Case Report: Primary squamous cell carcinoma of the pancreatic segment of the extrahepatic bile duct: a rare tumor easily misdiagnosed as pancreatic malignancy.

Background: Cholangiocarcinoma is a malignant tumor arising from the bile duct epithelium and may occur anywhere along the biliary tree, from intrahepatic ducts to the distal common bile duct. Most bile duct malignancies are histologically adenocarcinomas, whereas squamous cell carcinoma (SCC) is exceedingly rare. Herein, we report a case of extrahepatic bile duct SCC arising in the pancreatic segment, which is prone to misdiagnosis as pancreatic cancer. Case presentation: A 70-year-old male patient with no history of malignancy presented with worsening right upper abdominal pain and postprandial jaundice that worsened after meals, and underwent percutaneous transhepatic biliary drainage. The patient was clinically stable. Subsequently, he underwent open radical pancreaticoduodenectomy with cholecystectomy. Postoperative pathological examination confirmed primary moderately differentiated extrahepatic SCC of the bile duct. Conclusions: We report a rare case of primary pure SCC located in the pancreatic segment of the common bile duct, without adenocarcinoma components or metaplastic changes. This case underscores its rarity and diagnostic challenges, and highlights the critical role of histopathology and immunohistochemistry in confirming the diagnosis and excluding secondary tumors. Moreover, it broadens the clinicopathologic spectrum of extrahepatic bile duct malignancies and emphasizes that primary SCC should be considered in the differential diagnosis of distal biliary lesions mimicking pancreatic cancer.