The atoll sign

Clinical History

An elderly man with non-productive cough and slight fever for three months was sent to our institute to undergo a thoracic CT. No drug consumption was reported nor were there any blood count changes.

Imaging Findings

The CT study was performed without contrast medium injection and it showed some irregular areas of parenchymal consolidation (Fig. 1, 2, 3); subpleural and peribronchovascular ground glass opacities, were also associated (Fig. 1, 2, 4). Besides, there were also some millimetric ill-defined centrilobular nodules (Fig. 4) and the atoll sign, described as a circular area of parenchymal opacity surrounding normal parenchyma (Fig. 1, 3). There was no evidence of nodular walls or nodules inside the atoll sign. Based on these imaging findings a diagnosis of cryptogenic organizing pneumonia was made.

Discussion

In 1996, Voloudaki et al. [1] described two cases of OP with areas of central ground-glass-like opacity surrounded by a ring of increased density. The central area corresponds to a histological finding of inflammation of the alveolar septa with cellular debris, and the peripheral ring of consolidation corresponds to OP in the alveolar ducts. This presentation is exactly the opposite to that of nodules with a halo sign and has therefore been called “reversed halo sign” [2], or atoll sign [3]. A wide spectrum of diseases can manifest with the reversed halo sign on chest HRCT. This pattern is most often an indication of cryptogenic organizing pneumonia, but it may also be associated with other conditions such as inflammatory, neoplastic, or infectious diseases. Although the reversed halo sign is relatively nonspecific, correlation of clinical findings and other HRCT patterns with this finding can sometimes help the physician to narrow the differential diagnosis. Because cryptogenic organizing pneumonia may be idiopathic or may be secondary to infections, collagen vascular diseases, eosinophilic lung diseases, Wegener granulomatosis, and neoplasms, knowing whether the reversed halo sign is a manifestation of primary disease or secondary organizing pneumonia is difficult. Morphologic aspects of the halo, particularly the presence of small nodules in the wall or inside the lesion, usually indicate an active granulomatous disease (infection or sarcoidosis) rather than a primary cryptogenic organizing pneumonia [4]. Although COP is considered the most frequent cause of the reversed halo sign, the presence of nodular walls or nodules inside the reversed halo strongly favours a diagnosis of active pulmonary tuberculosis rather than COP [5]. The presence of reticulation inside the "reversed halo sign", outer rim thickness > 1 cm, and associated pleural effusion strongly suggest the diagnosis of invasive fungal infection rather than OP [6]. The cryptogenic organizing pneumonia in the majority of cases is an idiopathic disease, while less frequently it can be associated with infectious pneumonias, airway obstruction, aspiration, drug reaction, or collagen vascular disease associated with an intake of toxic substances. The prognosis is favourable because there is a good response to steroid therapy [3].

Differential Diagnosis List

Final Diagnosis

Cryptogenic organizing pneumonia

URL:	https://www.eurorad.org/case/10591
DOI:	10.1594/EURORAD/CASE.10591
ISSN:	1563-4086