OBJECTIVE: To study the clinicopathologic features and immunophenotype of desmoplastic small round cell tumor (DSRCT), and to assess the feasibility of reverse transcriptase-polymerase chain reaction (RT-PCR) as a diagnostic adjunct for DSRCT in routine practice. METHODS: The clinical (number = 15), cytologic (number = 1) and histopathologic (number = 14) features of 15 cases of DSRCT were investigated. The immunophenotype was studied by LSAB method using a panel of antibodies. RT-PCR was performed in one case using formalin-fixed, paraffin-embedded tissue for EWS-WT1 fusion gene mRNA. RESULTS: Among the 15 patients studied, 13 were males and 2 were females. Their age ranged from 12 to 38 years (mean age = 23.8 years). Most presented with vague abdominal discomfort, distension or pain, accompanied by nausea, constipation and weight loss. Physical examination showed a palpable abdominal mass with ill-defined borders and tenderness. Ultrasound and computerized tomographic examination revealed single or multiple nodular tumor mass(es) in the peritoneal cavity, measuring 3 cm to 25 cm in greatest diameter (mean tumor diameter = 8.6 cm). Cytologic examination in 1 case showed clusters of small round cells in a hemorrhagic background. The tumor nuclei were hyperchromatic and contained inconspicuous nucleoli. Mitotic figures were readily identified. The cytoplasm however was scant. Histologically, the tumor was composed of small, round, ovoid to spindled cells arranged in nests of various shapes and sizes, embedded in a desmoplastic and focally hyalinized stroma. Immuno- histochemically, all cases showed diffuse and strong staining for AE1/AE3, vimentin, desmin and neuron-specific enolase. Some of them also expressed CAM5.2, epithelial membrane antigen, CD57, chromogranin A, synaptophysin and WT1. They were all negative for myogenin, CK5/6, CD117, calretinin and CD99. RT-PCR successfully amplified the EWS-WT1 chimeric mRNA in 1 case using paraffin-embedded tissue. Subsequent DNA sequencing showed that the gene fusion involved exon 7 of EWS and exon 8 of WT1 genes. The fusion gene contained KTS sequence. CONCLUSIONS: DSRCT is a highly malignant small round cell tumor occurring predominantly in the abdominal or pelvic cavity of young to middle-aged males. It is characterized by multiphenotypic differentiation. The peculiar perinuclear dot-like staining pattern for vimentin and desmin is characteristic for DSRCT. EWS-WT1 fusion transcript can be detected in formalin-fixed, paraffin-embedded tissue by RT-PCR, which may thus serve as a useful diagnostic adjunct for DSRCT.

An intra-abdominal small round cell neoplasm with features of primitive neuroectodermal and desmoplastic round cell tumor and a EWS/FLI-1 fusion transcript.Hum Pathol. 1997 Apr;28(4):502-9.

We report an intra-abdominal round cell tumor in a young man which exhibited the light and electron microscopic appearance of a peripheral primitive neuroectodermal tumor (PNET), in addition to the clinical and topographic characteristics, desmoplasia and a complex immunophenotypic profile of the intra-abdominal desmoplastic round cell tumor (DSRCT). Reverse transcription polymerase chain reaction revealed a EWS/FLI-1 fusion transcript as in PNET/Ewing's sarcoma, instead of the EWS/WT1 transcript of DSRCT. The tumor was also strongly positive for the mic2 protein. This is a unique case of a hybrid tumor arising in the peritoneal cavity of a young male. The existence of such a hybrid tumor in this location suggests that DSRCT and PNET may be related and possibly share a common histogenesis.