Radiological Criteria

A hole in the lung with a wall, lumen and contents. Focus of increased density whose central portion has been replaced by air.

The following characteristics help in the differential diagnosis.

Number:

Multiple bilateral cavities would raise suspicion for either branchiogenous or hematogenous process. You should consider:

• Aspiration lung abscess
• Septic emboli
• Metastatic lesions 
• Vasculitis (Wegener's)
• Coccidioidomycosis, tuberculosis

            Single cavity

• Primary lung cancer
• Post-traumatic lung cyst
• Many other diseases

Size:

A large cavity encompassing the entire lobe or lung should raise suspicion for gangrene of lung.

Location:

• Classical locations for aspiration lung abscess are superior segment of the lower lobes and axillary subsegments of anterior and posterior segments of upper lobes. 
• Tuberculous cavities are common in superior segments of upper and lower lobes. 
• When a cavity in anterior segment is encountered, a strong suspicion for lung cancer should be raised. TB and aspiration lung abscess are rare in anterior segments. Cancer lung can occur in any segment.

Wall Thickness:

• Thick walls are seen in: 
  • Lung abscess 
  • Necrotizing squamous cell lung cancer
  • Wegener's granulomatosis
  • Blastomycosis
• Thin walled cavities are seen in:
  • Coccidioidomycosis
  • Metastatic cavitating squamous cell carcinoma from the cervix
  • M. Kansasii infection
  • Congenital or acquired bullae
  • Post-traumatic cysts
  • Open negative TB

Lining of Wall:

The wall lining is irregular and nodular in lung cancer or shaggy in lung abscess . The appearance is akin to stalactites and stalagmites.

Contents:

• The most common cause for air fluid level is lung abscess. Air fluid levels can rarely be seen in malignancy and in tuberculous cavities from rupture of Rasmussen's aneurysm. 
• A fungous ball should make you consider aspergillosis. A blood clot and  fibrin ball will have the same appearance.
• Floating Water Lily: I have never seen this. The collapsed membrane of a ruptured echinococcal cyst, floats giving this appearance.

Associated Features: 

Ipsilateral lymph nodes or lytic lesions of the bone is seen with malignancy.

Evolution of Lesion: 

Many times review of old films to assess the evolution of the radiological appearance of the lesion extremely helpful. Examples

• Infected bullae
• Aspergilloma
• Sub acute necrotizing aspergillosis
• Bleeding from Rasmussen's aneurysm in a tuberculous cavity

Etiology: 

Cavity can be encountered in practically most lung diseases.

Common diseases and their characteristics include:

• Primary Lung Cancer
  • Thick wall
  • Shaggy lumen
  • Eccentric cavitation
• Necrotizing Pneumonia
• Lung abscess
  • Gravity dependant segments
  • Thick wall
  • Air-fluid levels
• Tuberculosis
  • Superior segments
  • Infiltrate around
  • Bilateral
• Fungal infections
  • Aspergillus
    • Fungous ball
    • Sub acute invasive aspergillosis
• Metastatic disease
  • Thin walled (Squamous cell)
  • Thick wall (Adenoma)

Comprehension of the Above Principles:

• Rationale for multiple bilateral cavities.
• Why does reactivation TB occurs in superior segments?
• Why does aspiration lung abscess occur in the superior segment of lower lobes?
• What is the criteria for thick and thin wall ?
• What is the pathogenesis of stalactites and stalagmites?
• What is crescentic sign?
• How do you differentiate between aspergilloma and sub acute necrotizing aspergillosis?
• Does the location of cavity in a density have diagnostic significance?
• What is open negative TB?
• In metastatic disease, when do you get thin walled cavities and when do you get thick walled cavities?