Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: PA and lateral chest radiographs were obtained . There is a new subtle retrocardiac opacity that is most obvious as a spine sign on the lateral radiograph . Minimal atelectasis is seen at the left base . There is no effusion pneumothorax . Cardiac and mediastinal contours are normal .
    OUTPUT:
    
Retrocardiac opacity may be due to summation of shadows , but a focal infiltrate can not be excluded .