Primary mucinous adenocarcinoma of the common bile duct with signet-ring cell features: a rare case report

Introduction and importance: Primary biliary carcinoma with signet-ring cell morphology is exceptionally rare and may be difficult to distinguish from metastatic gastrointestinal or adjacent pancreatobiliary malignancies. Establishing a primary bile duct origin requires integration of endoscopic, radiological, and histopathological findings. Presentation of the case: A 70-year-old woman presented with progressive jaundice, right upper quadrant pain, pruritus, generalized weakness, and loss of appetite. Ultrasonography demonstrated biliary dilatation, and endoscopic retrograde cholangiopancreatography showed severe distal common bile duct narrowing. SpyGlass cholangioscopy identified abnormal intraductal tissue, and targeted biopsy confirmed adenocarcinoma. Preoperative upper gastrointestinal endoscopy and colonoscopy were documented as normal, and cross-sectional staging showed no distant metastatic disease. The patient underwent pancreaticoduodenectomy. Histopathology demonstrated a 2.5 cm poorly differentiated mucinous adenocarcinoma with signet-ring cell features centered in the common bile duct, with 4 mm depth of invasion and microscopic pancreatic extension. Lymphovascular and perineural invasion were present; surgical margins were negative; and 4 of 10 regional lymph nodes were metastatic, with extranodal extension. Adjacent high-grade biliary intraepithelial neoplasia/carcinoma in situ supported a primary biliary origin. The tumor was classified as pT1 pN2 cM0. Clinical discussion: Despite limited local invasion, the substantial nodal involvement and extranodal extension suggested aggressive biological behavior. Direct cholangioscopy-guided biopsy facilitated preoperative diagnosis, while the tumor epicenter and adjacent biliary precursor lesion were critical in supporting a primary biliary origin. Conclusion: This case highlights the value of targeted cholangioscopic biopsy and meticulous pathological assessment in diagnosing rare common bile duct carcinomas with signet-ring cell features.