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 char-acteristics 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 Can-cer 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 land-mark 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 avail-able-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.