Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: PA and lateral views of the chest provided . Left lung base mass with fiducial markers is again noted . Elevated right hemidiaphragm is again seen . There is a small left pleural effusion and a trace right pleural effusion . There is no pneumothorax . Left lower lobe opacity is best seen on the lateral view . The cardiomediastinal silhouette is normal . Imaged osseous structures are intact . No free air below the right hemidiaphragm is seen .
    OUTPUT:
    
Left lower lobe opacity is most likely atelectasis . In the appropriate clinical setting , pneumonia can not be excluded , but this is thought to be less likely .