Primary intra-renal desmoplastic small round cell tumor: expanding the histologic spectrum, with special emphasis on the differential diagnostic considerations.

Intra-abdominal desmoplastic small round cell tumor (DSRCT) is a rare, aggressive tumor affecting adolescent and young males. DSRCT presenting as a primary renal mass in the absence of visceral or serosal involvement is extremely rare. Herein, we present the pathologic and molecular findings in the case of a young man who presented with a large renal mass without any visceral or serosal involvement. Noticeably, the tumor lacked prominent desmoplastic stroma and only focally expressed cytokeratin, both of which are considered characteristic histologic features for this tumor. Fluorescence in situ hybridization studies using an EWSR1 break-apart probe confirmed the presence of a rearrangement involving the EWSR1 locus and RT-PCR demonstrated the presence of an EWSR1-WT1 fusion transcript associated with the t(11;22) rearrangement, which supported a diagnosis of DSRCT. We also discuss the differential diagnostic considerations faced by the pathologist in the workup of small round cell neoplasms of the kidney.