Durvalumab-Induced Pneumonitis CT Patterns
Durvalumab-induced pneumonitis demonstrates multiple CT patterns, most commonly organizing pneumonia (OP), followed by nonspecific interstitial pneumonia (NSIP), hypersensitivity pneumonitis (HP), diffuse alveolar damage (DAD), and rarely simple pulmonary eosinophilia. 1
Primary CT Patterns
According to the 2021 Fleischner Society position paper, immune checkpoint inhibitor (ICI) pneumonitis, including durvalumab, presents with five main radiologic patterns 1:
Organizing Pneumonia (OP) Pattern - Most Common
- Multifocal patchy alveolar opacities with peribronchovascular and/or peripheral distribution
- May demonstrate reversed halo sign
- This is the most frequently observed pattern with ICI therapy including durvalumab 1
Nonspecific Interstitial Pneumonia (NSIP) Pattern
- Patchy areas of ground-glass opacity (GGO) progressing to irregular reticular opacities
- Architectural distortion and traction bronchiectasis with or without consolidation
- Bilateral and symmetric distribution with predominant lower-lung involvement 1
Hypersensitivity Pneumonitis (HP) Pattern
- Poorly defined small centrilobular nodules
- Bilateral ground-glass opacity
- Large or lobular areas of decreased attenuation and vascularity (mosaic attenuation) 1
Diffuse Alveolar Damage (DAD/ARDS) Pattern - Most Severe
- Extensive bilateral areas of GGO and dependent airspace consolidation in exudative phase
- Traction bronchiectasis and decreased lung volumes in organizing/fibrotic phases
- Associated with serious clinical outcomes and requires immediate recognition 1
- This pattern has been specifically reported with durvalumab therapy 1
Simple Pulmonary Eosinophilia Pattern - Rare
- Nonsegmental consolidation or GGO, unilateral or bilateral
- Transient and migratory
- Spontaneous resolution within 4 weeks is common 1
Key Distinguishing Features for Durvalumab
When durvalumab follows chemoradiotherapy (as in stage III NSCLC), distinguishing radiation pneumonitis from ICI pneumonitis is particularly challenging 2, 3:
- ICI-related pneumonitis (durvalumab alone): Involves more lobes (average 4.06), more frequently bilateral (83%), and shows less bronchiectasis (6%) 2
- Combined RT + ICI: Higher incidence of pneumonitis (47.2% vs 22.1% RT-only vs 15.5% ICI-only), with synergistic lung damage in high-dose radiation regions 2
- Radiation-only pneumonitis: More bronchiectasis (47%), more localized to radiation field 2
Critical Clinical Correlations
High-Grade Pneumonitis Warning Signs
- Bilateral involvement (100% in high-grade cases) 2
- Earlier onset (median 52.5 days vs 133 days for low-grade) 2
- Pre-existing interstitial lung disease (14% vs 2%) 2
- Lower baseline diffusion capacity (52.2% vs 71.0%) 2
Rare but Severe Patterns
- Diffuse alveolar hemorrhage: Presents with rapidly spreading bilateral GGO, hemosiderin-laden macrophages in BAL, CD3+/CD8+ lymphocytic infiltration 4
- Progressive fibrotic pneumonitis: Steroid-refractory evolution to fibrosis despite initial response, may require antifibrotic therapy (nintedanib) 5
Important Diagnostic Caveats
CT patterns are nonspecific - the same drug can cause multiple patterns, and patterns cannot distinguish between different drugs 1. Diagnosis requires:
- Temporal relationship between durvalumab administration and symptom onset
- Exclusion of infections (particularly viral - HMPV has been reported with durvalumab) 3
- Exclusion of metastatic disease and lymphangitic carcinomatosis 1, 6
Mixed presentations occur frequently - when multiple patterns coexist, report the dominant pattern 1. CT and histologic patterns coincide in only 50% of cases 1.
Imaging Protocol Recommendations
- Thin-section CT (≤2.0-2.5 mm thickness) with IV contrast 1
- Coronal reformatted images in addition to transverse views 1
- Systematically describe: distribution, ground-glass opacities, consolidation, reticular opacities, centrilobular nodules, septal thickening, honeycombing, and traction bronchiectasis 1