3  Imaging Approach to Cavitary Lung Disease

The article presents a comprehensive approach to evaluating cavitary lung lesions on imaging, with a focus on how to narrow down the differential diagnosis based on imaging characteristics and clinical context.

3.1 Definition and Pathology

A cavity is formally defined by the Fleischner Society as a gas-filled space within a pulmonary mass, nodule, or consolidation. The cavity wall typically measures at least 2mm in thickness (distinguishing it from cysts which are usually thin-walled, <2mm). Pathologically, cavities form through liquefactive necrosis and subsequent expulsion of debris via the bronchial tree.

3.2 Key Imaging Features for Evaluation

The article emphasizes several imaging characteristics that can help distinguish between different etiologies:

  1. Number of lesions:
    • Solitary cavitary lesions often suggest primary lung cancer or pulmonary abscess
    • Multiple lesions are more characteristic of septic emboli, granulomatosis with polyangiitis (GPA), rheumatoid nodules, and metastases
  2. Location of lesions:
    • Upper lobes: Commonly affected by lung cancer and reactivation tuberculosis
    • Lower lobes: More commonly involved by septic emboli and metastatic disease
  3. Wall characteristics:
    • Wall contour: Malignant lesions typically show irregular internal walls and focal indentation of outer contours, while benign lesions tend to have smooth walls
    • Wall thickness:

      • 24mm is reported as 100% specific for malignancy

      • <7mm is 97% specific for benign lesions
      • 5-15mm: approximately 50% malignant, 50% benign
  4. Ancillary findings:
    • Perilesional consolidations and centrilobular/tree-in-bud nodules support benign etiologies, particularly infectious causes

3.3 Major Diagnostic Categories

The article categorizes the differential diagnoses of cavitary pulmonary lesions into three main groups:

3.3.1 Infectious Causes

  • Mycobacterial:
    • Reactivation TB: Upper lobe cavitary disease with thick, irregular walls
    • Mycobacterium avium complex: Classic form resembles TB; nonclassic form presents with bronchiectasis in middle lobe/lingula
  • Pyogenic:
    • Lung abscess: Typically solitary, round, in dependent portions of lungs
    • Necrotizing pneumonia: Parenchymal consolidation with multiple areas of cavitation
    • Septic emboli: Randomly distributed peripheral nodules of varying size, “feeding vessel sign”
  • Fungal:
    • Aspergillus: Simple aspergilloma (mycetoma) in preexisting cavities vs. invasive aspergillosis with “halo sign” and “air crescent sign”
    • Mucormycosis: Aggressive infection, more likely to cavitate than aspergillosis
    • Others: Histoplasmosis, coccidioidomycosis, cryptococcosis

3.3.2 Malignant Causes

  • Primary lung cancer: Squamous cell carcinoma most commonly cavitates, thick walls with irregular internal contours
  • Metastatic disease: Randomly distributed nodules of varying size, often from squamous cell carcinomas of head and neck

3.3.3 Rheumatologic Causes

  • Rheumatoid arthritis: Multiple well-defined lung nodules, occasionally with central necrosis/cavitation
  • Granulomatosis with polyangiitis (GPA): Multiple round/ovoid nodules (2-4cm) that can coalesce and cavitate, ground-glass opacities suggesting hemorrhage