Lymph node morphology as an MRI biomarker for microsatellite instability in rectal cancer

Microsatellite instability (MSI) status in rectal cancer has significant implications for treatment selection, particularly regarding immunotherapy eligibility. This study aimed to evaluate the preoperative magnetic resonance imaging (MRI) features, specifically regional lymph node morphology, in predicting MSI status in rectal cancer patients.

This retrospective, single-center observational study analyzed 80 patients with histopathologically confirmed rectal cancer who underwent preoperative MRI and MSI/mismatch repair protein expression analysis between August 2021 and January 2025. Two experienced radiologists independently assessed radiological features through consensus reading. Evaluated parameters included tumor localization, T-stage, growth pattern, volume, apparent diffusion coefficient values, T2 signal characteristics, and lymph node features encompassing necrosis presence, contour characteristics, location, and short-axis diameter. Statistical analysis employed univariate and multivariate logistic regression to identify independent predictors of MSI status.

Among 80 patients, 23 (28.8%) were MSI-positive and 57 (71.2%) were MSI-negative. Multivariate analysis identified three independent predictors of MSI positivity: lymph node necrosis on T1-weighted contrast-enhanced MRI (OR: 7.86, 95% CI: 1.68-36.79, p = 0.010), intermediate-risk tumor T-stage (T3b) (OR: 6.19, 95% CI: 1.43-26.68, p = 0.015), and polypoid growth pattern (OR: 6.14, 95% CI: 1.06-35.54, p = 0.043). Lymph node necrosis demonstrated significantly higher prevalence in MSI-positive patients compared to MSI-negative patients (26.1% vs. 7.0%, p = 0.029).

Lymph node necrosis on T1-weighted contrast-enhanced MRI, intermediate-risk T-stage (T3b), and polypoid growth pattern serve as valuable radiological biomarkers for predicting MSI status in rectal cancer. These imaging features enable preoperative patient stratification, facilitating personalized treatment planning and appropriate selection of candidates for immunotherapy.