Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: There is subtle opacification of the left retrocardiac region , which is not seen on the lateral . Lungs are otherwise clear . Heart size is normal . The mediastinal and hilar contours are normal . The pulmonary vasculature is normal . No pleural effusion or pneumothorax is seen . There are no acute osseous abnormalities .
    OUTPUT:
    
Subtle opacification in the left retrocardiac region , which could represent atelectasis or pneumonia in the appropriate clinical setting .