Letter to the Editor: “Chest imaging using signs, symbols, and naturalistic images: a practical guide for radiologists and non-radiologists”

Authors: Masahiro Kitami (, Junya Tominaga, Yoshinao Sato, Hajime Tamura
Affiliation: Department of Diagnostic Radiology, Tohoku University Graduate School of Medicine, 2-1 Seiryo-machi, Sendai, 980-8574 Japan

Dear Editor,

In the recent issue of Insights into Imaging, Chiarenza and colleagues outlined chest imaging signs, which provide an excellent practical guide for radiologists and non-radiologists [1]. As discussed in the article, the galaxy sign [1, 2], and the cluster sign [3] are suggestive of sarcoidosis. We would here like to discuss how these terms are misnomers, and therefore represent confusing nomenclature.

The galaxy sign is a large nodule surrounded by many tiny satellite nodules resembling a cluster of stars (Figure 1) [2]. On the other hand, the cluster sign is a collection of multiple small punctiform nodules without a large nodule (Figure 2) [3]. These star cluster signs were initially termed the sarcoid galaxy [2] and the sarcoid cluster [3]. Eventually, since these signs were not specific to sarcoidosis [4, 5], the term “sarcoid” was removed from these names [5-15]. However, the terms galaxy and cluster are still inappropriate in astronomical terms.

Figure 1. Globular cluster sign (or galaxy sign) in a 35 year-old man with sarcoidosis. An axial CT image shows a large nodule with small peripheral nodules in the right upper lobe, which resembles a globular star cluster.

Figure 2. Open cluster sign (or cluster sign) in a 33 year-old woman with sarcoidosis. An axial CT image shows clusters of multiple small punctiform nodules without a large nodule in the right upper lobe, which resembles an open star cluster.

The galaxy is disk-shaped and is surrounded by a spherical halo (Figure 3). New stars born in this galactic disk form loose (open) clusters (Figure 3) due to their weak mutual gravitational attraction. On the other hand, old stars exist in the halo, forming denser (globular) clusters (Figure 3). The characteristic appearance of a sarcoid nodule resembles the globular cluster and was, therefore, named the galaxy sign [2]. However, this is a misnomer due to the confusion between the globular cluster and the galaxy. The sarcoid nodule corresponds to the globular cluster and not the galaxy. Nakatsu et al. presented the image of a globular cluster M92 (NGC 6341) while explaining the galaxy sign [2]. Therefore, the globular cluster sign is the appropriate term for a sarcoid nodule instead of the galaxy sign. The cluster sign is also a confusing term because the star clusters include both globular and open clusters (Figure 3). The cluster sign was originally named so due to its resemblance to the open cluster (NGC 2194) [3]. Therefore, the open cluster sign is the appropriate term for the cluster sign.

Figure 3. Schematic drawing of the galaxy and star clusters (globular clusters and open clusters). The appearance of star clusters is comparable to the appearance of the sarcoid nodules in the globular clusters and open clusters.

Thus, the terms globular cluster and open cluster signs should be used instead of the galaxy and cluster signs, respectively, to avoid confusion.



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