Factors Explaining Socio-Economic Inequalities in Survival from Colon Cancer: A Causal Mediation Analysis

Background: Socio-economic inequalities in colon cancer survival exist in high-income countries, but the reasons are unclear. We assessed the mediating effects of stage at diagnosis, comorbidities, and treatment (surgery and intravenous chemotherapy) on survival from colon cancer. Methods: We identified 2,203 people aged 15 to 79 years with first primary colon cancer diagnosed in Victoria, Australia, between 2008 and 2011. Colon cancer cases were identified through the Victorian Cancer Registry (VCR), and clinical information was obtained from hospital records. Deaths till December 31, 2016 (n = 807), were identified from Victorian and national death registries. Socio-economic disadvantage was based on residential address at diagnosis. For stage III disease, we decomposed its total effect into direct and indirect effects using interventional mediation analysis. Results: Socio-economic inequalities in colon cancer survival were not explained by stage and were greater for men than women. For men with stage III disease, there were 161 [95% confidence interval (CI), 67- 256] additional deaths per 1,000 cases in the 5 years following diagnosis for the most disadvantaged compared with the least disadvantaged. The indirect effects through comorbidities and intravenous chemotherapy explained 6 (95% CI, -10-21) and 15 (95% CI, -14-44) per 1,000 of these additional deaths, respectively. Surgery did not explain the observed gap in survival. Conclusions: Disadvantaged men have lower survival from stage III colon cancer that is only modestly explained by having comorbidities or not receiving chemotherapy after surgery. Impact: Future studies should investigate the potential mediating role of factors occurring beyond the first year following diagnosis, such as compliance with surveillance for recurrence and supportive care services.