Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The cardiomediastinal silhouette and pulmonary vasculature are stable and unremarkable since prior examination . Better evaluated on the lateral film , a retrocardiac opacity is noted , which , in the appropriate clinical context , may represent atelectasis . There is no pleural effusion or pneumothorax .
    OUTPUT:
    
Lower lung volumes on lateral examination may account for atelectasis . No definite pneumonia .