Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: There has been interval development of a retrocardiac opacity at the left lung base obscuring the left medial hemidiaphragm . Linear scar or atelectasis in the right mid and left lower lungs are unchanged . Cardiomediastinal contours are stable There is no evidence of pulmonary vascular congestion .
    OUTPUT:
    
New left retrocardiac opacity which may be due to atelectasis or pneumonia .