Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: In comparison to the prior radiograph , there is a new retrocardiac opacity which may represent atelectasis or infection . The lungs are otherwise clear and moderate cardiomegaly is unchanged . No pleural effusion or pneumothorax .
    OUTPUT:
    
New left retrocardiac opacity may represent atelectasis or infection . Moderate stable cardiomegaly .