Patchy peripheral and peribronchovascular consolidation that migrates, with a perilobular pattern or a reversed halo (atoll) sign — a pattern with many causes, so name the likely cause and the mimic that must be excluded.
Orient first
- Organising pneumonia is a pattern of lung injury (buds of granulation tissue in the airspaces): cryptogenic or secondary to infection, drugs, connective tissue disease, radiation or aspiration.
- It responds to steroids — which is why calling it correctly, and not missing a mimic, matters.
- The reversed halo sign (central ground glass ringed by consolidation) is suggestive but not specific — invasive fungal infection (mucormycosis) can do it.
Acquire the study
- Thin-section CT chest (non-contrast adequate); comparison with any previous CT for migration.
The manoeuvre
- Lung window, axial and coronal: subpleural and/or peribronchovascular consolidation, lower-zone predominant, often bilateral.
- Perilobular pattern: arcade-like opacities outlining secondary lobules.
- Reversed halo (atoll) sign — describe its ring and its central ground glass.
- Migration: compare with prior imaging — new areas while others clear.
- Look for the cause (drug history, CTD features such as NSIP fibrosis, radiation field) and the mimic (persistent consolidation → lepidic adenocarcinoma or lymphoma).
What confirms it
- A typical peripheral/peribronchovascular migratory pattern with a compatible cause, confirmed by response or biopsy.
What licenses you to exclude it
- Consolidation that persists unchanged for months without migration needs tissue — organising pneumonia is not the default for a persistent opacity.
The classic misread
- Calling a reversed halo in a neutropenic patient organising pneumonia — think angioinvasive fungal infection first.