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Hepatology: Pancreatic lymphoepithelial cyst mimicking mucinous cystic neoplasm

A well-defined 6 × 3 × 3.7 cm multilocular cystic lesion at the pancreatic tail (Figure 1a) was found incidentally on computed tomography in a 70-year-old woman who presented with melena. Physical examination was unremarkable and both of upper and lower endoscopies were normal. The cyst was homogeneously hypo-intense on T1-weighted and hyper-intense on T2-weighted magnetic resonance images. Further evaluation with endoscopic ultrasonography (EUS) showed the presence of echogenic spots and cyst-in-cyst lesion within the cyst, and the margin was irregular (Figure 1b). Serum amylase, carcinoembryonic antigen (CEA), and carbohydrate antigen (CA) 19-9 were normal. Based on these imaging features, the cyst was thought to be a mucinous cystic neoplasm (MCN) and the patient underwent a distal pancreatectomy and splenectomy. Fluid cytology of the resected cyst showed only lymphocytes and histiocytes, with cystic fluid CEA level of 215 ng/ml and CA19-9 of more than 100,000 ng/ml. On histology, the walls of the cyst were lined with stratified squamous epithelium, which was surrounded by a layer of lymphoid tissue composed of small lymphocytes, plasma cells, and germinal centers without signs of atypia. Further immunohistochemical studies, the epithelium of the cyst diffusely expressed CK5/6, p63, CK7, CA19-9 (Figure 1c), and more focally, CEA (Figure 1d). Although keratinization was not classically present, these findings support the diagnosis of a lymphoepithelial cyst, which has a mixed squamous–transitional–ductal phenotype. Outside of Japan, EUS guided cyst aspirate for fluid analysis of tumor markers, such as CEA and CA19-9, and cytology is often performed to aid the differentiation between mucinous and nonmucinous lesions. MCNs are more likely to express high levels of cystic fluid CEA and Ca19.9. This case illustrated that high levels of CEA and CA19-9 on EUS guided fluid analysis can also be found in lymphoepithelial cysts, and can be mis-diagnosed as MCNs. The suspicion of lymphoepithelial cyst should be raised if cytology only reveals the presence of lymphocytes and histiocytes. Given surgery is not generally indicated for lymphoepithelial cysts of the pancreas, it is important to recognise the potential similarities in the imaging and fluid analysis profile of lymphoepithelial cysts to MCNs, and avoids unnecessary resection.