Fine needle aspiration biopsy of a small round cell tumor exhibiting both neural and myogenic differentiation : A case report

Divergent differentiation may not be detected in the limited material available in a fine needle aspiration biopsy (FNAB). 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. 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.