This tumor family tends to involve the diaphysis of long bones but also affects the pelvis and ribs. It is made up of undifferentiated mesenchymal cells growing in dense solid sheets with minimal intervening stroma. The nucleus is round and central, and possesses little associated cytoplasm. The chromatin pattern is finely granular. Nucleoli are present and may be multiple but not prominent (Fig. 21). Sometimes a population of light and dark cells is seen with the dark cells representing apoptotic cells. Rosette formation is occasionally noted but if it is more than 20%, one should consider a diagnosis of PNET. Necrosis is a frequent feature. The tumor cells permeate the cortex and involve the haversian system and Volkman canals. Some tumors have slightly larger tumor cells, contain vacuoles or are spindle shaped, and have been termed "atypical" Ewing's sarcoma. Intracellular glycogen is found in most tumors and can be demonstrated by a PAS stain. Immunohistochemical stains will be positive for the M1C2 gene product (CD99) in approximately 90% of tumors; however, this marker is also positive in some pediatric l ymphomas and rhabdomyosarcoma as well as mesenchymal chondrosarcoma. Thus a panel of stains tailored to rule out these other entities should be performed. 1f tissue is adequate, molecular studies can identify the Ewing's sarcoma-associated reciprocal translocation between chromosome 11 and 22. For a tumor to
be classified as a PNET, there must be positivity for neural markers, with greater than 20% rosette formation (27,69).
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