Chromosomal instability (CIN) refers to ongoing chromosome segregation errors throughout consecutive cell divisions. CIN is a hallmark of human cancer, and it is associated with poor prognosis, metastasis, and therapeutic resistance. Analyzing CIN of the DNA extracted from bile tract exfoliated cells in bile samples seems a promising method for diagnosing, monitoring, and predicting the prognosis of patients with malignant biliary obstruction, including biliary tract cancer (BTC), pancreatic head carcinoma. CIN can be assessed using experimental techniques such as bulk DNA sequencing, fluorescence in situ hybridization (FISH), or conventional karyotyping. However, these techniques are either time-consuming or non-specific. The investigators here intend to study whether a new method named Ultrasensitive Chromosomal Aneuploidy Detection (UCAD), which is based on low-coverage whole-genome sequencing, can be used to analyze CIN thus helping diagnose malignant biliary obstruction and assessing follow-up.