Authors: Masahiro Kitami (firstname.lastname@example.org), 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
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 . As discussed in the article, the galaxy sign [1, 2], and the cluster sign  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) . On the other hand, the cluster sign is a collection of multiple small punctiform nodules without a large nodule (Figure 2) . These star cluster signs were initially termed the sarcoid galaxy  and the sarcoid cluster . 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.
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 . 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 . 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) . Therefore, the open cluster sign is the appropriate term for the cluster sign.
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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