Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Subtle patchy left base retrocardiac opacity is seen which may be due to atelectasis but subtle infectious process is not excluded . No pleural effusion or pneumothorax is seen . The cardiac and mediastinal silhouettes are unremarkable .
    OUTPUT:
    
Subtle patchy left base retrocardiac opacity , which may be due to atelectasis but subtle infectious process is not excluded .