Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Degree of retrocardiac opacity has increased since prior and there is silhouetting of the descending thoracic aorta . Elsewhere , lungs are clear . Cardiomediastinal silhouette is within normal limits . No acute osseous abnormality .
    OUTPUT:
    
Retrocardiac opacity which could be atelectasis versus pneumonia in the proper clinical setting .