Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: AP upright and lateral views of the chest provided . Subtle retrocardiac opacity is most compatible with atelectasis , difficult to exclude an early pneumonia in the correct clinical setting . Elsewhere lungs are clear . No large effusion or pneumothorax . Cardiomediastinal silhouette appears stable . Bony structures are intact .
    OUTPUT:
    
Subtle retrocardiac opacity may represent atelectasis versus an early pneumonia .