PURPOSE: The purpose of the study was to examine if a minimum of 12 lymph nodes (LNs) is still valid in rectal cancer after neo-adjuvant treatment.

METHODS: An analysis was carried out in a nationwide Danish cohort of 6793 patients, treated by curative resection of stage I-III rectal cancer during the period 2003-2011. The cohort was divided into two groups according to whether neo-adjuvant treatment had been given. The groups were analysed separately and were further analysed according to four lymph node yield (LNY) groups 0-5, 6-11, 12-17 and ≥18.

RESULTS: Two thousand one hundred twenty-three patients (31.0 %) received neo-adjuvant treatment. A median LNY of 10 and 15 (p < 0.0001) and rates of node-positive (N-positive) disease of 31.6 and 36.7 % (p < 0.001) were observed with and without (+/-) neo-adjuvant treatment, respectively. The rate of N-positive disease according to tumour stage ranged from 4.8 %/11.4 % (ypT0/pT1) to 42.1 %/64.1 % (ypT4/pT4). The rate of N-positive disease according to LNY ranged from 19.5 %/16.8 % (0-5 LNs) to 42.6 %/37.9 % (≥18 LNs) (-/+neo-adjuvant treatment). In a logistic regression analysis, a significant association was found between N-positive disease and pT/ypT stage as well as between N-positive disease and LNY.

CONCLUSIONS: A significantly smaller ratio of N-positive disease was observed in the group of patients who had received neo-adjuvant treatment. The ratio of N-positive disease increased significantly with more advanced tumour stage and increasing LNY irrespective of neo-adjuvant treatment. A minimum of 12 LNs is needed to ensure N-negative disease, irrespective of neo-adjuvant treatment.