Paraduodenal/pancreatic Ewing sarcoma is very rare and therefore may be mistaken for neuroendocrine carcinoma.

A diagnosis of Ewing sarcoma (ES) is suggested by certain morphological and immunohistochemical findings and supported by demonstration of EWSR1 translocation,1 as is usually demonstrated generically with a break-apart fluorescence in situ hybridisation (FISH) probe. However, because there are differential diagnoses that also harbour EWSR1 translocations (eg, desmoplastic small round cell tumour, DSRCT), ES can only be confirmed by demonstrating a fusion partner regarded to be ES specific.1 The most common of these partners are FLI1 (85% of ES) and ERG (10% of ES).1 Most ESs arise in bone or soft tissues1 and here, pathologists are primed to consider other neoplasms which mimic ES morphologically and/or immunohistochemically. However, there are visceral sites from where ES can rarely arise and may not, therefore, be suspected. The following report shows one such site to be in/around the pancreas and how ES here may especially be mistaken for neuroendocrine carcinoma (NEC). Distinguishing between the two diagnoses is clinically crucial in dictating different patient management, including chemotherapeutic regimes.2 3\n\nPatient A presented in his early 40s with abdominal pain. His cross-sectional imaging showed a 7 cm diameter mass between the duodenum and pancreatic head. Endoscopy revealed duodenal ulceration and biopsies from here contained neoplastic epithelioid cells with scant cytoplasm, closely packed, small nuclei (figure 1) and an immunoprofile presented in table 1. …