Over the past 20 years, China's obesity rates have surged, increasing cancer burden. Obesity links to 13 cancers via metabolic effects of visceral fat and insulin resistance, while sarcopenic obesity (BMI-independent) may worsen outcomes. Traditional BMI lacks precision; advanced methods (e.g., BIA/CT) are needed. Existing studies show inconsistent results, possibly due to heterogeneity. This multicenter prospective cohort study uses imaging to assess body composition changes (fat/muscle) in obesity-related tumors and their impact on survival, recurrence, and quality of life, and explore the underlying mechanism.