Background: Pancreatic cancer accompanied by a moderate-sized pseudocyst with extrapancreatic growth is\nextremely rare. Diagnosis of pancreatic cancer on preoperative imaging is difficult when the pancreatic parenchyma is\ncompressed by a pseudocyst and becomes unclear. Despite advances in imaging techniques, accurate preoperative\ndiagnosis of cystic lesions of the pancreas remains difficult. In this case, it was challenging to diagnose pancreatic\ncancer preoperatively as we could not accurately assess the pancreatic parenchyma, which had been compressed by a\nmoderate-sized cystic lesion with extrapancreatic growth.\nCase presentation: A 63-year-old woman underwent investigations for epigastric abdominal pain. She had no history\nof pancreatitis. Although we suspected pancreatic ductal carcinoma with a pancreatic cyst, there was no mass lesion or\nlow-density area suggestive of pancreatic cancer. We did not immediately suspect pancreatic cancer, as development\nof a moderate-sized cyst with extrapancreatic growth is extremely rare and known tumor markers were not elevated.\nTherefore, we initially suspected that a massive benign cyst (mucinous cyst neoplasm, serous cyst neoplasm, or\nintraductal papillary mucinous neoplasm) resulted in stenosis of the main pancreatic duct. We were unable to\nreach a definitive diagnosis prior to the operation. We had planned a pancreaticoduodenectomy to reach a\ndefinitive diagnosis. However, we could not remove the tumor because of significant invasion of the surrounding\ntissue (portal vein, superior mesenteric vein, etc.). The fluid content of the cyst was serous, and aspiration\ncytology from the pancreatic cyst was Class III (no malignancy), but the surrounding white connective tissue\nsamples were positive for pancreatic adenocarcinoma on pathological examination during surgery. We repeated\nimaging (CT, MRI, endoscopic ultrasound, etc.) postoperatively, but there were neither mass lesions nor a\nlow-density area suggestive of pancreatic cancer. In retrospect, we think that the slight pancreatic duct dilation\nwas the only finding suggestive of pancreatic cancer.\nConclusions: It is difficult to diagnose pancreatic cancer with pseudocyst preoperatively. If a pancreatic cyst is\nfound in patients who had normal tumor marker levels or no history of pancreatitis, we should always consider\nthe possibility of pancreatic cancer. In such cases, slight pancreatic duct dilation may be a diagnostic clue.
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