History: A 30 year-old otherwise healthy female presented with a slow growing painless mass on her left shoulder that has recently become painful (Figs. 1 and 2).

Fig. 1
figure 1

a Ultrasound of the left shoulder mass. b-d MRI images of the left shoulder: (b) T1 weighted sagittal image, (c) T2 weighted fat suppressed axial image, (d) Post-contrast T2 weighted fat suppressed sagittal image

Fig. 2
figure 2

a-c: Hematoxylin and eosin stain. a 2x Magnification, (b) 4x Magnification, (c) 10x Magnification