MRI metastatic lymph nodes detection in rectal cancer staging: retrospective analysis of intra- and inter-observer variability in an academic health centre.

This study aimed to evaluate the agreement among observers with different abdominal MRI expertise and intra-observer reliability in metastatic lymph nodes detection and features definition. In this retrospective study, MRI examinations were performed in 88 patients with rectal adenocarcinoma treated with primary surgery. Four obser-vers, two senior and two junior physicians, analyzed MRI scans in two sessions 30 days apart and determined size and morphological pattern of regional lymph nodes. Statistical analysis included the determination of the Fleiss kappa (k) coefficient, Cohen's Kappa coefficient, and confidence intervals (CI). The inter-observer reproducibility for MRI N-staging was good among the four physicians (kappa = 0.65; CI 0.45-0.77). Reproducibility between the two senior physicians had a kappa of 0.68 (CI 0.62-1.00), while between the two junior physicians had a kappa of 0.61 (CI 0.33-0.89). The intra-observer reproducibility progressively decreased between observers with different experience. There was a significant difference in lymph nodes measurements between the first and second sessions in observer A (p ≥ 0.05). Excellent intraobserver reproducibility was found for mesorectal lymph nodes while the lowest ones were found for presacral and lateral sacral lymph nodes. This study demonstrates good interobserver reliability among physicians with different abdominal MRI experiences.