Rectal cancer is one of the most common malignant disease worldwide. In the National Comprehensive Cancer Network’s (NCCN) clinical practice guidelines, neoadjuvant chemoradiotherapy (nCRT) followed by total mesorectal excision (TME) is recommended as the standard of care for locally advanced rectal cancer (LARC) (1). It has been confirmed that nCRT can improve the local control and overall survival in patients with LARC without increasing the risk of serious complications compared with TME with no prior treatment (2–5). The American Joint Committee on Cancer (AJCC) recommends that at least 12 lymph nodes be examined to ensure accurate tumor staging (6). However, nCRT is known to significantly reduce the number of lymph node yield (LNY) in surgical specimens (7–10). Although this finding has been interpreted by some investigators as indicating a good response to nCRT and therefore a predictor of positive outcome (11, 12), others have suggested that detection of fewer lymph nodes may lead to understaging and staging migration, affecting patient outcomes (13, 14).

In recent years, several multicentre studies, such as the MRC-CLASSIC, COLOR II, and COREAN trials, have demonstrated that laparoscopic resection of rectal cancer is superior to open surgery in short-term efficacy with no significant difference in the aspect of oncological results (15–17). Therefore, laparoscopic surgery in rectal cancer has been rapidly promoted worldwide in the past decade. Laparoscopic surgery has a better surgical field of view than open surgery, making it easier for surgeons to identify anatomical landmarks and enter into the correct anatomical plane. In theory, laparoscopic techniques may potentially lead to some changes in lymph node dissection.

In this study, we investigated the relationship between LNY and survival in LARC patients who underwent laparoscopic TME following nCRT. In addition, characteristics associated with LNY were analyzed to determine whether laparoscopic surgery-related factors have an impact on LNY.

Accurate lymph node pathologic assessment is essential to ensure correct staging and treatment of rectal cancer, and it is the strongest predictor of long-term survival (10, 12). Insufficient LNY may result in tumor understaging and suboptimal treatment, ultimately increasing the risk of tumor recurrence (10, 12–14). Moreover, once tumor recurrence occurs, treatment will be extremely difficult (22, 23). nCRT followed by TME surgery is currently considered the standard of care for LARC, however, it has been demonstrated that nCRT may reduce the number of lymph nodes retrieved (7–10). Mechera et al. demonstrated nCRT resulted in a mean reduction of 3.9 lymph nodes and an average reduction in harvested positive lymph nodes of 0.7 compared with patients who received no neoadjuvant therapy (10). This phenomenon may be due to stromal fibrosis and lymph node shrinkage caused by the inflammatory response induced by neoadjuvant radiotherapy, making it difficult to identify lymph nodes in the resected specimen (24, 25). In addition, there are many factors that may affect the number of lymph nodes retrieved, such as the experience and specialization of the surgeon, the experience and work attitude of the pathologist, the characteristics of the patient (age, sex, obesity, etc) and the disease (stage, site, etc) (26).

In our cohort, the median number of lymph nodes retrieved was reduced to 10 (7 to 13) after receiving nCRT, consistent with previous studies (7–9), and approximately 62% of the patients had fewer than 12 lymph nodes retrieved. In order to specifically explore the correlation between the number of lymph nodes dissected and the prognosis of patients after laparoscopic TME, all patients enrolled in this study underwent laparoscopic surgery. Compared with previous reports (16, 17, 27, 28), this group of patients achieved satisfactory results, with a complication rate of 11.3% and mortality of 0.4% within 30 days, the 3-year OS and PFS of 89.9% and 81.1%, and the 5-year OS and PFS of 83.7% and 76.9%, respectively. In recent years, several multicenter studies, such as the COLOR II and COREAN trials, have confirmed that laparoscopic resection of rectal cancer is superior to open surgery in terms of short-term efficacy, and there are no significant differences in oncologic outcomes (16, 17). Data from the COLOR II study showed a complication rate of 40% after laparoscopic surgery, the median number of lymph node dissected was 13 (10 to 18), and the 3-year OS and PFS were 86.7% and 74.8%, respectively (16). In COREAN study, all patients underwent nCRT, the complication rate after laparoscopic surgery was 21.2%, the median number of lymph nodes dissected was 17 (12 to 22), and the 3-year OS and PFS were 91.7% and 72.5%, respectively (17). Clearly, long-term survival was not worse in our cohort than in these two studies, despite relatively fewer lymph nodes being detected.

At present, the impact of the total number of lymph nodes retrieved on the prognosis of rectal cancer patients after nCRT is still controversial. Some studies have suggested that LNY <12 have no effect on the survival of patients receiving nCRT (29, 30), while other studies have come to the opposite conclusion that LNY <12 is a poor prognostic factor for patient survival (31–33). Based on an analysis of 495 rectal cancer patients treated with nCRT, Wang et al. concluded that an LNY of at least 12 indicated an improved survival, so sufficient LNY was still required after nCRT, especially in patients with potentially poor tumor response (29). Lin et al. reported a cohort study of 837 patients underwent nCRT and showed no significant improvement in OS or PFS with ≥12 lymph nodes dissected compared with less than 12 lymph nodes dissected (33). In our study, after propensity score matching, the clinical characteristics and treatment options of the two groups were generally comparable, and there was no significant difference in 5-year OS and PFS between LNY <12 and LNY ≥12 groups. Therefore, the standard of at least 12 lymph nodes being dissected after nCRT remains to be discussed. Some researchers even suggest that since nCRT can reduce tumor size and result in down-grading of lymph node stage, the reduction of LNY is correlated with overall tumor pathologic regression (11, 12). Bustamante-Lopez et al. reported that the number of lymph nodes retrieved was positively associated with laparoscopic surgery and upper rectal cancer, but negatively related to complete or nearly complete pathologic regression, and TRG was the most important factor for decreased LNY (12).

