Figure 1.

Fluorescence in situ hybridisation for EWSR1 gene rearrangement, and histology of variant Ewing sarcoma types. (A) Fluorescence in situ hybridisation using dual-colour break-apart probes which flank the EWSR1 breakpoint region on chromosome 22q12. The nucleus of this neoplasm contains separated (split) red and green signals indicating a rearrangement involving the EWSR1 gene at 22q12. A fused normal signal is also present, denoting the site of the EWSR1 gene. (B and C) Ewing sarcoma. These are examples of Ewing sarcomas with morphologic features that can cause diagnostic difficulty, particularly as they can show immunophenotypical overlap with other round cell neoplasms. (B) Ewing sarcoma with a well-defined nested architecture and areas of cellular discohesion mimicking alveolar rhabdomyosarcoma; (C) Ewing sarcoma with irregular cellular nests in prominent desmoplastic stroma, mimicking desmoplastic small round cell tumour, and (D) large cell Ewing sarcoma. RT–PCR in each case was diagnostically contributory, as this showed the presence of EWSR1-FLI1 fusion transcripts diagnostic of Ewing sarcoma.