OBJECTIVETo 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.

METHODSThe 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.

RESULTSAmong 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.

CONCLUSIONSDSRCT 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.