Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Frontal view of the chest was obtained . The heart is normal size . Increased retrocardiac opacity is compatible with known left hilar mass and left lower lobe collapse . Left main stem bronchus stent is in similar position to prior . The right lung is grossly clear , with known nodules better appreciated on CT . No pneumothorax .
    OUTPUT:
    
Retrocardiac opacity , which correlates to known left lower lobe collapse and perihilar mass better seen on prior chest CT . Superimposed pneumonia would be difficult to exclude .