Nests of small blue round cells are separated by fibrovascular septae. This tumor arose in the chest wall of a 6 year old girl.

Some rosette formation (albeit imperfect rosettes) can be appreciated. The tumor cells have a speckled chromatin and small nucleoli.

The classification of primitive neuroectodermal tumors can be confusing. Broadly, they can be divided into two main groups: PNETs of the CNS and peripheral PNETs. Some authorities add neuroblastoma as a third group, likening them PNETs of the autonomic system. PNETs of the CNS can be supratentorial (pinealblastomas) or infratentorial (medulloblastomas). Peripheral PNETs arise in the soft tissues, and are basically indistinguishable from Ewing sarcomas, which arise in the bone.

Peripheral PNETs/Ewing sarcoma family of tumors, characterized by neuronal differentiation and EWS gene rearrangements. The most common cytogenetic lesion is t(11;22), creating the EWSR1-FLI1 fusion gene. Additional translocations have been discovered, the most common of which is t(21;22) which creates the EWSR1-ERG fusion gene. Differences in translocation partners and breakpoints have been associated with better prognosis.

Askin tumor (peripheral neuroepithelioma of the thoracopulmonary region) describes a PNET that arise in the chest wall. These tumors often extend into the ribs, pleura and lung.

Ewing sarcoma is considered to be at the undifferentiated end of the spectrum, and ultrastructurally, they have few organelles and abundant glycogen. PNETs are more differentiated with formation of HW rosettes and presences of microtubules and neurosecretory granules. Neuroepithelioma are the most differentiated with plentiful rosettes and well-formed neuritic processes ultrastructually.

In terms of IHC, the Ewing sarcoma family of tumors/PNET are positive for CD99, NSE, neurofilament, Leu7 and synaptophysin.

• Neural : Ewing Sarcoma