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Background & Aims: While skeletal muscle index (SMI) is the most widely used indicator of low muscle mass (or sarcopenia) in oncology, optimal cut-offs (or definitions) to better predict survival are not standardized., Methods: We compared five major definitions of SMI-based low muscle mass using an Asian patient cohort with gastrointestinal or genitourinary cancers. We analyzed 2015 patients with surgically-treated gastrointestinal (n = 1382) or genitourinary (n = 633) cancer with pre-surgical computed tomography images. We assessed the associations of clinical parameters, including low muscle mass by each definition, with cancer-specific survival (CSS) and overall survival (OS)., Results: During a median follow-up period of 61 months, 303 (15%) died of cancer, and 147 died of other causes. An Asian-based definition diagnosed 17.8% of patients as having low muscle mass, while the other Caucasian-based ones classified most (>70%) patients as such. All definitions significantly discriminated both CSS and OS between patients with low or normal muscle mass. Low muscle mass using any definition but one predicted a lower CSS on multivariate Cox regression analyses. All definitions were independent predictors of lower OS. The original multivariate model without incorporating low muscle mass had c-indices of 0.63 for CSS and 0.66 for OS, which increased to 0.64-0.67 for CSS and 0.67-0.70 for OS when low muscle mass was considered. The model with an Asian-based definition had the highest c-indices (0.67 for CSS and 0.70 for OS)., Conclusions: The Asian-specific definition had the best predictive ability for mortality in this Asian patient cohort., (Copyright © 2023 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved.)