Primary breast Ewing sarcoma in a young adult: diagnostic challenges and neoadjuvant chemotherapy success

Extra-skeletal Ewing Sarcoma (EES) is a malignant soft-tissue tumor morphologically indistinguishable from osseous Ewing Sarcoma (ES). The Ewing family includes ES, EES, Askin tumor, and peripheral primitive neuroectodermal tumor. About 85% harbor the t(11;22) (q24;q12) fusion by fluorescence in situ hybridization producing a chimeric protein central to pathogenesis. EES is rare, highly aggressive, and prone to recurrence, typically affecting adolescents and young adults and arising in the trunk or lower limbs; primary breast origin is exceptionally uncommon and carries a poor prognosis relative to other breast malignancies and versus other extra-skeletal ES sites. We report the case of a 37-year-old woman with a rapidly enlarging breast mass. Imaging suggested a cyst-like lesion; a core biopsy was non-diagnostic. Wide excision established ES. She received neoadjuvant chemotherapy with complete response, followed by mastectomy. This case and literature review underscore the rarity of primary breast ES and the value of neoadjuvant chemotherapy in management.