Radiological criteria

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 Focus of increased density whose central portion has been replaced by air.

Following characteristics help in differential diagnosis.

Number

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

• Aspiration lung abscess
• Septic emboli
• Metastatic lesions 
• Vasculitis (Wegners)
• Coccidiomycosis, 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 lower lobes and axillary sub segments 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
  • Wegners granulomatosis
  • Blastomycosis
• Thin walled cavities Extremely thin walled cavities are seen in
  • Coccidiomycosis
  • Metastatic cavitating squamous cell carcinoma from Cervix
  • M. Kansasii infection
  • Congenital or acquired bullae
  • Posttraumatic cysts
  • Open negative TB.

Lining of wall

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

Contents

• 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. Blood clot and   fibrin ball will have the same appearance.
• Floating Water Lily: I have never seen this. The collapsed membrane of a ruptured echynococcal cyst, floats giving this appearance 

Associated Features

Ipsilateral lymph nodes or lytic lesions of 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

• 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 (Adenoca)

Comprehension of the above principles.

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