Lymph node metastasis is the most common and main metastasis pathway of colorectal cancer, and it is also an important indicator of staging and prognosis of colorectal cancer . The value of lymph node dissection around root of IMA is still disputed. Many studies have reported that D3 dissection can reduce paraaortic recurrence and systemic metastasis , and improve the prognosis [3, 4]. On the other side, some studies believe that the lymph node metastasis rate of IMA-LNs is relatively low, even after resection this kind of patients suggest poor prognosis, so it is of less significance to be resected [5–7].
1.Risk factors of IMA-LN
Our study showed that the positive rate of IMA-LN was related to distance from anal verge, CEA level, tumor size, pathological type, differentiation, nerve invasion, T stage and N stage. The result is similar to the previous reports . Sun et al.  pointed out that for rectal cancer, neoadjuvant chemoradiotherapy can reduce the lymph node metastasis rate of IMA-LNs. For the patients who received neoadjuvant chemoradiotherapy, the location of the lesion above peritoneal reflexes, low degree of tumor differentiation and high preoperative serum CEA level were the risk factors of positive IMA-LNs. Nagasaki et al.  found that for patients with stage III colon cancer, serum CEA level, T stage, number of lymph node dissection will significantly affect the positive situation of the third station lymph nodes (including IMA-LNs). Multivariate Logistic regression analysis showed that only four factors (distance from anal verge, CEA level, differentiation, and T stage) were independent risk factors for positive IMA-LNs. there is no clear evidence that different locations of the lesion in the rectum affect the lymph node metastasis rate. It is worth noting that the lymph node metastasis rate of sigmoid colon tumor is significantly higher than that of rectal tumor [6, 11].
2. The influence of IMA-LN on TNM staging
The AJCC staging is determined by the number of lymph nodes rather than the distance from the tumor [12, 13]. There are few studies and reports about the effect of IMA-LNs on TNM staging . The positive IMA-LNs can aggravate the severity of the original stage III patients [15, 16]. Some surgeons believe that IMA-LNs metastasis can occur in T2, 3, 4 colorectal tumors, and there may be skip metastasis. Therefore, IMA-LNs should be routinely removed for colorectal tumors beyond T1 [17, 18]. But in this study, we did not find N positive was caused by only IMA-LN, that means no skip metastasis. It also means that if IMA-LNs turns negative, TNM staging will not be reduced. Does it mean that IMA-LN is not the origination of metastasis, but just the destination or interchange station?
3. Benefits of high ligation
Whether lymph node dissection around IMA can benefit patients is still uncertain, which may be the reason why the guidelines differ in this respect. Since there is no clear evidence that D3 lymph node dissection can benefit patients, the European and American guidelines do not consider it necessary to perform routine third station lymph node dissection . High ligation has been reported to be effective in oncology, it can reduce paraaortic recurrence and systemic metastasis, and improve the prognosis of some patients . But from the point of view of complications such as anastomosis leakage and postoperative physiological dysfunction, it seems that high ligation is slightly worse than low ligation [21, 22]. For laparoscopic or robotic assisted radical surgery for colorectal cancer, the guidelines are conservative and not recommended as a routine recommendation. Only doctors with relevant experience should be recommended. At the same time, tumor staging, lymph node metastasis and surgical difficulty should be considered comprehensively . Many studies think that there is no significant difference between high and low ligation [24–26]. In our study, from the pathological features, the benefit of high ligation with low tumor location is limited.
4. The area to be resected in high ligation
Although the range of lymphadenectomy is controversial in different guidelines, the importance of lymphadenectomy is consistent. According to Japanese Society for Cancer of the Colon and Rectum (JSCCR) guidelines for the treatment of colorectal cancer, IMA-LNs are defined as the lymph nodes from the root of IMA to the beginning of LCA and along the IMA . Similarly, follow the principles of CME, the scope of dissection is around the root of IMA, but it often goes beyond the boundary in real operation. It is possible that part of the retroperitoneal tissue may be removed due to excessive traction. So we need further research to define such a region.