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

Abstract

A diagnosis of Ewing sarcoma (ES) is suggested by certain morphological and immunohistochemical findings and supported by demonstration of EWSR1 translocation, as is usually demonstrated generically with a breakapart 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. The most common of these partners are FLI1 (85% of ES) and ERG (10% of ES). Most ESs arise in bone or soft tissues 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. 3 Patient A presented in his early 40s with abdominal pain. His crosssectional 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. Chemotherapy was commenced for a favoured diagnosis of small cell NEC but because the tumour showed minimal response to this therapy, it was resected by pancreaticoduodenectomy. In this specimen, the neoplasm showed not only the epithelioid cells, but also a second population of widerspaced cells with longer nuclei (figure 1) and some immunohistochemical differences (table 1). Differential diagnoses of gastrointestinal stromal tumour, synovial sarcoma and ES were considered. The latter was favoured after EWSR1 rearrangement (figure 1) was generically demonstrated and a EWSR1exon 7/FLI1exon 6 type 1 fusion transcript then confirmed by RTPCR analysis. Patient B presented in her early 20s with obstructive jaundice. Her crosssectional imaging showed a 6 cm diameter mass in the same location as above. An endoscopic ultrasound guided biopsy (EUSB) of the mass contained epithelioid cells Correspondence