Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Lungs are clear except for a patchy left retrocardiac opacity . The left costophrenic angle is unchanged and could be due to pleural thickening are pleural effusion . The cardiomediastinal silhouette is unchanged as compared to prior .
    OUTPUT:
    
Patchy left retrocardiac opacity , which may represent focal atelectasis , aspiration , or focal pneumonia . Short term radiographic followup is recommended to ensure resolution appear .