Efficacy of adjuvant chemotherapy in patients with late-onset stage II/III colon cancer: a retrospective cohort study.

Objectives: Studies providing more evidence to guide adjuvant chemotherapy decisions in colon cancer patients aged 50 years and older are expected. Methods: Eligible patients were recruited from the Surveillance, Epidemiology and End Results (SEER) database between 2010 and 2015. The cancer-specific survival (CSS) rate was analyzed using the Kaplan-Meier method, and comparisons of survival difference between different subgroups were performed using the log-rank test. Multivariate Cox proportional hazards regression models were built to assess hazard ratios (HRs) of different variables with 95 % confidence intervals (95 % CIs). Results: In stage II colon cancer patients aged 50 years or older, the Kaplan-Meier survival analysis showed that the 5-year CSS rates of chemotherapy and no chemotherapy groups were 83.5 % and 85.6 %, respectively (p=0.0047). Patients with chemotherapy receipt were associated with a higher cancer-specific mortality rate (HR=1.134, 95 % CI: 1.027-1.252, p=0.013); however, further subgroup analyses indicated that this finding is highly likely driven by unmeasured confounding and selection bias rather than direct treatment-related harm. For stage III patients of the same age group, 5-year CSS was 74.2 % (chemotherapy) vs. 56.4 % (no chemotherapy, p<0.0001), and chemotherapy was linked to a 49.3 % lower cancer-specific mortality (HR=0.507, 95 % CI: 0.482-0.534, p<0.001). Conclusions: Adjuvant chemotherapy should be considered during the treatment of stage III colon cancer patients aged 50 years or older. For stage II patients, a generalized survival advantage from chemotherapy was not observed, which highlights the profound impact of selection bias in observational data and strongly emphasizes the necessity of precise, individualized risk stratification before administering treatment.