Associations between comorbidities and survival outcomes in older adults with cancer: a scoping review of international and Japanese evidence

Abstract Background Advances in cancer treatment have improved survival among older adults with cancer, many of whom experience multiple comorbidities. This scoping review synthesized international and Japanese evidence regarding associations between comorbidities and survival outcomes in older adults with cancer. Methods We conducted a two-stage scoping review using PubMed and Ichushi. In the primary analysis, we searched for large-scale population- or registry-based studies globally to examine whether the pathways through which comorbidities influence survival outcomes in older adults with cancer, previously described for diabetes, are similarly applicable to comorbidities in general. In the secondary analysis, we searched for Japan-based studies to examine whether similar associations and mechanisms were observed in smaller clinical cohort settings in Japan. Data extraction focused on comorbidity measures, survival outcomes, and proposed mechanisms linking comorbidities to prognosis, which were categorized according to six conceptual pathways. Results Eighteen international studies and 17 Japan-based studies met the eligibility criteria. Across both evidence streams, greater comorbidity burden was consistently associated with poorer overall survival. The Charlson Comorbidity Index was the most frequently used comorbidity measure. International studies more commonly suggested that comorbidities contributed to cancer-related mortality through restricted access to standard treatment (Pathway II). By contrast, Japanese studies more often reported associations with competing non-cancer mortality (Pathway I). Evidence remained limited for mechanisms involving increased malignancy, reduced treatment responsiveness, and delayed cancer detection, highlighting substantial gaps in the literature. Conclusions Comorbidities were consistently associated with poorer survival outcomes among older adults with cancer. International and Japanese studies appeared to emphasize different mechanisms linking comorbidities to prognosis; however, these differences should be interpreted cautiously owing to heterogeneity in study design, outcome definitions, and data sources. Further large-scale studies incorporating standardized comorbidity assessment are needed to clarify the mechanisms through which comorbidities affect prognosis and to inform survivorship care for aging cancer populations.