Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Compared with prior radiographs on ___ , there is an asymmetric opacity at the left lung base , which obscures the left heart border . There is no pneumothorax or pleural effusion . Cardiomediastinal silhouette is unchanged .
    OUTPUT:
    
Left basilar opacity , could represent atelectatic changes , however in view of clinical history , pneumonia should be considered . PA and lateral radiographs would provide better evaluation if able to obtain .