Microsatellite instability as a predictive marker for response to neoadjuvant chemoradiation in locally advanced rectal cancer.

Abstract

806 Background: Microsatellite instability (MSI) is a marker for hypermutability due to impaired DNA mismatch repair and has been shown to be a favorable prognostic marker in colon cancer. We examined the role of MSI in locally advanced rectal cancer (LARC) as a predictive marker for response to neoadjuvant chemoradiation (CRT). Methods: We identified T3-T4 or ≥ N1 rectal cancer with reported MSI status in the National Cancer Data Base from 2007 – 2012. Patients were eligible if they were treated with CRT followed by surgical resection. Squamous cell and carcinoid histologies were excluded. The primary outcome was neoadjuvant rectal (NAR) score, categorized as low ( < 8), intermediate (8-16), and high ( > 16), representing a good, intermediate, and poor response to CRT, respectively. The secondary outcomes were complete pathologic response (ypCR) rate and overall survival (OS). Chi-square, Fisher’s exact, and independent sample t-tests were used to compare MSI status. Multivariable logistic regression models were used to examine factors associated with increasing level of NAR score and ypCR. OS was examined using multivariable Cox proportional hazards models. Results: A total of 1849 patients were eligible for the study and were classified as MSI-high (n = 52) or MSI-low/stable (n = 1797). Patients in the MSI-high group were younger (median age 50.5 vs 57.0, p = 0.005) and had higher rates of mucinous and signet ring histology (p < 0.001), clinical T4 disease (p = 0.001), and clinically positive lymph nodes (p = 0.049). The two groups did not differ in gender, race, grade, or resection margin status. While clinical T and N stage, histology, LVI, tumor grade, and PNI were correlated with the NAR score groups, MSI-high status was not significantly associated with a higher NAR score group on multivariable analysis (OR 1.168, 95% CI 0.673 – 2.028). The MSI-high group also did not differ significantly from MSI-low/stable group in ypCR rate (OR 0.748, 95% CI 0.179 – 3.115) or OS (HR 1.727, 95% CI 0.864 – 3.453) on multivariable analysis. Conclusions: MSI status in LARC was not associated with NAR score, ypCR rate, or OS. Our results do not support the use of MSI status as a predictive marker for response to CRT in LARC.