Diagnostic Performance and Misclassification Patterns of Preoperative MRI in Rectal Cancer: A Real-World Study

Introduction: Magnetic resonance imaging (MRI) is the reference imaging modality for locoregional staging and restaging of rectal cancer (RC). However, its agreement with surgical pathology in real-world practice is limited. We aimed to assess the agreement and diagnostic performance of preoperative MRI for dichotomized T and N staging in RC. Secondarily, we explored the direction of MRI misclassification and potential preoperative factors associated with discordance. Methods: We conducted a retrospective real-world study on 152 consecutive patients with pathologically confirmed RC who underwent surgery between September 2019 and June 2025 in our institution. Two cohorts were analyzed separately: patients treated without neoadjuvant therapy (non-NAT, n = 70) and patients treated with NAT followed by restaging MRI and surgery (NAT, n = 82). The main staging outcomes were dichotomized into T0-T2 vs. ≥T3 and N0 vs. N+, using final pathology as the reference standard. Agreement, Cohen's kappa, sensitivity, specificity, predictive values, McNemar's test, and exploratory regression analyses for misclassification were performed. Results: In the overall cohort, agreement was 72.4% for T staging and 73.0% for N staging, with moderate agreement for T (kappa = 0.452) and fair-to-moderate agreement for N (kappa = 0.349). Sensitivity and specificity were 80.3% and 67.0% for T staging and 54.5% and 80.6% for N staging, respectively. T-stage errors were mainly associated with overstaging. In NAT-treated patients, baseline MRI showed markedly poorer agreement with final pathology than restaging MRI, particularly for T stage (45.1% vs. 72.0%). Exploratory analyses did not identify strong or reproducible predictors of misclassification. Conclusions: This real-world study provides a contemporary estimate of MRI-pathology agreement for dichotomized T and N staging in routine RC care. Agreement was moderate, and performance was more consistent for advanced T-category assessment than for nodal staging. These findings support MRI as a practical tool for multidisciplinary risk stratification and highlight the need for continued monitoring of MRI usage and performance in clinical practice.