Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The cardiac , mediastinal and hilar contours appear unchanged since at least the prior radiographs . There is no definite pleural effusion or pneumothorax . Lung volumes are low . An interstitial abnormality appears increased since the prior CT , and in particular , there is prominent posterior basilar opacity in the left lower lobe in the retrocardiac region . Radiographs are more difficult to compare directly to the recent prior CT although the distribution of the background interstitial abnormality appears fairly similar .
    OUTPUT:
    
1 . Increased interstitial abnormality since prior radiographs . Although an etiology such as mild vascular congestion or atypical infection could explain the change , since the distribution is very similar , worsening of a underlying interstitial disease is probably the most likely reason . 2 . New or increased retrocardiac opacity . Differential includes atelectasis , focal area of interstitial change and fibrosis , but possibly superimposed infectious process . Correlation with clinical presentation is suggested .