Materials Fully covered SEMS: Niti-S Biliary ComVi Stent; Uncovered SEMS: Niti-S (D type) stent Sample size calculation: The primary end point of the study is stent occlusion. The number of patients in each group required to demonstrate a statistically significant difference in SEMS patency with an 80% power is 70 for a 22% difference,121 for a 17% difference, and 248 for a 12% difference in the obstruction rate between the two groups. Estimated sample size is 121 for a 17% difference, and 70 for a 22% difference.