Acinar cell carcinoma of the pancreatobiliary system is a relatively rare malignant neoplasm arising usually in the pancreatic parenchyma. We experienced a 68-year-old woman who presented with obstructive jaundice due to an ampullary mass 1.0 cm in diameter, detected by abdominal computed tomography and endoscopic examination. The patient underwent a curative surgical operation, and histopathological examination revealed that the tumor was confined to the ampulla of Vater with no continuity to the pancreatic parenchyma. The tumor cells showed acinar or tubular arrangement with eosinophilic to basophilic granular cytoplasm, findings identical to those of acinar cell carcinoma of the pancreas. Immunohistochemically, the tumor cells were positive for lipase. From these findings, we concluded that the tumor was primary acinar cell carcinoma arising in the ampulla of Vater, probably originating from heterotopic pancreatic tissue. This is the first reported case of primary acinar cell carcinoma in the ampulla of Vater.
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