Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Linear opacity at the left lung base . There is also blunting of the lateral costophrenic angle on the left . There is no visualized pneumothorax . The cardiomediastinal silhouette is within normal limits . Old posterior left rib fractures are identified .
    OUTPUT:
    
Possible left - sided pleural effusion and left basilar atelectasis .