• Occurs as a consequence of diseases with chronic pleural scarring, especially asbestos-related pleural disease and TB
• Most often at the lung bases, posteromedially
• Must be subpleural in position
   

Pathophysiology
 

• A rapidly forming pleural effusion produces an adjacent area of passive atelectasis
• A groove of visceral pleura may infold into the area of atelectasis and come to surround a part of it
• If the effusion recedes at once, the lung will probably re-expand
• If fibrinous adhesions form or if there is preexistent chronic pleural disease, then the atelectatic area of lung remains trapped by the enfolded visceral pleura
• Asymptomatic: important because it resembles a bronchogenic ca

Imaging Findings 

• Rounded density at lung base
• Contiguous to area of pleural disease or superimposed on apparent asbestos-related pleural disease or TB
• Comet tail on CT: vessels and bronchi converge upon and appear to swirl around mass
• Crow’s feet — linear bands radiating from mass into lung parenchyma
• Linear densities radiate back toward hilum
• May have air bronchogram