CYTOLOGY OF TYPICAL AND ATYPICAL EWINGS-SARCOMA PNET

The cytologic appearance of both typical and atypical Ewing's sarcoma (GS)/PNET has been described, but predominantly in small series. The a uthors reviewed the cytology of 22 casts (5 fine-needle aspirates, 17 intraoperative smears) of confirmed ES/PNET. Fifteen cases had typical cytologic features, including small round cells, scant cytoplasm, rou nd nuclei, fine chromatin, and inconspicuous, but distinct basophilic nucleoli. Two cases were atypical (large cell) variants, with abundant eosinophilic cytoplasm, large irregular nuclei, vesicular chromatin, and prominent eosinophilic nucleoli, Both cases had the immunophenotyp e of ES/PNET; including reactivity for CD99. Five cases had features i ntermediate between typical and atypical ES/PNET, including abundant c ytoplasm, intranuclear grooves, and pale vesicular nuclei. AII cases h ad the characteristic immunophenotype of ES/PNET, including reactivity for CD99 or demonstrated a t(11;22)(q24;q12) cytogenetically. The aut hors conclude that although atypical ES/PNET is rare, a significant pr oportion of cases may exhibit unusual cytologic features. Ancillary st udies are recommended to establish the correct diagnosis.