Aim: To explore whether the size and characteristics of the largest regional lymph node in patients with rectal cancer, based on magnetic resonance imaging (MRI), following neoadjuvant therapy and before surgery, is able to identify patients at high risk of developing metachronous metastases.

Patients and Methods: A retrospective case–control study with data from the Swedish Colo-Rectal Cancer Registry. Forty patients were identified with metachronous metastases (M+), and 40 patients without metastases (M0) were matched as controls.

Results: Patients with M+ disease were more likely to have a regional lymph node measuring ≥5 mm than patients with M0. (87% vs. 65%, p=0.02). There was also a significant difference between the groups regarding the presence of an irregular border of the largest lymph node (68% vs. 40%, p=0.01).

Conclusion: Lymph nodes measuring ≥5 mm with/without displaying irregular borders at MRI performed after neoadjuvant therapy emerged as risk factors for metachronous metastases in patients with rectal cancer. Intensified follow-up programmes may be indicated in these patients.

Objective: To assess the accuracy of MRI to predict remaining lymph node metastases in patients with complete pathological luminal response (ypTO) after neoadjuvant therapy. Methods: Data from a national registry were used. 19 patients with histopathologically remaining lymph node metastases (ypTON+) were identified. Another 19 patients without lymph node metastases (ypTONO) were used as matched controls. Two radiologists blinded to all patient information evaluated staging and restaging MRI that was compared to histopathological findings of the resected specimen. Results: The average size of the largest lymph node on restaging MRI was significantly larger (4.5 mm) in the ypTON+ group than in the ypTONO group (2.6 mm) (p = 0.04). Presence of ypN+ was correctly predicted by MRI in 7 of 19 patients. In patients without lymph node metastases (ypTONO), these were correctly classified by MRI in 16 of 19 patients. All patients who had MR-identified lymph nodes larger than 8mm at restaging were ypTN+. The sensitivity, specificity, positive predictive value and negative for prediction of remaining lymph node metastasis with MRI were 37, 84, 70 and 57%. Conclusion: In patients with ypTO in rectal cancer after neoadjuvant treatment, remaining regional lymph node metastases cannot safely be predicted by restaging MRI alone using presently known criteria. Presence of a lymph node over 8mm on restaging MRI strongly indicates yPN+. Advances In knowledge: This is one of the first studies on MRI lymph node assessment after chemo- radiotherapy (CRT) in luminal complete response.