Impact of Changes in Physical Activity on Dementia Risk Among Cancer Survivors: A Nationwide Cohort Study Comparing Chemotherapy and Non-Chemotherapy Groups.

PURPOSE: Chemotherapy-associated cognitive decline is well recognized; however, the role of changes in physical activity in reducing dementia risk among cancer survivors remains unclear. This study evaluated the association between physical activity changes and dementia risk. MATERIALS AND METHODS: This nationwide retrospective cohort study used health screening and claims data from the Korean National Health Insurance Service. A total of 260,659 adults diagnosed with cancer between 2009 and 2015 who survived ≥3 years and underwent both pre- and post-diagnosis health examinations were included. Individuals with prior dementia or missing data were excluded. Physical activity was assessed as metabolic equivalent task-minutes per week at two time points and categorized as decreased, maintained, or increased. The primary outcome was incident all-cause dementia (ICD-10 codes F00-F03, G30). Adjusted hazard ratios (aHRs) and 95% confidence intervals (CIs) were estimated using Cox proportional hazards models, stratified by chemotherapy exposure. RESULTS: Decreased physical activity was associated with increased dementia risk compared with maintaining high activity (aHR, 1.46; 95% CI, 1.31-1.62), whereas maintaining or increasing activity was associated with reduced risk (aHR, 0.82; 95% CI, 0.75-0.91). The protective association was stronger among chemotherapy-treated patients (aHR, 0.80; 95% CI, 0.72-0.90). No significant association was observed among those not receiving chemotherapy. CONCLUSION: Maintaining or increasing physical activity after cancer diagnosis was associated with lower dementia risk, particularly among chemotherapy-treated patients.