What is the main finding?
Well circumscribed relatively low density mass abutting the mediastinum.
Can you tell where it is (anterior to, at or posterior to the hilum)? How?
The mass has an ill defined medial margin and the right heart border (especially SVC) cannot be seen. In addition the hilar vessels can be seen through the mass, indicating the mass is not at the hilum. It is therefore located in the anterior mediastinum.
The ability to visualize the hilar vessels through the mass indicates that the mass is not at the hilum. What is this sign known as?
Hilum overlay sign.
Well defined relatively low attenuation anterior mediastinal mass. On the frontal projection the hilar vessels can be seen distinctly through the mass.