Findings: the most obvious finding is a retro-tracheal mass in the lateral view (B, arrow). The answer to the etiology of the mass lies in the PA view, which shows an imprint in the right wall of the trachea (A, red arrow) and the descending aorta on the right side (A, arrows). In a patient without significant symptoms the most likely diagnosis is a right aortic arch.

Coronal and axial enhanced CT confirms the diagnosis of right aortic arch with a Kommerel diverticulum (C-D, arrows) that explains the anterior tracheal displacement.

Final diagnosis: right aortic arch with Kommerel diverticulum

Congratulations to Diogo, who was the first to suggest the correct diagnosis

Teaching point: remember that right aortic arch occurs in up to 0.1% of the population. A posterior tracheal imprint in an asymptomatic patient should prompt a search for a right aortic arch in the PA view.