Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: AP chest radiograph . There is a new left retrocardiac opacity with obscuration of the medial left hemidiaphragm . Left hemidiaphragm is mildly elevated . However , there is no pleural effusion or pneumothorax . The heart size is normal .
    OUTPUT:
    
New left retrocardiac opacity may represent atelectasis , aspiration or developing infectious pneumonia . PA and lateral chest radiographs may be helpful for more complete evaluation . Alternatively , short - interval followup is recommended .