The histologic and immunohistochemical differentiation of Ewing' s sarcoma/primitive neuroectodermal tumor (ES/PNET) from other small, blue, round cell tumors may be difficult. Despite initial promise, CD99 (MIC2) has not proven to be a specific marker. Approximately 90% of ES/PNET have a specific t(11; 22)(q24;q12) that results in fusion of the EWS and FLI-1 genes, and overexpression of FLI-1 protein. A recent study has shown immunohistochemical FLI-1 expression in five of seven of the ES/PNET cases tested. We evaluated FLI-1 expression in 132 well-characterized small, blue, round cell tumors. All tumors were immunostained for FLI-1 (1:40, Sc 356 polyclonal, Santa Cruz Biotechnology) using steam heat for epitope retrieval. Only nuclear staining was accepted as positive. Endothelial cells were strongly positive in all cases and served as an internal control. In many cases, a subset of lymphocytes also stained positive. No staining was seen in any other normal tissue. FLI-1 expression was seen in 29 of 41 (71%) ES/PNET, 7 of 8 (88%) lymphoblastic lymphomas, 0 of 8 poorly differentiated synovial sarcomas (PDSS), 0 of 32 rhabdomyosarcoma (RMS), 0 of 30 neuroblastomas, 0 of 8 esthesioneuroblastomas, 0 of 3 Wilms' tumors, 0 of 1 mesenchymal chondrosarcoma, and in 1 of 1 desmoplastic round cell tumor. This last case was known to have an EWS/WT-1 fusion. Although the EWS/FLI-1 fusion gene is specific for ES/PNET, FLI-1 protein expression is not. Significantly, the great majority of lymphoblastic lymphomas (also CD99-positive) are strongly FLI-1-positive. Immunohistochemical detection of FLI-1 may be valuable in confirming the diagnosis of ES/ PNET in cases in which molecular genetic evaluation is not feasible. FLI-1 protein expression is also helpful in distinguishing ES/PNET from other tumors that may be CD99-positive, such as PDSS and RMS. It is not surprising that some ES/ PNET are FLI-1-negative, because not all ES/PNET have the classic EWS/FLI-1, and some cases of ES/PNET may produce either low levels of protein or idiotypically different protein.

FLI-1 nuclear transcription factor has been proposed as a useful tool in the differential diagnosis of small round cell sarcomas. Recently, FLI-1 has been reported as the first nuclear marker of endothelial differentiation. However, its clinical use has been hampered by major interpretation problems, due to the presence of background staining as well as staining variation between different lots of the same antiserum. In this study, a novel monoclonal antibody raised against the carboxyl terminal of the FLI-1 protein (clone GI146-222, BD Pharmingen) was tested in a series of small round cell and vascular neoplasms. Furthermore, in order to assess FLI-1 specificity, we analyzed its expression in a series of common epithelial and nonepithelial malignancies. In total, 15 Ewing's sarcomas, 10 rhabdomyosarcomas, 5 desmoplastic small round cell tumors, 10 synovial sarcomas, 10 high-grade pleomorphic sarcomas, 10 malignant melanomas, 5 Merkel's carcinomas, 10 colonic adenocarcinomas, 10 breast carcinomas, 10 lung adenocarcinomas, 20 angiosarcomas, 5 epithelioid hemangioendotheliomas, 10 Kaposi's sarcomas and 10 benign hemangiomas, were stained. A strong FLI-1 immunoreactivity was detected in all Ewing's sarcomas and vascular neoplasms, highlighting the high sensitivity of FLI-1 monoclonal antibody. However, 2/5 Merkel's carcinomas and 1/10 malignant melanomas showed a strong nuclear immunostaining, suggesting that FLI-1 may not be so helpful in the differential diagnosis of cutaneous Ewing's sarcoma. In addition, a weak immunoreactivity was found in 3/5 Merkel cell carcinomas, 3/10 synovial sarcomas, 5/10 malignant melanomas, 6/10 lung adenocarcinomas and in 1/10 breast carcinomas. In contrast, all the rhabdomyosarcomas, desmoplastic small round cell tumors, high-grade pleomorphic sarcomas and colonic adenocarcinomas tested were negative. Importantly, in contrast with previous studies, no background staining was observed. Our results indicate that FLI-1 monoclonal antibody can be reliably applied to the differential diagnosis of small round cell neoplasms of soft tissue, and confirm its important role as nuclear marker of endothelial differentiation, mainly helpful in those cases in which technical artifacts are seen by using the traditional membranous and cytoplasmic endothelial markers.