Histologic classification of Ewing’s sarcoma includes three major subtypes: classic or conventional type, primitive neuroectodermal tumor, and a typical Ewing’s sarcoma. These tumors share the same immunohistochemical and molecular features, differing only in the extent of neural differentiation.
Classic Ewing’s sarcoma is composed of small round uniform cells. The nuclei are round, and the nucleoli are inconspicuous. The cytoplasmic boundaries are indistinct, such that, the cytoplasm of several cells seems to form a syncytium with nuclei are embedded in it. Mitotic activity is usually not prominent.
PNET: the diagnosis requires the presence of Homer Wright rosettes with central core of neuropil, the background contains monotonous fields of conventional Ewing’s Sarcoma.
A typical Ewing’s sarcoma difficult to recognize because these tumors have a greater degree of cytologic variability or growths patterns bringing a broad variety of primary or metastatic small round cell tumors in differential diagnosis.