Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Positional differences limit comparison of a right - sided pleural effusion , which is still at least moderate in size . There is mild right lower lung atelectasis . The left lung is clear . There is no definite left pleural effusion . The cardiomediastinal silhouette is unchanged . There is no pneumothorax .
    OUTPUT:
    
Moderate right pleural effusion is difficult to compare to the prior radiograph from ___ , given differences in patient positioning .