Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: There is mild cardiomegaly , similar in degree when compared to prior exam . Subtle opacity in the retrocardiac region in the posterior left costophrenic angle on the lateral view is new since prior . Elsewhere , the lungs are clear . No acute osseous abnormality .
    OUTPUT:
    
Subsegmental retrocardiac opacity , potentially atelectasis although infection or infarct in the setting of sickle cell disease would be possible .