Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Postoperative mediastinum with prominent aortic knob consistent with known aortic aneurysm is unchanged in appearance . The left lower lobe opacity has improved but not as much as would be expected and now contains a questionable cavitation seen best on frontal view .
    OUTPUT:
    
Partial clearing of left lower lobe pneumonia with new questionable cavitation raising the possibility incompletely treated or atypical infection . Chest CT could further characterize .