Solid-Pattern Desmoplastic Small Round Cell Tumor.

Desmoplastic small round cell tumor (DSRCT) is an aggressive small round cell sarcoma that typically occurs intra-abdominally in adolescents and young adults, and is characterized by a recurrent t(11;22)(p13;q12) translocation leading to generation of the EWSR1-WT1 fusion gene, which codes for a chimeric protein with transcriptional regulatory activity. DSRCT has a characteristic histologic appearance of nests of uniform small cells within prominent fibroblastic stroma and immunohistochemically it shows multidirectional differentiation, with expression of epithelial, neural, and muscle markers. We illustrate a case of DSRCT that presented as a large intra-abdominal mass, which harbored EWSR1 rearrangement by fluorescence in situ hybridization and EWSR1-WT1 fusion transcripts by reverse transcription-polymerase chain reaction (RT-PCR), and which histologically had an entirely solid morphology, lacking evidence of desmoplastic stroma. This purely solid variant emphasizes that even when occurring at a typical location, DSRCT may be difficult to recognize when lacking nonclassical morphology. This is of clinical relevance, as DSRCT with this pattern could be misdiagnosed as Ewing sarcoma if RT-PCR is not performed, with resulting prognostic and therapeutic implications.