RESUMO
A desmoplastic small round cell tumor (DSRCT) is a rare, aggressive neoplasm that occurs predominantly in children and young men. It presents as a large mass inside the abdomen, particularly within the pelvis, and may be accompanied by extensive tumor implants throughout the peritoneum. Microscopically, it typically appears as nests of small undifferentiated cells within a desmoplastic stroma. A DSRCT shows a special immunohistochemical staining pattern, expressing epithelial, neural, and muscle markers. A DSRCT is associated with a specific chromosomal translocation, t (11;22) (p13;q12), resulting in a chimeric EWS/WT1 transcript that is helpful for diagnosing this tumor. We experienced a case of DSRCT in a 19-year-old man who had been diagnosed with Down's syndrome.