Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: There is a retrocardiac opacity which corresponds to an area of increased opacity over the spine on the lateral view . Cardiomediastinal silhouette is unremarkable . Bones are intact . No pleural effusions are seen .
    OUTPUT:
    
Left lower lobe / retrocardiac opacity most likely due to atelectasis , but pneumonia can not be excluded in the appropriate clinical setting .