Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: There is very subtle retrocardiac opacification , which is not definitively seen on the PA view , and may represent an early developing pneumonia . The 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:
    
Retrocardiac opacification , which may represent an early developing pneumonia .