N-terminus WT1 immunohistochemistry is negative in desmoplastic small round cell tumour

Desmoplastic small round cell tumour (DSRCT) is a rare, aggressive sarcoma which occurs in young patients. It is characterised by a t(11;22)(p13;q12) chromosomal translocation which produces a EWSR1-WT1 fusion protein containing the EWSR1 N-terminus domain and the WT1 C-terminus domain. We present a case masquerading as an advanced ovarian tumour in a 15-year-old female patient with ascites and a pelvic mass, who underwent omentectomy and salpingo-oophorectomy. Extensive intra-abdominal tumour deposits were present. Histopathological examination showed nests of small round blue cells in desmoplastic stroma. Immunohistochemistry for N-terminus WT1 expression was negative, and EWSR1-WT1 gene fusion was demonstrated by FISH. As a result of the characteristic EWSR-WT1 fusion mutation in DSRCT, IHC for WT1 only demonstrates positivity with use of an antibody targeting the WT1 C-terminus. Understanding of the characteristic immunohistochemical and molecular profile of DSRCT is key to the accurate diagnosis of this entity.