Microsatellite Instability Analysis and Its Prognostic Value in Invasive Nonampullary Duodenal Adenocarcinoma

Purpose: Nonampullary duodenal adenocarcinoma (NADA) is a rare disease. Although several prognostic factors have been reported for this disease, they remain controversial due to their rarity. In this study, we retrospectively analyzed 54 cases of invasive NADA, focusing on the microsatellite instability (MSI) phenotype, programmed cell death-ligand 1 (PD-L1) expression, and prognostic factors. Methods: Expression of the PD-L1 protein and cell differentiation markers in tumors was detected by immunohistochemistry. Microsatellite markers (NR-21, NR-22, NR-24, BAT-25, and BAT-26) were amplified for MSI assessment by PCR. Results: The incidence of MSI in invasive NADA was 35.2%. No significant correlation between the MSI phenotype and clinicopathological factors was observed. Positive expression of PD-L1 by immune cells was common in advanced-stage disease (p = 0.054), and positive expression of PD-L1 in cancer cells correlated significantly with the histologically undifferentiated type (p = 0.016). Kaplan-Meier survival analysis demonstrated a significantly better overall survival (OS) in patients with MSI (p = 0.013) and at early-stage disease (p = 0.000) than in those with microsatellite-stable or at late tumor stages. Univariate and multivariate analyses showed that MSI (hazard ratio [HR]: 0.282, 95% confidence interval [CI]: 0.106-0.751, p = 0.011) and early tumor stage (stage I-II) (HR: 8.81, 95% CI: 2.545-30.500, p = 0.001) were independent better prognostic factors of OS. Conclusions: MSI and early tumor stage (stage I-II) were independent better prognostic factors of OS. A high proportion of MSI phenotypes and positive PD-L1 expression may be helpful for identifying immune checkpoint inhibitors as a novel therapeutic strategy.