6-mm vs 10-mm diameter fully covered self-expandable metal stents in patients with unresectable malignant distal bile duct stricture (COSMIC UNISON): study protocol for a multicenter, randomized controlled trial.

BACKGROUND: Unresectable malignant bile duct stricture (often caused by unresectable pancreatic cancer and cholangiocarcinoma) can be drained via insertion of self-expandable metal stents (SEMS) during endoscopic retrograde cholangiopancreatography (ERCP). Because recurrent biliary obstruction (RBO) and complications following stent insertion can delay chemotherapy and other treatments, a longer time to RBO (TRBO) is desirable. Although a longer TRBO has been reported among patients who undergo insertion with larger diameter SEMS, patients who undergo insertion with smaller diameter fully covered SEMS (FCSEMS) may have a lower incidence of complications than those with larger diameter FCSEMS. The aim of this study is to determine the TRBO and incidence of complications with 6-mm FCSEMS vs 10-mm FCSEMS in patients with unresectable malignant distal bile duct stricture.

METHODS: In this multicenter, open-label, randomized controlled, non-inferiority trial (COSMIC UNISON), a target of 250 patients over 23 locations in Japan will receive either the 6-mm FCSEM or the standard 10-mm FCSEM during ERCP, with 125 patients in each group. The observation period will be 24 months, and patients will be enrolled from 15 March 2024 and assessed until the date of RBO or the study end (31 March 2029). The primary endpoint is TRBO, with RBO defined as the coexistence of abnormal liver enzyme values and dilation of the common bile duct and intrahepatic bile duct upstream of the stent. The secondary endpoints are the incidence and rates (at 3, 6, and 12 months) of non-RBO events, overall survival, cause of RBO, and symptomatic stent deviation. Adverse events from endoscopic procedures will be classified by the Lexicon Classification from the American Society of Endoscopy, and all other adverse events will be classified per the Japanese translation of the Common Terminology Criteria for Adverse Events version 5.0.

DISCUSSION: The COSMIC UNISON study is anticipated to provide evidence regarding the efficacy and safety of 6-mm vs 10-mm FCSEMS to inform the use of 6-mm FCSEMS for the treatment of unresectable malignant distal bile duct stricture.

TRIAL REGISTRATION: Japan Registry of Clinical Trials identifier: jRCT1042230170. Prospectively registered on 15 March 2024.