Congenital undifferentiated sarcoma associated to BCOR-CCNB3 gene fusion.

Small round cell sarcomas are aggressive bone and soft tissue tumors that predominantly affect children and young adults. A new group of sarcomas with a recurrent BCOR-CCNB3 gene fusion has been recently identified in previously unclassifiable small round cell sarcomas. BCOR-CCNB3 sarcomas share clinical and pathologic similarities with Ewing sarcoma, but show a stronger male predilection and less aggressiveness, as well as distinct gene expression profiling and pangenomic SNP array analyses. We report the unusual case of a congenital BCOR-CCNB3 retroperitoneal sarcoma in a female born at 34th gestational week, which was diagnosed in necropsy after 21hours of life. Immunohistochemical analysis showed diffuse expression of CD99 and CCNB3. SNPa showed two focal segmentary deletions at 5q34 and 22q11.23, the latter harboring among others the SMARCB1/INI1 tumor suppressor gene. Immunohistochemistry confirmed loss of INI1 in tumor cells, which has not been previously reported in this type of undifferentiated sarcomas.