Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Subtle retrocardiac opacity may be due to atelectasis , though pneumonia is possible . Follow - up chest radiograph is recommended . There is no pulmonary edema or pleural effusions . Cardiomediastinal and hilar contours are within normal limits for this portable examination . There is no pneumothorax .
    OUTPUT:
    
Small retrocardiac opacity , possible atelectasis or pneumonia . Follow - up chest radiograph recommended .