The atoll sign or reversed halo sign refers to focal ground-glass opacity with a peripheral ring of consolidation which may be complete or incomplete. The sign was originally described in organizing pneumonia but can also occur in infections, infarction, granulomatous disease, inflammation and tumor.
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Pathology
The central area (ground-glass opacity) corresponds to alveolar septal inflammation and cellular debris in alveolar spaces, while the peripheral consolidation corresponds to granulomatous tissue within the distal airspaces 5.
Etiology
The sign is most commonly seen in organizing pneumonia (OP), which is most commonly cryptogenic (COP) but can also be secondary to specific causes. However, the sign is only seen in about one-fifth of patients with cryptogenic organizing pneumonia 1.
The sign has also been described on CT with the following pathologies 3,4,7:
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fungal pneumonia
mucormycosis (most common)
histoplasmosis (uncommon)
cryptococcosis (uncommon)
pneumocystis pneumonia (rare)
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other infections
coronavirus in the context of COVID-19
pulmonary infarction due to pulmonary thromboembolism
granulomatosis with polyangiitis (formerly known as Wegener granulomatosis)
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pulmonary neoplasms
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iatrogenic
radiofrequency/microwave ablation of pulmonary malignancies
Clinical significance
In severely immunocompromised patients, the sign has been demonstrated as highly suggestive of early infection by an angioinvasive fungus. Suggesting the diagnosis might prove life-saving in patients with prolonged neutropenia or graft-vs-host disease 3,4.
When associated with nodular walls, nodules inside the reversed halo or even centrilobular nodules and pattern of endobronchial spread (tree-in-bud sign), active pulmonary tuberculosis should be high on the list of differential diagnoses 2,3.
Practical points
Integrating the ancillary radiological and clinical data (as exemplified above) should enable substantial narrowing of differential diagnoses. Providing a presumptive final diagnosis may obviate the need for biopsy in selected cases, especially when dealing with immunocompromised patients 3.