Epithelial marker-negative desmoplastic small round cell tumor with atypical morphology: definitive classification by fluorescence in situ hybridization

Desmoplastic small round cell tumor typically presents with abundant desmoplastic stroma containing nested primitive round cells bearing a polyphenotypic immunohistochemical profile. Lesions with minimal classic morphology pose a formidable diagnostic challenge. The current case represents one such example, arising as a large abdominal-pelvic mass in a 17-year-old female patient. The tumor was composed of a monomorphous population of small round cells lining microcystic structures and forming pseudoacini and fine anastomosing trabeculae and cords. The stroma was abundantly myxoid with only occasional thick desmoplastic septa. The tumor cells were variably immunopositive for vimentin, desmin, smooth muscle actin, synaptophysin, neuron-specific enolase, Bcl-2 and WT1 (nuclear); epithelial markers were negative. The definitive diagnosis of desmoplastic small round cell tumor was rendered with the demonstration of the characteristic EWS-WT1 gene fusion by fluorescence in situ hybridization. The current case emphasizes the utility of fluorescence in situ hybridization to demonstrate EWS-WT1 gene fusion in desmoplastic small round cell tumor with nonclassic morphologic and immunohistochemical features to avoid potential misdiagnosis.