavicles levels. A lateral radiograph, not obtained, would be useful in confirming the location.
Which are the possible differentials?
Foregut duplication cysts (e.g. oesophageal duplication or bronchogenic cysts), lymphadenopathy, goitre. There are no right lung volume reduction or evidence of fissure/hilum displacement to consider right upper lobe collapse as a possibility.
The cardiac contour is within normal limits. There is enlargement of the right superior mediastinal contour with tracheal displacement to the left. The lungs and pleural spaces are clear.