Lymphoepithelial cyst of the pancreas. Clinical, morphological, and immunohistochemical findings.

Lymphoepithelial cyst of the pancreas, formerly also termed branchial cyst, is an extremely rare tumor of uncertain histogenesis. Our case, that of a 53-year-old man, is the fourth to be described. Fluid aspirated from the cyst exhibited a very high concentration of carcino-embryonic antigen (CEA; 5000 ng/ml), and a high level of carbohydrate antigen 19-9 (CA 19-9; 187 U/l), suggesting a diagnosis of carcinoma of the pancreas. However, the serum CEA and CA 19-9 levels were only slightly elevated (5.5 ng/ml and 125 U/l, respectively). Histologic investigation revealed a cyst lined by squamous epithelium with closely associated lymphoid tissue, without cellular atypia. Numerous lymphocytes, mainly T cells (UCHL1 positive), were present in the lining epithelium. The lymphoid tissue surrounding the lining epithelium was composed of germinal centers and T regions. Epithelial cords contiguous with the squamous epithelium lining the cyst radiated out through the lymphoid tissue towards the pancreatic parenchyma, which suggests that lymphoepithelial cyst of the pancreas is a true pancreatic cyst. Since the excretory ducts of the normal pancreatic tissue and some of the epithelial cells lining the cyst were immunoreactive for CEA and CA 19-9, it can be concluded that CEA and CA 19-9 in the cyst contents are probably produced by cells derived from the exocrine pancreas. The histogenesis of lymphoepithelial cyst of the pancreas remains unclear, but it is probable that it derives from the duct system of the pancreas.