What is the most likely diagnosis? What about the differential diagnosis?
In an older adult, the most common mass in this location would be a goitre. The differential includes lymph node enlargement (e.g. lymphoma / metastases) and a thymic mass. An aneurysm should be kept in the back of the mind as not considering it can eb disastrous (especially if a biopsy is performed), but in this case is unlikely.
Other than the displacement of the trachea, what suggests this mass is anterior?
The preservation of the the aortic knuckle and descending aortic silhouette.
How can the nature of this mass be confirmed?
Both ultrasound and CT would easily confirm the diagnosis. The inferior portion of this mass would be difficult to image with ultrasound due to the overlying sternum.
Chest x-rays demonstrate a large mass in the superior mediastinum, displacing to the right and somewhat narrowing the trachea. There is thickening of the right paratracheal stripe suggesting that it extends to the right of the trachea also. The outline of the mass fades out inferiorly, and does not obscure the arch or the aorta or the descending aorta.