Cytology of typical and atypical Ewing's sarcoma/PNET

The cytologic appearance of both typical and atypical Ewing's sarcoma (ES)/PNET has been described, but predominantly in small series. The authors reviewed the cytology of 22 cases (5 fine-needle aspirates, 17 intraoperative smears) of confirmed ES/PNET. Fifteen cases had typical cytologic features, including small round cells, scant cytoplasm, round 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 immunophenotype of ES/PNET, including reactivity for CD99. Five cases had features intermediate between typical and atypical ES/PNET, including abundant cytoplasm, intranuclear grooves, and pale vesicular nuclei. All cases had the characteristic immunophenotype of ES/PNET, including reactivity for CD99 or demonstrated a t(11;22)(q24;q12) cytogenetically. The authors conclude that although atypical ES/PNET is rare, a significant proportion of cases may exhibit unusual cytologic features. Ancillary studies are recommended to establish the correct diagnosis.