Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The heart is normal in size . The mediastinal and hilar contours appear within normal limits . There is no pleural effusion or pneumothorax . Slight residual left lower lung opacity remains but improved since the prior examination from ___ , with no definite new focal opacity . An exostosis along the course of the superior right second rib appears unchanged .
    OUTPUT:
    
Substantial improvement in left lower lung opacity .