Galaxy sign | Radiology Signs

Answer:

The galaxy sign, or sarcoid galaxy, is a large upper-zone parenchymal mass formed by the coalescence of innumerable small granulomas, ringed by a halo of discrete satellite micronodules. The granulomas are most densely packed at the centre, giving a solid, often ill-defined nodular mass, and become progressively sparser towards the periphery, where individual perilymphatic micronodules remain separate. It was described in pulmonary sarcoidosis, where it is typically bilateral, perihilar and upper-lobe predominant and accompanied by the other perilymphatic features of the disease, but it is not pathognomonic and is well documented in tuberculosis, including multidrug-resistant disease. The closely related cluster sign describes grouped small nodules that remain distinct and have not yet coalesced into a mass. Because the appearances of sarcoidosis and tuberculosis overlap, the distinction rests on clinical context, microbiology and ancillary findings rather than the sign itself.

Why is it called so?

The central conglomerate mass with its surrounding scatter of small satellite nodules resembles a galaxy: a bright dense core with stars trailing off into the surrounding space.

Pathophysiology

In sarcoidosis, non-caseating granulomas form along the perilymphatic interstitium of the bronchovascular bundles, interlobular septa and subpleural regions. Where granulomas are numerous they merge into a confluent nodular mass, while at the advancing margin they remain individually resolvable as micronodules. The same architecture arises in tuberculosis when caseating granulomas coalesce along the same lymphatic routes, which is why the sign cannot separate the two.

Alternative names: Sarcoid galaxy sign

Other associated named signs: Cluster sign (non-coalescent grouped micronodules), perilymphatic nodules, and the reversed halo sign

References

1. Nakatsu M, Hatabu H, Morikawa K, Uematsu H, Ohno Y, Nishimura K, et al. Large coalescent parenchymal nodules in pulmonary sarcoidosis: “sarcoid galaxy” sign. AJR Am J Roentgenol. 2002;178(6):1389-93. PMID: 12034602.