Clinical, pathological, and adjuvant chemotherapy use differences among microsatellite unstable and microsatellite stable colon cancers

Background: Colon cancers are categorized into mismatch repair deficient/microsatellite unstable (MSI-H) and mismatch repair proficient/microsatellite stable (MSS) cancers. This study aims to compare the disease characteristics and trends in the utilization of cancer therapies across different age groups and stages in these two groups. Methods: MSI-H and MSS colon adenocarcinomas from 2010 to 2016 were identified using the National Cancer Database. We compared patient and disease characteristics between the two groups and evaluated the use of adjuvant chemotherapy across age groups and cancer stages. Within MSI-H and MSS groups, we conducted a landmark analysis after propensity score matching for adjuvant chemotherapy versus no chemotherapy to determine its effect on survival. Results: Of the 542,368 patients that met inclusion criteria, 120,751 (22%) had mismatch repair results available—out of these 96,928 (80%) had MSS colon cancers while 23,823 (19.7%) had MSI-H cancers. MSI-H disease had a bimodal age distribution (<40 years = 22%; ≥75 years = 26%) and was frequent among females (22%) and non-Hispanic Whites (20%). Among those < 65 years, 15% of low-risk stage 2 MSI-H patients and 40% of high-risk stage 2 MSI-H patients received adjuvant chemotherapy. More than two-thirds of stage 3 patients <65 years received adjuvant chemotherapy in both groups. After conducting propensity-score matching for age, gender, and co-morbidities, we found that adjuvant chemotherapy use had a trend towards lower overall survival (OS) in low-risk stage 2 MSI-H (HR = 1.8 [95% CI, 0.8–4.02]) and high-risk stage 2 MSI-H (HR = 1.42 [95% CI, 0.96–2.12]) groups. Adjuvant chemotherapy significantly improved OS in stage 3 colon cancer patients irrespective of microsatellite status or risk category of disease. Conclusions: MSI-H colon cancer had bimodal age distribution. Among stage 2 MSI-H patients <65 years, a notable proportion received adjuvant chemotherapy. Among MSI-H stage 2 patients, adjuvant chemotherapy use was associated with lower survival while it significantly improved survival for stage 3 patients, irrespective of MSI status