EWSR1::BEND2 fusion sarcoma of the urinary bladder - a case report and review of literature.

In this case report we describe a Ewing-like high grade small round cell sarcoma of the urinary bladder in which an extremely rare EWSR1::BEND2 fusion was found. A 28-year-old male patient presented with hematuria and in the following examinations a large necrotic bladder tumor with spreading to adjacent prostatic tissue and multiple lung metastases were found. Histology showed a poorly differentiated small round cell tumor with perivascular rosettes and moderate membranous positivity for CD99. The methylation profile of the tumor did not match with any of the tumor entities grouped by the sarcoma classifier. With tumor agnostic methods, mainly next generation sequencing, novel fusions are being found at an accelerating rate. Our case adds to the expanding group of EWSR1 fusion neoplasms, and describes the effects of a Ewing sarcoma treatment protocol on this type of sarcoma. The relevance of traditional methods for detecting Ewing sarcoma with fluorescence in situ hybridization is decreasing as EWSR1 rearrangements are detected in tumors that show different clinical behavior and morphology. The classification of these tumors into WHO defined entities to guide treatment is a challenge.