Question:

A 16-year-old girl presented with a 2 month history of a painless lump in the buttock.

Fig. 1
figure 1

a Sagittal T1W SE, (b) sagittal STIR and (c) axial PDSW FSE MR images of the sacrum

Fig. 2
figure 2

a Microphotograph (H&E, 2x magnification). b Microphotograph (H&E, 20x magnification) (c) Immunohistochemistry for GFAP. d Immunocytochemistry for S100