Background. Recent immunocytochemical and cytogenic studies have shown that Ewing's sarcoma and peripheral primitive neuroectodermal tumor (PNET) are probably related neoplasms of common neuroectodermal origin. The purpose of this study was to investigate the morphological characteristics of neural differentiation in these tumors.
Methods. We investigated the ultrastructure of 17 cases of Ewing's sarcoma and related tumors in the specimens of 17 patients, and carried out immunohistochemical analysis.
Results. Ultrastructurally, the cell process and basal laminae were frequently observed in all cases of an unclassified small round-cell type of malignant peripheral nerve sheath tumor (MPNST-related tumor). In contrast, a poorly developed cell process and fragmented basal lamina were rarely observed in Ewing's sarcoma or in PNET. Cytoplasmic neurosecretory granule-like particles were found in two specimens of Ewing's sarcoma and two of PNET, and were abundant in one MPNST-related tumor specimen. These ultrastructural neural features were not common in Ewing's sarcoma, and were occasionally present in PNET. There was a discrepancy between positivity for immuno-neural markers and ultrastructural neural features.
Conclusion. The differential diagnosis between Ewing's sarcoma and PNET was based on the degree of their morphological neural differentiation. Thus, we consider that the diagnosis of PNET should be made on the basis of ultrastructural neural features, taking into account immunohistochemical and histological considerations.
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