Microsatellite instability and survival after adjuvant chemotherapy among stage II and III colon cancer patients: results from a population‐based study

Previous studies have reported conflicting results regarding the benefit of administering 5‐FU‐based chemotherapy to colon cancer (CC) patients with microsatellite‐instable (MSI‐high) tumors, and results from stage‐specific analyses are scarce. Patients with stage II or III CC were recruited as part of a population‐based study between 2003 and 2015. The Cox regression models including propensity score weighting were used to calculate hazard ratios and confidence intervals for the association between chemotherapy and cancer‐specific (CSS), relapse‐free (RFS), and overall survival (OS) by stage of disease and MSI status of the tumor. Median follow‐up was 6.2 years. A total of 1010 CC patients were included in the analysis (54% stage II, 46% stage III, 20% MSI‐high). Adjuvant chemotherapy was administered to 48 (8.7%) stage II and 366 (79%) stage III patients. Overall, patients who received adjuvant chemotherapy had better CSS [HR = 0.65 (0.49–0.86)] than those who received surgery alone. Among stage II patients, only 64 (12%) cancer‐related deaths occurred, none of which in MSI‐high patients who received chemotherapy. Patients with MSI‐high tumors who received adjuvant treatment showed better CSS and a tendency toward better RFS compared to MSI‐high patients who did not receive chemotherapy [HR(CSS) = 0.36 (0.15–0.82), HR(RFS) = 0.49 (0.22–1.06)]. Patients with microsatellite‐stable (MSS) tumors receiving adjuvant chemotherapy also had significantly better survival [HR(CSS) = 0.65 (0.48–0.87) and HR(RFS) = 0.68 (0.52–0.88)]. In this population‐based study including stage II and III CC patients, we observed a survival benefit of adjuvant chemotherapy for both MSS and MSI‐high tumors. Adjuvant chemotherapy seemed to be beneficial among high‐risk stage II patients with MSI‐high tumors.