Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: There is a left retrocardiac opacity which obscures the left heart border and diaphragm . There is no pleural effusion or pneumothorax . The heart size is normal and the left mediastinal structures are unremarkable .
    OUTPUT:
    
Left lower lung opacity is thought to represent a post-obstructive infection in the setting of a known hilar mass .