In order to determine which factors might influence the number of lymph nodes dissected after nCRT in patients with rectal cancer, especially those laparoscopic surgery-related parameters, we conducted a multivariable linear regression analysis. The results showed that only uncontrollable factors such as age, TRG score and ypN stage were closely related to the number of lymph node dissected, while laparoscopic surgery-related factors had no effect on this. Here, we took into account the time span of the operation, the qualification of the surgeon, the type of surgery, the quality of TME, the duration of the operation and the like, but none of these controllable factors showed an association with the number of lymph nodes retrieved. The implication may be that in centers where laparoscopic TME can be performed routinely, the number of lymph nodes retrieved after nCRT in rectal cancer is largely determined by patient or disease factors, and laparoscopic surgery itself has little effect. Therefore, it is questionable whether the detection of at least 12 lymph nodes after nCRT can be used as a standard to measure the quality of laparoscopic surgery, and whether it can be considered for cancer-specific prediction. In our study, we further explored the factors affecting tumor recurrence, and the results showed that only presurgery CEA level and ypN stage were independent predictors of tumor relapse, while whether LNY ≥12 or not showed no connection to tumor recurrence.

In view of the fact that LNY <12 may not be a poor prognostic factor for rectal cancer with nCRT, some researchers have proposed that different criteria should be set for this type of patients compared with those without nCRT (33, 34). Lin et al. recommended that at least 7 harvested lymph nodes may be more appropriate for LARC patients with nCRT (33). La Torre el al pointed out that node-negative patients with six or fewer lymph nodes after nCRT were significantly associated with a poor DFS and OS (34). Other researchers have improved pathologic testing reagents or methods to increase the number of LNY, so as to meet the requirements of the current guidelines (35, 36). Dias et al. reported a randomized trial comparing the Carnoy’s solution and formalin concerning LNY in specimens of LARC patients after nCRT, the results showed that the Carnoy’s solution increased lymph node count and reduced the cases with <12 lymph nodes, and it reduced the formalin cases with <12 lymph nodes from 33.8% to 4.6% and upstaged 2 patients (35). In addition, several studies have suggested that other lymph node-related indicators, such as positive lymph node ratio (LNR) or lymph node regression grade (LRG), are more effective in predicting survival of patients with nCRT (37–40). Sun et al. explored the prognostic impact of LRG in LARC patients following nCRT and radical surgery, which ultimately concluded that higher LRG score was associated with higher TRG, more advanced ypT and ypN stages, and poorer OS and DFS, and LRG was an independent prognostic indicator for DFS in LARC patients after nCRT (39).

To our knowledge, this is one of very few studies specifically focusing on whether LNY <12 has adverse impact on the prognosis of LARC patients undergoing laparoscopic surgery after nCRT. In the meantime, the related factors affecting LNY during laparoscopic surgery were analyzed to determine whether surgery-related parameters would have influence on the number of lymph nodes harvested. Therefore, this study could provide more data for these controversial issues. However, the present study has several limitations. First, due to the retrospective design and small sample size, there must be inherent selection bias, and the generality of the conclusion is uncertain. Second, the time span of this study was 9 years, and the development of new radiotherapy techniques, chemotherapy regimens and laparoscopic techniques may have an impact on the prognosis of patients. However, the deviation caused by these factors was offset to some extent by multivariate analysis. Finally, more than 60% of the patients in this cohort had less than 12 lymph nodes retrieved, which may be related to the sampling method and work attitude of pathologists, and this could be changed by some methods.

Our study identified that for LARC patients who underwent laparoscopic surgery after nCRT, the number of lymph nodes dissected less than 12 has not been proved to be an adverse predictor for long-term survival. There were a number of factors associated with LNY after laparoscopic TME, but mainly patient and disease related factors, such as age, TRG score and ypN stage, while laparoscopic surgery-related factors had no effect on this. Given the relatively small sample size of this study, more convincing conclusions need to be confirmed by more large-scale clinical studies.

The datasets presented in this article are not readily available because: Due to the sensitive nature of the clinical data collected in this study, patients were assured raw data would remain confidential and would not be shared, except under specific request. Requests to access the datasets should be directed to suxiangqian@bjmu.edu.cn.

The studies involving human participants were reviewed and approved by The Research Ethics Committee of Peking University Cancer Hospital & Institute. The patients/participants provided their written informed consent to participate in this study. Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

Study conception and design: HY and XS. Acquisition of data: HY, CZ, ZY, and XW. Analysis and interpretation of data: HY, JX, MC, and BJ. Writing manuscript: HY. All authors contributed to the article and approved the submitted version.