Extra-skeletal Ewing's sarcoma with EWSR1::ERG translocation of the breast: a case report.

Background: Ewing's sarcoma (ES), first described by Ewing in 1921, is a highly malignant small blue round cell tumor that arises in bone or extraskeletal soft tissues. It predominantly affects patients aged 10 to 30 years. Approximately 12% of cases involve solid organs, often presenting as rapidly growing masses in deep soft tissues. The most common genetic alteration in ES is the t(11;22)(q24;q12) translocation, resulting in the EWSR1::FLI1 fusion gene, which accounts for 90% of tumors. Extraskeletal Ewing's sarcoma (EES) presented as a breast mass is extremely rare, with only a handful of cases documented in the literature. Notably, a primary breast ES harboring the EWSR1::ERG fusion gene has never been reported previously.

Case Presentation: A 23-year-old woman presented to our hospital with a rapidly growing, palpable left breast mass. Histopathological examination of the surgical specimens (including histological features, immunohistochemical staining, and molecular analysis) confirmed the diagnosis of primary EES with EWSR1::ERG translocation in the breast. Despite receiving multimodal adjuvant therapy (surgery, chemotherapy and radiotherapy), the patient experienced two disease relapses within 15 months.

Conclusions: Our report establishes the first molecularly confirmed case of ES harboring the rare EWSR1::ERG translocation presenting as a primary breast mass. Primary breast ES can demonstrate significant morphologic and immunohistochemical overlap with other small round cell tumors. This case highlights that next-generation sequencing (NGS) is the gold standard for definitive diagnosis, as it reliably detects fusion partners-especially in rare tumors arising in uncommon locations.