Molecular diagnosis of Ewing sarcoma/primitive neuroectodermal tumor in routinely processed tissue: A comparison of two FISH strategies and RT-PCR in malignant round cell tumors

Ewing sarcoma/primitive neuroectodermal tumor (EWS/PNET) is a diagnostically challenging malignant round cell tumor with signature translocations involving the EWS gene. These translocations are detectable with both reverse transcriptase-polymerase chain reaction (RT-PCR) and fluorescence in situ hybridization (FISH) in formalin-fixed paraffin-embedded tissue. However, RT-PCR is less sensitive in formalin-fixed paraffin-embedded than frozen tissue. Similarly, commercial FISH probes have recently become available, but have yet to be rigorously tested in the clinical setting. Therefore, we have compared RT-PCR with FISH using 'home brew' fusion probes for Ewing sarcoma (EWS)-FLI1 and a commercial EWS break apart probe set in 67 archival round cell tumors, including 27 EWS/PNETs. Sensitivities and specificities for both FISH assays were 91 and 100%, respectively, whereas RT-PCR had a sensitivity of 54% and a specificity of 85%. The break apart strategy was easier to interpret than probe fusion approach. We conclude that FISH is a more sensitive and reliable ancillary technique than RT-PCR for the diagnosis of EWS/PNET in formalin-fixed paraffin-embedded tissue, although the latter provides additional information regarding fusion transcript subtype and prognosis. The commercial break apart probe set is both readily available and easy to interpret, making it particularly attractive. Nonetheless, complex round cell tumors often benefit from molecular testing with multiple methods.