Purpose: We examined the association between pathological lateral pelvic lymph node (LPLN) metastasis and the LPLN diameter in patients with locally advanced rectal cancer (LARC) who received a neoadjuvant chemotherapy (NAC) regimen based on oxaliplatin as induction chemotherapy. We aimed to determine whether or not the LPLN size predicts LPLN metastasis in NAC cases. Methods: We retrospectively examined data from 3 institutes for 60 patients with LARC who received mesorectal excision and LPLN dissection after NAC. We evaluated the LPLN size on magnetic resonance imaging (MRI) scans acquired before and after NAC. We performed multivariate analyses to analyze the relationship between the LPLN size and clinicopathological factors. Results: For patients with visible LPLNs, the median short-axis diameter (SA) was significantly reduced from 5.1 mm (range 2.0–17.4) before NAC to 3.7 mm (range 2.1–19.0) after NAC (p = 0.0479). SA diameters were significantly larger in pathological LPLNs than in healthy LPLNs, both before (p = 0.0002) and after NAC (p < 0.0001). A SA cut-off value of 7 mm before NAC was able to independently predict lymph node metastasis (p = 0.0178). Conclusions: We showed that MRI-based evaluations of LPLN size were able to predict metastasis in patients who underwent NAC for LARC. This finding might be useful when considering selective LPLN dissection in NAC cases.
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