Stratification for the risk of lateral lymph node metastasis of locally advanced rectal cancer

Aim: The metastatic rate of lateral lymph node (LLN) of the locally advanced rectal cancer is not necessarily high according to the current Japanese guideline. We aimed to stratify the LLN metastatic risk more accurately based on the factors that can be found on high-resolution MRI.
Method: A total of 110 patients who underwent rectal resection with LLN dissection at our institution from 2016-2020 were included. The risk factors for pathological LLN metastasis were analyzed. The diameter of short-axis of lymph node in mesorectum and lateral pelvic cavity, MRI findings including extramural vascular invasion (EMVI) and mesorectal fascia (MRF) involvement were analyzed.
Results: The overall rate of LLN metastasis was 15.5%. Of the factors, baseline mrEMVI positivity (P=0.0019) and the short-axis of lymph node ≥5mm (P<0.0001) were correlated with LLN metastasis, which were confirmed in multivariate analysis (P=0.0080 and <0.0001). When the patients were stratified into three groups (group A: EMVI negative and short-axis <5mm, group B: either EMVI positive or short-axis ≥5mm, and group C: EMVI positive and short-axis ≥5mm), the metastatic rate to LLN was 0%, 8.9%, and 46.4%, respectively. 
Conclusion: It might be possible to stratify the metastatic risk to LLN based on EMVI and the diameter of short-axis of lateral lymph node. The efficacy of LLN dissection is needed to be investigated on each stratified group.