Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The heart size is top normal . The hilar and mediastinal contours are normal . Note is made of atherosclerotic calcifications at the aortic knob . Interval increase in density at the retrocardiac region not clearly present on prior . Lungs are otherwise clear . There is no pleural effusion or pneumothorax . The visualized osseous structures are unremarkable .
    OUTPUT:
    
New retrocardiac opacity , potentially atelectasis .