Does comorbidity interact with colorectal cancer to increase mortality? A nationwide population-based cohort study

Background: It is unknown whether comorbidity interacts with colorectal cancer (CRC) to increase the rate of mortality beyond that explained by the independent effects of CRC and comorbid conditions. Methods: We conducted a cohort study (1995-2010) of all Danish CRC patients (n = 56 963), and five times as many persons from the general population (n = 271 670) matched by age, gender, and specific comorbidities. To analyse comorbidity, we used the Charlson Comorbidity Index (CCI) scores. We estimated standardised mortality rates per 1000 person-years, and calculated interaction contrasts as a measure of the excess mortality rate not explained by the independent effects of CRC or comorbidities. Results: Among CRC patients with a CCI score = 1, the 0-1 year mortality rate was 415 out of 1000 person-years (95% confidence interval (CI): 401, 430) and the interaction accounted for 9.3% of this rate (interaction contrast = 39 out of 1000 person-years, 95% CI: 22, 55). For patients with a CCI score of 4 or more, the interaction accounted for 34% of the mortality (interaction contrast = 262 out of 1000 person-years, 95% CI: 215, 310). The interaction between CRC and comorbidities had limited influence on mortality beyond 1 year after diagnosis. Conclusion: Successful treatment of the comorbidity is pivotal and may reduce the mortality attributable to comorbidity itself, and also the mortality attributable to the interaction.