Oligometastatic renal Ewing sarcoma with FUS::ERG fusion: a case report and literature review

Ewing sarcoma (ES) is an undifferentiated round cell sarcoma arising in skeletal and extraskeletal locations usually diagnosed in the second decade of life. It is typically characterized by a monomorphic round blue cell morphology, positive membranous CD99 immunohistochemical staining, and EWSR1 gene fusion with a partner gene from the ETS family. Less frequently, other genes from the FET family of genes substitute for EWSR1. We report a case of oligometastatic Ewing sarcoma with FUS::ERG fusion with an unusual primary location in an older patient. This case highlights the application of ancillary tests, especially next generation sequencing, as an adjunct to the morphological assessment of undifferentiated round cell tumours. We also discuss potential diagnostic pitfalls and literature review of Ewing sarcomas with FUS::ERG fusion.