WT1 staining reliably differentiates desmoplastic small round cell tumor from Ewing sarcoma/primitive neuroectodermal tumor - An immunohistochemical and molecular diagnostic study

Differentiating desmoplastic small round cell tumor (DSRCT)from another sim ilar small round cell tumor of childhood the Ewing sarcoma/primitive neuroe ctodermal tumor (EWS/PNET), can be difficult because morphologic and immuno histochemical features overlap. We studied the predictive value of immunohi stochemistry with an antibody to the C-terminal region of the Wilms turner (WT1) protein for differentiating DSRCT from EWS/PNET in 24 malignant small round cell tumors that had been previously diagnosed as DSRCT or EWS/PNET by standard methods. We performed reverse transcriptase-polymerase chain re action (RT-PCR) analysis in cases with available tissue as a confirmatory m easure: 6 of 13 DSRCTs were informative by RT-PCR, and 6 of 6 showed an EWS -WT1 fusion; all 13 DSRCTs showed strong, definitive nuclear staining with the WT1 antibody. All II EWS/PNETs were WT1 antibody negative; 7 of 11 case s classified as EWS/PNET were informative by RT-PCR, and 7 of 7 showed an E WS-FLI-1 fusion. For cases in which the morphologic and immunohistochemical features are consistent with a diagnosis of DSRCT, WT1 antibody staining p redicts the EWS-WT1 translocation with high sensitivity and specificity and is, therefore, useful for differentiating DSRCT from EWS/PNET when genetic information is unavailable.