Risk Factors for Lateral Pelvic Lymph Node Metastasis in Rectal Cancer

This study aimed to assess the pre-treatment prevalence of lateral pelvic lymph node metastasis (LPLNM) in patients with rectal cancer and identify the associated risk factors.

We retrospectively evaluated 241 patients who underwent neoadjuvant chemoradiotherapy for rectal cancer between 2014 and 2024. All patients were staged using contrast-enhanced rectal MRI. The lower, middle, and upper rectum were defined as 0–5 cm, 5–10 cm, and 10–15 cm from the anorectal junction, respectively. Pelvic lymph nodes outside the mesorectum were considered metastatic if they had a short axis diameter of ≥1 cm or a diameter of 5–9 mm with at least two of the following MRI characteristics: heterogeneous signal intensity, round shape, or irregular borders. Mesorectal fascia involvement (MRFI) was defined as a tumor distance of ≤1 mm from the mesorectal fascia on MRI. Categorical and continuous variables were analyzed using the Chi-square (χ²) test and the Mann-Whitney U test, respectively.

Fifty-two LPLNMs were detected in 41 out of 241 patients (17%) with a median age of 63 years (range: 23–90). Among them, 35 patients (85.4%) had unilateral LPLNM, while 6 (14.6%) had bilateral involvement. The median number of LPLNMs was 1 (range: 1–7). The median short-axis diameter of metastatic lymph nodes was 11 mm (range: 7–20 mm). The distribution of LPLNMs sites was as follows: 22 (42.3%) internal iliac, 21 (40.4%) obturator, 7 (13.5%) common iliac, and 2 (3.8%) external iliac nodes. No statistically significant association was found between LPLNM and age (p = 0.205), gender (p = 0.271), pre-treatment CEA levels (p = 0.589), extramural venous invasion (p = 0.580), tumor growth pattern (p = 0.994) or tumor morphology on MRI (p = 0.939). However, LPLNM was significantly associated with larger tumor diameter (p < 0.001), an increased number of mesorectal lymph node metastases (p = 0.033), advanced T stage (p = 0.041), and tumor location (p = 0.013). The rate of LPLNM was significantly higher in tumors with MRFI compared to those without MRFI (26.3% vs. 12.2%, p = 0.006), in tumors ≥T3d compared to <T3d (28.3% vs. 14.3%, p = 0.024), in tumors involving both the lower and middle rectum compared to those not involving both regions (23.7% vs. 8.5%, p = 0.002). The risk of LPLNM was significantly associated with tumor differentiation, with rates of 7.6% (9/118) in well-differentiated tumors, 29.6% (16/54) in moderately differentiated tumors, and 77.8% (7/9) in poorly differentiated tumors (p < 0.001).

LPLNM was found to be significantly associated with larger tumor diameter, advanced T stage, an increased number of mesorectal lymph node metastases, involvement of both the lower and middle rectum, MRFI, and poorer tumor differentiation. These findings highlight the importance of comprehensive pre-treatment imaging assessment to identify high-risk patients who may benefit from intensified treatment strategies.