Nowadays, CC is associated with a higher incidence of gastrointestinal cancers and poses a major public health challenge due to its high mortality rate . The AJCC TNM staging system is the most widely applied system in clinical practice to evaluate the survival status, treatment and prognosis of patients. Among them, N stage was divided mainly according to whether there was lymph node metastasis or the number of positive lymph nodes: N0 (no metastatic LNE), N1 (N1a: 1 metastatic LNE; N1b: 2-3 metastatic LNE; N1c: cancer nodule formation) and N2 (N2a: 4-6 metastatic LNE; N2a ≥ 7 metastatic LNE). It can be seen that there is no further stratification in N0 stage. Hence, pN0 stage patients were only stratified according to the T stage, remains a controversial issue.
At present, the number of LNE has been shown to be an independent prognostic factor in multiple cancer types, especially in CC. Higher LNE has been associated with improved survival of pN0 CC patients but the mechanism of the relationship between the two is unclear [6, 9, 14]. Several hypotheses have been proposed. One possible reason is that the greater the number of LNE is associated with a greater chance of a positive node being examined and a more accurate tumor stage [15, 16]. Assessing the number of LNE helps with reducing the likelihood of misclassifying stage III disease as stage I or II and improve prognosis, particularly for pN0 CC patients [17-19]. In addition, an increase in the number of LNE may be an indicator of better treatment, including complete tumor resection and adequate pathological evaluation. Another explanation is that the increase in the number of negative lymph nodes indicates a stronger immune response. Once the immune system detects the presence of tumor cells, local lymph nodes will increase, and more lymph nodes will be easier to be examined in postoperative pathology. Studies have found that LNE are correlated with local neutrophil and lymphocyte infiltration by analyzing the tumor microenvironment . All the above studies proved the relationship between LNE and prognosis through data analysis, but did not specify the optimal stratification of LNE in pN0 CC patients. In this study, the optimal stratification of LNE for CSS was achieved by the X-tile software (N0a: LNE ≥ 26, N0b: LNE = 10-25 and N0c: LNE < 10) and the Kaplan-Meier survival analysis results showed that there were significant differences in prognosis among the three LNE groups (P < 0.001) that proves that our results are meaningful.
The AJCC 8th TNM classification system recommends a minimum of 12 lymph nodes to effectively assess patient survival benefits. The number of LNE can be used effectively as a marker of surgical and pathological adequacy. But LNE are often influenced by tumor location, tumor size and patient age, and especially by the skill of the surgeon and the diligence of the pathologist [12, 20-22]. When the number of LNE is insufficient, the conventional TNM system is used for staging, and patients may be misjudged, especially for those determined as N0 stage cases. The inclusion of the number of LNE in the modified staging system could better stratify patients compared with conventional method to some extent.
In addition, there is a great deal of debate about the number of LNE at least 12. Ning et al. found that the optimal cut-off value of LNE should be 18 in pN0 CC patients .Therefore, the cut-off value of the number of LNE is still controversial. We urgently need a new and convincing staging system for clinical use.
In this study, the optimal stratification of LNE was achieved by the X-tile software (nN stage: (N0a: LNE ≥ 26, N0b: LNE = 10-25 and N0c: LNE < 10) and there were significant statistical differences between the three groups. Subsequently, a modified TNM stage was constructed based on conventional T stage and nN stage. To make the new system more rational in distinguishing patients with different outcomes, all patients were unified into six modified stages (mStage) according to the HRs and survival curves. The KM CSS curves show that the mStage can better classify patients with similar prognosis than the conventional stage. In addition, the AUC and C-index of mStage were significantly higher than those of conventional TNM staging system in both development and validation cohorts, indicating that the mStage has potential advantages over conventional stage in predicting survival.
There are several innovations in our research. First of all, the selection of LNE cut-off value took into account the patients with insufficient LNE, making the nN stage system more universal. Then, we further analyzed the prognostic interaction between nN stage and conventional T stage and constructed a modified staging system for pN0 CC patients, which showed superior predictive power compared with conventional TNM staging system. Finally, we did validation cohort to make our results more convincing.
This study has several limitations. Firstly, we proposed stratification of LNE for the first time, while there was no consensus on stratification results, which may limit the application and promotion of the mStage system. Secondly, this study is a retrospective analysis, which needs to be further verified by some prospective clinical studies. Thirdly, the sample size of the validation cohort seems to be insufficient, requiring a larger sample analysis to verify the accuracy of the modified staging system in the future.
In conclusion, the mStage system could predict the prognosis of pN0 CC patients and showed superior predictive power compared with conventional TNM staging system.