CT demonstration of empyema necessitatis.

Empyema necessitatis is a collection of fluid that has, by direct extension from the pleural cavity, penetrated the thoracic wall to form a mass in the extrapleural soft tissues of the chest wall. This is a rare condition and usually is associated with tuberculous effusion but may be associated with malignancy, blastomycosis, actinomycosis, and may even follow thoracentesis.