Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The heart size is normal . The hilar and mediastinal contours are normal . There is a retrocardiac opacity , overall similar to the prior exam . There is no pleural effusion or pneumothorax . The visualized osseous structures are unremarkable .
    OUTPUT:
    
Subtle retrocardiac opacity , overall similar to the prior exam . This could be secondary to scarring from prior infection , however an acute superimposed infectious process can not be excluded .