Background: Primitive neuroectodermal tumors (PNET)
Ewing sarcoma (ES):
- is a type of Primitive NeuroEctodermal Tumor
- ES family of tumors (ESFT), include ES (Ewing sarcoma) and pPNET
- are a group of aggressive malignant tumors of childhood and young adults which may arise in bone of soft tissue
ES is predominantly of skeletal origin while pPNET usually presents in soft tissues.
PNETs are a group of small round cell tumors which share the following features:
- morphological
- immunohistochemical
- ultrastructural
- cytogenetic
They comprise two main groups:
- Central. A central PNET (cPNET) arises within the central nervous system - medulloblastoma is the prototype.
- Peripheral. A peripheral PNET (pPNET) refers to a group of tumors including neuroblastoma, peripheral PNET, and Ewing Sarcoma (ES).
Cell of origin could possibly be:
- Pluripotential neural crest cells.
- Mesenchymal stem cell that differentiates along neural crest lineage.
pPNETs
pPNET by definition shows evidence of neural differentiation either by light microscopy, immunohistochemistry or electron microscopy.
A range of appearances, depending on the extent of neural differentiation, are apparent in ESFT, from ‘undifferentiated’ Ewing sarcoma to neural differentiation pPNET.
Immunohistochemistry:
Typical Immunohistochemical profile of ES and pPNET:
| Immunohistochemistry | Ewing Sarcoma |
pPNET |
Vimentin |
+ |
+ |
CD99 |
+ |
+ |
NSE |
- |
+ |
Pgp 9.5 |
- |
+ |
S100 |
- |
+ |
Chromagranin |
- |
+ |
Leu 7 |
- |
+ |
It is impossible to distinguish between members of the Ewing sarcoma family based on staining alone. Ultrastructural and cytogenetic differences are used to further characterize these tumors.
Typical ultrastructural findings in ES and pPNET:
| Ultrastructure | Ewing sarcoma |
pPNET |
Cytoplasmic glycogen |
+++ |
+ |
Adhesion areas |
Few |
More common |
Organelles |
rare |
More common |
Cell processes |
- |
+ |
Neurosecretory granules |
rare |
+ |
Histology shows sheets of small round blue cells:
