Loco-regional N staging in Rectal Cancer with Magnetic Resonance Imaging: A Study of Inter- and Intra-Observer Variability

Background: Colorectal cancer is one of the most common tumors for both men and women: in the United States, it represents the third leading cause of new cancer cases and cancer-related deaths. The prognosis is directly related to tumor infiltration in the mesorectum and lymph node metastases. In particular, it’s important to define the distance between lymphadenopathy and mesorectal fascia, as this has repercussions on surgical planning. This study aimed to evaluate the agreement among observers with different abdominal MRI expertise and intra-observer reliability in lymph nodes size and feature definition. Methods: In this retrospective study, MRI examinations were performed in 88 patients with rectal adenocarcinoma treated with primary surgery. Four observers, two senior physicians, and two junior physicians, analyzed MRI scans in two sessions 30 days apart and determined the size and morphological pattern of regional lymph nodes. Statistical analysis included the determination of Fleiss kappa (k) coefficient, Cohen's Kappa coefficient, and confidence intervals (CI). Results: 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). Inter-observer reproducibility was excellent for mesorectal, inferior mesenteric, and internal iliac lymph nodes (kappa values of 0.89, 0.82, and 0.80 respectively). For the other two nodal stations (superior and middle rectal lymph nodes, sacral lymph nodes), there was a good interobserver reproducibility (kappa between 0.70 and 0.77).The intra-observer reproducibility of interpretations of the MRI overall N staging progressively decreased among observer B (kappa= 0.85), observer C (kappa= 0.59), and the other two physicians. 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; the lowest intraobserver reproducibility values were found for presacral and lateral sacral lymph nodes.Conclusions: Although the low accuracy of MRI in assessing the involvement of metastatic lymph nodes in rectal cancer, this study demonstrates good interobserver reliability among physicians with different abdominal MRI experiences.