Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Compared to most recent prior exam , there has been interval development of a retrocardiac opacity and small left pleural effusion . No pneumothorax is detected . There is no evidence for pulmonary edema . Heart and mediastinal contours are within normal limits .
    OUTPUT:
    
New retrocardiac opacity , which could represent atelectasis , pneumonia or aspiration . New small left pleural effusion . Findings reported to Dr . ___ by Dr . ___ by telephone at 9 :49 a .m . on ___ at the time of discovery of these findings .