Can you determine which part of the mediastinum the mass abuts? How?
The mass does not abut the descending aorta, as its outline is clearly visible. The hilar vessels are not visible through the mass (no hilum-overlay-sign) and as such the mass must involve the hilum.
Chest x-ray (slightly rotated to the right) demonstrates a large opacity obliterating the left hilum. It is associated with some volume loss in the left hemithorax and what appears to be a pleural effusion.