Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Cardiomediastinal silhouette and hilar contours are normal and stable . A right central venous catheter is unchanged in position with the tip projecting over the low SVC . There is a vague retrocardiac density in the left lower lung . Right lung is clear . There is no pleural effusion or pneumothorax .
    OUTPUT:
    
Vague retrocardiac opacity which may represent focal atelectasis , but PA and lateral chest radiographs would be helpful for further characterization if there is clinical suspicion for pneumonia .