Randomization and masking
After biliary cannulation was obtained, patients were randomly assigned (1:1) by the coordinating investigator to either the intervention group (dense brush) or the control group (conventional brush) with the use of sealed opaque envelopes. The randomization sequence was computer-generated before trial commencement by SL. Patients were enrolled and assigned to trial groups by the coordinating investigators (SL, NvH, MG). Participants were masked to group assignment (they were not told which group they were allocated to), whereas the endoscopist was not masked to the outcome of randomization. Pathologists were blinded to group assignment. The study coordinator was not blinded to treatment allocation during the assessment of the outcomes and the analyses of the study data. The members of the DSMB were blinded to group allocation.