Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: There is again a large mass occupying the left upper lobe , but what is new is a substantial pleural effusion on the left and probable associated atelectasis of at least parts of the left lower lobe . Overall , there is mildly positive mass effect toward the right side . The right lung remains clear .
    OUTPUT:
    
New pleural effusion and basilar opacification on the left , the latter probably compatible with atelectasis . New coinciding infection in the setting of a known left upper lobe mass is not excluded , however , including the potential for post - obstructive pneumonia .