Impact of timing of adjuvant chemotherapy initiation on survival for elderly patients with stage III colon cancer in Norway – a register-based cohort study

Standard treatment for stage III colon cancer is major surgical resection followed by adjuvant chemotherapy (ACT). Norwegian guidelines recommend initiation of ACT within 4–6 weeks after resection, but consensus regarding optimal timing of ACT is lacking. To investigate the impact of ACT timing on 5-year overall survival (OS) of elderly patients with stage III colon cancer in Norway. This population-based retrospective cohort study included patients aged 70 years or older who underwent major surgical resection for stage III colon cancer diagnosed between 2011 and 2021. Patients were grouped by ACT timing after resection: ≤ 6, 7–8, 9–10, and 11–13 weeks, in addition to those with resection only. The 5-year OS was assessed using Kaplan-Meier analysis and Cox proportional hazards models. Among 4 075 patients included, 1 408 were provided ACT. Median timing of ACT was 6.4 weeks after resection. Initiation of ACT in weeks 7–10 after resection was not associated with increased mortality risk compared to initiation within the first 6 weeks. Delaying the initiation of ACT beyond 10 weeks after resection was associated with worse 5-year OS (hazard ratio 1.49, 95% confidence interval 1.04–2.12). The association between survival benefit and ACT timing varied based on risk levels: For low risk patients, there was an association of improved survival benefit when ACT was initiated within 8 weeks after resection compared to resection alone, whereas for high risk patients, the association of survival benefit was better for those provided ACT in weeks 9–10 as well. Our findings support initiation of ACT within 8 weeks after major resection to maximise survival benefits in elderly patients with stage III colon cancer. However, for certain patient groups, initiation of ACT seems beneficial even up to 10 weeks after resection. The findings suggest greater flexibility in ACT initiation timing, benefiting both patients and health care services.