Ewing sarcoma family of tumours: unusual histological variants and immunophenotypic characteristics

Ewing sarcoma family of tumours are typified by small round blue cell morphology with variable but usually minimal neuroectodermal differentiation and non-random translocations involving the EWSR1 gene and a member of the ETS family of genes. Atypical morphological patterns may be seen in up to 20% of cases and include large cell, adamantinoma-like, spindle cell sarcoma-like (synovial sarcoma like), sclerosing, clear cell (hypernephroid)/anaplastic, and vascular like patterns. Immunophenotypically, CD99, vimentin, CAV1 and FLI1 are typically positive. Around half of the cases may express NSE and/or CD57. Low molecular weight cytokeratins (CK), S100, NB84, and CD117 are expressed less commonly (around 20%–40%). EMA, desmin, CD31, and synaptophysin expression are rare. CK7, CK20, D2-40, chromogranin, TDT, TTF, MYF4 appear to be consistently negative. Molecular confirmation is less sensitive. The correct diagnosis requires correlation between morphology and immunostaining, and may require molecular testing.