Diagnostic results at 3 years of age. (a) Thoracic magnetic resonance imaging at the time of initial surgery. (b) Macroscopic findings of the resected specimen at the time of initial surgery. (c) Sheets of small round cells with rosette formation (hematoxylin-eosin, ×400). (d) Immunohistochemical staining for CD99 antibody (×200). The tumor cell membrane stained positively for CD99.