Fine Needle Aspiration Biopsy of a Small Round Cell Tumor Exhibiting Both Neural and Myogenic Differentiation

BACKGROUND: Divergent differentiation may not be detected in the limited material available in a fine needle aspiration biopsy (FNAB). CASE: A small round cell malignancy showed neural features ultrastructurally on FNAB, in keeping with primitive neuroectodermal tumor, but desmin and actin positivity on surgical biopsy, suggesting rhabdomyosarcoma. CONCLUSION: Accurate classification of small round cell tumors by FNAB is more likely to occur when both electron microscopy and immunocytochemistry are employed since these tumors may express divergent differentiation.