Tumor deposits on MRI in rectal cancer—detection and differentiation from lymph nodes with histopathological correlation

OBJECTIVE: Tumor deposits (TDs) are a negative prognostic factor in colorectal cancer, independent of lymph node metastasis (LNM). This study aimed to investigate the ability of pre-operative MRI to identify TDs and differentiate them from LNM in rectal cancer. MATERIALS AND METHODS: Patients with histopathological N1-N2 rectal cancer with and without TDs who underwent curative-intent primary surgery between 2013 and 2023 were retrospectively included. Three radiologists independently reviewed pre-operative MR images for the presence of TDs and LNM. Histopathology served as the reference standard from which interobserver agreement by Cohen's kappa (κ) was calculated. Unblinded node-to-node comparison of MRI and histopathological images was used to identify sources of discrepancies between the modalities. Post hoc analysis stratified on nodal size (> or ≤ 5 mm) was performed. RESULTS: Eighty-eight patients (50% with TDs on histopathology, 74% male, median age 70 [20-89] years) were included, of whom 27 were excluded due to poor image quality. Agreement between MRI and histopathology (p), both for TD-presence (κ = -0.04) and LNM-presence (κ = 0.16), was low. In the node-by-node comparison, 43 out of 50 pTDs (86%) measured ≤ 5 mm, and 131 of 167 pLNM (78%) measured ≤ 5 mm. In post hoc analysis of smaller (≤ 5 mm) nodules, no pTDs and 3% of pLNM were diagnosed using MRI. CONCLUSION: MRI demonstrated limited ability in detecting and differentiating TDs and LNM in pre-operative low-risk patients. Diagnostic performance was particularly poor for nodules ≤ 5 mm, which constituted 174 out of 217 (80%) pathological nodular structures. KEY POINTS: Questions Tumor deposits and lymph node metastasis are important factors for staging rectal cancer, but the ability of pre-operative MRI to detect and differentiate them remains unclear. Findings In low-risk rectal cancer patients treated with primary surgery, tumor deposits and lymph node metastasis are often small (≤ 5 mm) and not detected by MRI. Clinical relevance MRI has a limited ability to identify and differentiate small (≤ 5 mm) tumor deposits and lymph node metastasis in rectal cancer, which must be considered during pre-operative staging and neoadjuvant treatment planning.