PERIPHERAL PRIMITIVE NEUROECTODERMAL TUMOR AND EXTRA-OSSEOUS EWINGS-SARCOMA - A HISTOLOGICAL, IMMUNOHISTOCHEMICAL AND DNA FLOW CYTOMETRIC STUDY

Although peripheral primitive neuroectodermal tumour (pPNET) and extra -osseous Ewing's sarcoma (EES) are thought to be closely related neopl asms, their clinical behaviour differs considerably. To determine the clinical relevance of the Schmidt classification scheme for differenti ating pPNET and EES, 20 tumour specimens of poorly differentiated roun d cell tumours were evaluated. In addition, the diagnostic value of se veral neural markers and the prognostic value of quantitative morpholo gical variables (DNA ploidy, S-phase fraction, and the mitotic activit y) were assessed. Homer-Wright rosettes were present in 9 tumours. Neu ron specific enolase (NSE) was expressed in 11 tumours, 8 of which exp ressed a second neural marker (CD57, S100, or neurofilament). Accordin g to the Schmidt classification, 11 pPNET and 5 EES were distinguished . HBA-71 was exclusively expressed in pPNET and EES. The remaining tum ours were classified as sarcoma not otherwise specified (n = 2), rhabd omyosarcoma (n = 1), and desmoplastic tumour with divergent differenti ation (n = 1). EES611 patients fared significantly better than the pPN ET patients (100% versus 42% 5-year survival). Neither DNA ploidy nor S-phase fraction assessed in 12 evaluative histograms (9 pPNET and 3 E ES), nor mitotic activity yielded information of additional prognostic value. On the basis of this study and the Schmidt classification sche me, it can be concluded that if the diagnosis of EES and pPNET is base d on light microscopy (Homer-Wright rosettes) and/or immunohistochemis try (at least two neural markers, i.e. NSE, S-100, CD57, and neurofila ment), the classification provides important clinical information. Fur thermore, positivity for HBA-71 is helpful in differentiating pPNET an d EES from all other small round cell tumours.