[Ewing's sarcoma and peripheral neuroectodermal tumors. Report of a case with laterothoracic localization].

The authors report the case of a 20-year old male patient presenting with a lateral thoracic tumour which at pathological examination looked like an extra-osseous Ewing's sarcoma. Strongly positive NSE immunolabeling suggested a recently described variant of this lesion: Askin's tumour. In 1979, F. B. Askin described a tumour made of round small cells, located on the side of the thorax and possibly derived from differentiated neuroectodermal tissue. Since that time, the ever wider use of electron microscopy and immunohistochemical methods has made it possible to demonstrate the existence of peripheral neuroectodermal tumours (PNETs) distinct from neuroblastomas. Askin's tumour being only, as it turned out, one of their clinical forms. Following a brief presentation of the clinical, pathological and therapeutic features of PNETs, the authors underline the great similarity between these tumours and Ewing's sarcoma. Owing to the successive discoveries of a cytogenetic abnormality common to these tumour--t(11;22)(q24;q12)--, of a similar expression of proto-oncogens and of identical neuroenzymatic characteristics, Ewing's syndrome can now be regarded as the most undifferentiated of all PNETs, probably arising from the postganglionic neuron of the parasympathetic system.