Extraosseous Ewing sarcoma of the distal pancreas: diagnostic pitfalls and the critical role of molecular testing

Primary pancreatic Ewing sarcoma (ES) is an exceedingly rare malignancy, with fewer than 100 confirmed cases reported worldwide. We report the case of a young adult woman presenting with a large solid-cystic pancreatic body-tail mass, initially diagnosed as solid pseudopapillary neoplasm and subsequently misinterpreted as metastatic endometrial stromal sarcoma following resection. Lack of adjuvant therapy and loss to follow-up preceded metastatic recurrence, whereupon targeted immunohistochemistry and molecular analysis established the definitive diagnosis of extraosseous ES with t(11;22)(q24;q12) translocation. Palliative chemotherapy yielded transient disease control before rapid progression and death. This case illustrates the diagnostic challenges posed by this rare entity due to overlapping clinicoradiologic and histologic features with more common pancreatic tumors, emphasizing the critical role of early inclusion of CD99, NKX2.2, and EWSR1 testing to facilitate appropriate multimodal management and improve outcomes.