Ewing sarcoma family of tumours includes Ewing sarcoma, peripheral primitive neuroectodermal tumor, neuroepithelioma, atypical Ewing sarcoma, and Askin tumour.      

 Case Presentation(Complejo Académico Hospitalario Nelson Mandela REPUBLICA SUDAFRICANA)

Peripheral primitive neuroectodermal tumours are characterized by chromosomal translocation t(11;22)(q24;q12) in almost 90% cases. Ewing sarcoma and the peripheral primitive neuroectodermal tumor were both found to contain the same reciprocal translocation between chromosomes 11 and 22, t(11;22). Both lesions have similar patterns of biochemical and oncogene expression.

Age: The tumour is common between 10 and 30 years.

Site:  Usually occurs in deep soft tissue and rarely involves dermis and subcutis. The  common sites include paravertebral and intercostal regions as well as on the lower limbs. Rarely pelvic and hip regions are involved.
Tumour located on the chest wall (may involve ribs, pleura and lung) is referred to as Askin tumour.

Clinical presentation: The tumour presents as a rapidly enlarging, pale, soft mass with extensive necrosis. One third of the cases are painful.

Microscopic features:

The tumour cells are arranged in a trabecular or lobular growth pattern. A prominent vascular network is present in the backround.

The microscopic features depends on the degree of neuroectodermal differentiation.

Malignant primitive neuroectodermal  tumours (peripheral neuroepithelioma)- well differentiated end of the spectrum.
The tumour cells show eosinophilic cytoplasm,coarse chromatin and prominent nucleoli.
Homer Wright rosette containing central core of neurofibrillary material.
Flexner-Wintersteiner rosette containing central lumen. 
PAS positivity is present in about 40% of cases.  A peripheral neuroepithelioma must demonstrate positivity with at least two neural markers.

Extraskeletal Ewing's sarcoma- poorly differentiated end of the spectrum. The tumour cells show scanty, pale, cytoplasm, fine dusty chromatin and inconspicuous nucleoli. The cells contain glycogen . PAS positivity is prominent. Immunohistochemistry shows characteristic positivity with the neural markers. 

Immunohistochemistry:  

The tumour cells show diffuse membrane positivity positivity with CD99 antigen (product of the MIC2 gene) .
The tumour cells are usually positive for neuron-specific enolase, PGP9.5, neurofilament , Leu-7  and synaptophysin.
Note:  About 10% cases of PNET may demonstrate dot like positivity with cytokeratin. PNET and Ewing's sarcoma are immunopositive to vimentin.

Differential diagnosis: (Small blue cell tumours):    

Lymphoblastic lymphomas/leukaemia ;  rhabdomyosarcoma ; mesenchymal chondrosarcoma ; desmoplastic small round cell tumor ; synovial sarcomas,   solitary fibrous tumor ; extrarenal malignant rhabdoid tumour ; neuroendocrine tumors - may demonstrate immunoreactivity to MIC-2. 

The MIC-2 immunostain should always be done in a panel, which usually includes muscle markers (desmin, muscle-specific actin, myoD1, myogenin), neural markers (protein gene product 9.5, S100 protein, neuron specific enolase, CD57, synaptophysin, neurofilament protein), epithelial markers (epithelial membrane antigen, cytokeratin), and lymphoid markers (CD45, CD30, Tdt, T-cell and/or B-cell markers).

 - Desmoplastic small blue cell tumour: click

- Poorly differentiated synovial sarcoma: click

- Merkel cell carcinoma: click . CK20 shows dot positivity and EMA positivity. -Neuroblastoma - CD99 negative.

-Solitary fibrous tumor:click

-Extrarenal malignant rhabdoid tumour:click