Unique cellular features of peripheral primitive neuroectodermal tumor: ultrastructural evidence of its unique cytodifferentiation.

We present a case of primitive neuroectodermal tumor (PNET) showing a unique differentiation phenotype based on ultrastructural observation. Rapidly growing tumor involving the retroperitoneum of a 68-year-old woman was characterized by histological findings including proliferation of diffuse poorly differentiated small round cells with scattered rosette formation similar to Homer-Wright type and by the ultrastructure demonstrating cytoplasmic neurosecretary granules and short cytoplasmic cilial structures. These cells revealed immunoreactivity only with neuron-specific enolase (NSE). We also observed a chromosomal translocation, t (11; 22) (q24; q12), which is an identical recurrent alteration found in the neoplastic cells in the spectrum of PNETs. These findings may support the explanation that tumor cells observed in our case shared phenotypes of both neuronal and ependymal cell lineages and give a unique insight suggesting the possible histogenesis of PNETs.