Our previous study reported the technique of fascial space priority approach in laparoscopy lateral pelvic lymph node dissection.  We found an interesting phenomenon, 5 cases in 42 patients (11.9%) showed lateral lymph node metastasis but no rectal mesenteric lymph nodes metastasis.This phenomenon shows that lateral lymph nodes could be one of the important ways of rectal lymph drainage,even one of the sentinel lymph nodes, lymph nodes should be brought to the equal attention with mesenteric lymph nodes. This phenomenon have been reported in previous studies. [7-9] Lymphatic drainage of the lower rectum passes to external pelvic (inguinal area) or pelvic (iliac vessels and anterior sacral) lymph nodes, or to the root of IMA(inferior mesenteric ) along the superior rectal artery.Akiyoshi found that the prognosis of patients with lymph node metastasis in the external iliac artery region was not statistically different from that of N2a (OS: 45% vs 45%, P = 0.9585;CSS: 51%vs 49%, P = 0.5742), while the prognosis of patients with lateral lymph node metastasis in the internal iliac artery region was not statistically different from that of N2b (OS: 32% vs29%, P = 0.3342).CSS: 37% vs 34%, P = 0.4347) . This suggests that the lateral lymph nodes should belong to local lymph nodes.Our study also confirms this conclusion. lymph nodes dyeing technology(lymphatic mapping technology ) can be adopted to study drainage direction of low rectal cancer. 
Studies about prognosis of patients with lateral lymph node metastasis but no mesorectum lymph nodes metastasis is rare.Takahashi found that 5-year survival rate of patients with lateral lymph node metastasis but no mesorectum lymph nodes metastasis is 75%, and 5-year survival rate of those with no lateral lymph node metastasis and mesorectum metastasis,mesorectum metastasis but no lateral lymph node metastasis,both lateral lymph node metastasis and mesorectum metastasis were 90.1% ,67.7% and 32%,respectively.This means that the prognosis of patients with lateral lymph node metastasis but no mesorectum metastasis may be worse than that of patients with no lateral lymph node metastasis and mesorectum metastasis but better than that of patients with both lateral lymph node metastasis and mesorectum metastasis.  Akiyoshi thought lymph node metastasis located in the medial of internal iliac artery should be classified as N2a and those located in the lateral of internal iliac artery artery should be classified as N2b .However, the prognosis of patients with lateral lymph node metastasis but no mesorectum metastasis is far better than that of patients with both lateral lymph node metastasis and mesorectum metastasis. So lateral lymph node staging should be more specific to verify a more accurate prognosis and formulate postoperative treatment strategy.But more studies about survival of patients with lateral lymph node metastasis but no mesorectum lymph nodes metastasis are needed to confirm this.
In addition, the phenomenon patients with lateral lymph node metastasis but no mesorectum lymph nodes metastasis also illustrates that the lateral lymph node dissection is of great significance for postoperative pathological diagnosis.If the lateral lymph nodes dissection were not undergone, the lymph node staging would be N0,and the pathology of tumor staging in patients would be stageⅡ. If there were no risk factors, adjuvant chemotherapy after surgery might not even be recommended . But the postoperative pathology showed lateral lymph node metastasis patients actually exised, and the tumor staging should be classified as stage Ⅲwith a similar prognosis to N2a or N2b tumors and standardized postoperative adjuvant chemotherapy after surgery. Misleading staging would affects the clinical doctors developing wrong postoperative adjuvant treatment plan and making wrong prognosis judgement.LLND can change the patient's pathological staging.for low rectal cancer, CRT should be adopted if preoperative imaging reveals lateral lymph node enlargement and no mesorectum lymph node enlargement,and LLND should follow the CRT if the lateral enlarge lymph nodes do not shrinkage.If postoperative pathology after TME shows no mesenteric lymph node metastasis, the possibility of postoperative follow-up of lateral lymph node enlargement still deserves attention.The principle of “watch and wait” was proposed in NCCN guideline update 2020 ,which suggested no surgery temporarily and strict observation and follow-up for patients with clinical tumor complete remission after neoadjuvant chemoradiation. Our study shows that lateral lymph node metastasis should be payed more attention after neoadjuvant chemoradiation in addition to focusing on local lesion and mesenteric lymph nodes metastasis.
Lymph nodes in the iliac artery region are classified as regional lymph nodes by AJCC colorectal cancer staging guidelines, but those in obturator artery region are regarded as distant metastasis lymph nodes,which means the tumor staging would be M1 if lymph node metastasis in obturator artery region exists.However,according to our study,there was 2 cases in the 5 patients with obturator lymph nodes metastasis but no iliac artery lymph nodes and mesenteric lymph nodes metastasis, which means lymph node metastasis in obturator region may be earlier than that in the iliac region.So according to our finding, obturator lymph nodes should also be classified as regional lymph nodes,but more studies will be needed to confirem this conclusion.
However there are some shortages of our study.We just found this phenomenon, conducted a retrospective study of a few of cases.Our sample size is small and the follow-up time is short,and the prognosis of these patients is unable to evaluate.The studies about the characteristics and prognosis of the patients with lateral lymph node metastasis but no mesorectum lymph nodes metastasis remain rare,and a lot of researches will be needed to clarify these problems. And it is well worth studying whether LLND surgery can improve the survival of these patients.