Which pathologic findings are characteristic of primitive neuroectodermal tumors (PNETs)?

Updated: Feb 07, 2019
  • Author: Carlo P Honrado, MD, FACS; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Answer

The information obtained with light microscopy, cytogenetic analysis, and the immunohistochemical profile of tumor cells is essential in diagnosing peripheral primitive neuroectodermal tumors (pPNETs). On light microscopy, peripheral primitive neuroectodermal tumors (pPNETs) appear as a monotonous collection of small, round, darkly stained cells. However, peripheral primitive neuroectodermal tumors (pPNETs) cannot be distinguished from other tumors with small round cells based on histologic studies alone.

Other tumors with a similar appearance on light microscopy include rhabdomyosarcoma, neuroblastoma, and non-Hodgkin lymphoma. In these tumors, Homer-Wright rosettes, in which the tumor cells are arranged about a central space filled with fibrillar extension of the cells, can be found. Examination of peripheral primitive neuroectodermal tumors (pPNETs) with electron microscopy reveals neurosecretory granules with microtubules and microfilaments. In addition, short dendritic processes lie between cells in peripheral primitive neuroectodermal tumors (pPNETs), in contrast to Ewing sarcoma, in which the dendritic processes are absent.

Cytogenetic analyses of peripheral primitive neuroectodermal tumors (pPNETs) reveal the close relationship among tumors in the Ewing family of tumors (EFTs). Approximately 85% of all Ewing family of tumors (EFTs) include a translocation that involves the EWS gene (22q12) and FLI-1 (11q24). [16] In the other 5-10% of cases, the translocation is found between EWS and ERG (21q22). [17]

The elucidation of the immunohistochemical profile allows the pathologist to distinguish peripheral primitive neuroectodermal tumors (pPNETs) and EFTs from other small, round cell tumors. Immunohistochemistry can be used to detect antibodies to FLI-1 in the gene fusion product of EWS. [18] The expression of the MIC2 gene produces an antigen, MIC2, which consistently identifies both Ewing sarcoma and peripheral primitive neuroectodermal tumors (pPNETs). In contrast, CNS PNETs and neuroblastomas uniformly lack the expression of the MIC2. [19] Furthermore, peripheral primitive neuroectodermal tumors (pPNETs) typically coexpress CD99 (the glycoprotein MIC2) and vimentin. [20] Other nonspecific markers include S-100, neuron-specific enolase, CD75, and synaptophysin.


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