Cavitation in tuberculosis is the result of necrosis of lung tissue that is then extruded through a communicating bronchus thus forming an air-filled cavity. In this specimen the fungus ball exhibits areas of retraction from the wall of the cavity resulting in the presence of air between the fungus ball and cavity wall; this forms the basis for the diagnostic "air-crescent sign" seen in imaging studies. The bronchus that communicates with the cavity is evident. Healed, calcified tuberculous lesions are also present.
Aspergilloma complicating tuberculosis.
Images supplied by Yale Rosen (http://www.flickr.com/photos/pulmonary_pathology/)