348/of 350 allowed) Background: Lung metastasectomy in the treatment of advanced colorectal cancer has been widely adopted without good evidence of survival or palliative benefit. We aimed to test its effectiveness in a randomised trial (RCT). Methods: Multidisciplinary teams in 13 hospitals recruited participants with potentially resectable lung metastases to a multicentre 2-arm RCT comparing active monitoring with or without metastasectomy. Other local or systemic treatments were decided by

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... e local team. Randomisation was remote and stratified by site with minimisation for age, sex, primary cancer stage, interval since primary resection, prior liver involvement, the number of metastases, and carcinoembryonic antigen. The central trial management group were blind to patient allocation until completion of the analysis. Analysis was on intention to treat with a margin for noninferiority of 10%. Results: Between December 2010 and December 2016, 65 participants were randomised. Characteristics were well-matched in the two arms and similar to those in reported studies: age 35 to 86 (Inter Quartile Range (IQR) 60 to 74); primary resection IQR 16 to 35 months previously; stage at resection T1, 2