Comparative analysis of electron microscopy and immunocytochemistry in the cytologic diagnosis of malignant small round cell tumors.

Objective: To compare the contributions of electron microscopy (EM) and immunocytochemistry (ICC) as adjuncts in the cytodiagnosis of malignant small round cell tumors (MSRCT).

Study Design: This prospective study included 57 cases with a preliminary aspiration diagnosis of MSRCT. The contributions of EM and ICC in arriving at a specific diagnosis were evaluated.

Results: The 57 cases included 22 cases of Ewing's sarcoma/peripheral neuroectodermal tumor (PNET), 12 neuroblastomas, 8 Wilms' tumors, 6 rhabdomyosarcomas, 5 lymphomas, 2 retinoblastomas and 1 synovial sarcoma. One case remained unclassified. Electron microscopy was crucial to the diagnosis in 38.4% cases as against 39.2% of cases by ICC. The light microscopic diagnosis was confirmed in 42.3% and 53.5% cases by EM and ICC, respectively. EM and ICC were inconclusive for a specific diagnosis in 19.2% and 7.1% of cases, respectively. Technically unsatisfactory preparations in EM and ICC accounted for 5 and 1 cases, respectively. The overall efficiency in making a diagnosis was 80.7% for EM versus 92.8% for ICC. Aberrant expression of antigens led to difficulties in interpretation of ICC, and EM was particularly helpful. The ultrastructural demonstration of neural differentiation in Ewing's sarcoma/PNET tumors helped place tumors in the PNET category.

Conclusion: While ICC is the ancillary method of choice in the cytologic diagnosis of MSRCT, EM contributes to the diagnosis and improves diagnostic accuracy.