Metastatic small cell tumor of bone with 'true' rosettes and glial fibrillary acidic protein positivity.

A 15-year-old black girl was admitted to St. Jude Children's Research Hospital with a lytic lesion involving the right fourth metacarpal bone with a solitary metastasis to the ipsilateral axillary lymph node. Histologic examination of both the primary lesion and metastasis revealed a primitive small round cell lesion. Abundant rosettes with central lumina were present in the metastasis. Immunohistochemical stains performed on sections of the metastasis were strongly positive for glial fibrillary acidic protein (GFAP) and negative for neuron-specific enolase (NSE), S-100 protein, and Leu-7. Electron microscopic examination of the metastasis revealed lumen formation with apical microvilli and juxtaluminal junctional complexes. This tumor exhibits features that are suggestive, but not definitive, of primitive ependymal differentiation and further expands the morphologic and immunophenotypic range of small round cell tumors of bone.