Microsatellite instability in Costa Rican patients with colorectal adenocarcinoma and its association with overall survival and response to fluoropyrimidine-based chemotherapy.

Background: Microsatellite instability (MSI) is a prognostic and predictive factor in colorectal cancer (CRC). Previous trials have acknowledged that MSI prevalence varies according to ethnicity. The aim of this study was to determine the prevalence of MSI among CRC patients from Costa Rica and to analyze its influence on overall survival (OS) and response to fluoropyrimidine-based chemotherapy.

Methods: We conducted a retrospective cohort study with all diagnosed CRC cases from 2010 to 2015 in a referral center in San José, Costa Rica. MSI was determined by immunohistochemical analysis. Clinical and epidemiological variables were retrieved from medical records. An univariate and multivariate COX regression analysis was performed to evaluate the association between MSI and mortality in the overall population and in those patients treated or not with fluoropyrimidine-based chemotherapy.

Results: 553 CRC patients were identified during the study timeframe, and 165 of them were diagnosed with MSI (29.84 %; 95 % Confidence Interval CI: 26.02-33.65%). MSI was associated with M1 disease, and right-sided tumor location. After adjusting for potential confounders, MSI was an independent prognostic factor for OS (Hazard Ratio (HR): 0.56; 95 %CI: 0.43-0.67; Log-rankp = 0.03). MSI status did not modify the response to fluoropyrimidine-based chemotherapy (HR: 0.56; 95 %CI: 0.18-1.80; Log-rank: p = 0.33).

Conclusions: MSI was detected in a higher proportion of CRC patients than previously reported for other non-Hispanic populations. MSI was an independent prognostic factor for OS, but did not predict the efficacy of cytotoxic treatment with Fluoropyrimidine-based chemotherapy.