Significance of accurate hilar and intrapulmonary lymph nodes examination in stage IA-IIA non-small cell lung cancer, a retrospective cohort study CURRENT STATUS:

Background The examination of lymph node plays an important part in the nodal staging of nonsmall cell lung cancer (NSCLC). Till present, on the role of hilar and intrapulmonary (N1) station lymph node (LN) examined is not fully appreciated. In this study, we aimed to confirm the significance of N1 lymph node examined in the long-term survival for stage IA-IIA NSCLC patients and find the minimum number of lymph nodes. Methods The data of patients who underwent radical lobectomy and confirmed as lymph node non-metastatsized from January 2008 to March 2018 were retrospectively screened. Pathology records were reviewed for the number of lymph nodes examined. Kaplan-Meier method and Cox regression model were used to identify survival and prognostic factors. Results The median number of resected N1 LNs was 8. The number of patients with 0-2 N1 LNs, 3-5 N1 LNs, 6-8 N1 LNs, 9-11 N1 LNs and more than 11 N1 LNs examined was 181, 425, 477, 414 and 531, respectively. Gender (P=0.004), age (P<0.001), tumor size (P=0.004), differentiation degree (P=0.001) and the number of N1 LNs examined (P=0.008) were the independent prognostic factors of overall survival. Gender (P=0.006), age (P=0.031), tumor size (P=0.001), differentiation degree (P=0.001), vascular invasion (P=0.034) and the number of N1 LNs examined (P=0.007) were the independent prognostic factors of disease-free survival. vascular invasion 95%CI 1.031-2.200) were negatively correlating were positively correlated with N1 LNs

Inadequate lymph node (LN) examination may be an important risk factor for recurrence. (6) The examination of LN plays an important role in the accurate node staging and node-positive patient needs adjuvant therapy to reduce their risk of recurrence. (7) There still is no consensus regarding the optimal number of perioperative LN to be retrieved for pathological examination. Some studies evinced that a minimum of 10 LNs should be examined to guarantee an accurate N stage. (8)

Follow-up
In this study, the dissection of N1 and N2 LN stations were both performed by the surgeons and reconfirmed by pathologists. The information about the number of lymph nodes examined and other pathological factors were reviewed from pathology reports. Follow-ups were performed every 3 months in the first two years, every 6 months until 5 years and once a year thereafter. Surgeons would prescribe between chest radiography, ultrasonography or computed tomography scan as needed during follow-up. The study endpoint was OS and DFS, which were defined as the time from surgery to death and the time from surgery to first locoregional or distant recurrence or to death, respectively.

Statistical analysis
All statistical analyses were performed using the SPSS software, version 22.0 (SPSS, Inc., Chicago, IL).
Analysis of Variance (ANOVA) was performed to compare quantitative data and Pearson's χ2 or Mann-Whitney U test were used to compare categorical data between five groups. The OS and DFS were estimated by using the method of Kaplan-Meier and compared using the log-rank test. Univariate and multivariate Cox proportional hazards regression analysis was used to identify prognostic factors for survival. Variables assessed in this study included: age, gender, tumor size, smoking history, 8th TNM stage, histology, differentiation degree, visceral pleura invasion, vascular invasion, adjuvant chemotherapy, number of N2 LNs examined, number of N1 LNs examined, thoracotomy or videoassisted thoracoscopic surgery (VATS). The number of N2 LNs examined was treated as rank variable categorized into 5 groups, namely, 0 to 2, 3 to 5, 6 to 8, 9 to 11, 12 to 15 and more than 15. The number of N1 LNs examined was also treated as rank variable categorized into 5 groups, namely, 0 to 2, 3 to 5, 6 to 8, 9 to 11 and more than 11. Variables whose P value were < 0.1 in univariate analyses were included in multivariate analyses. P values < 0.05 were considered statistically significant and all hypotheses were two-sided.
As shown in Table 2   There are some potential explanations for the survival advantage brought by the larger number of N1 LN examined. An increasing number of N1 LNs examined would lead to a greater probability of discovering metastasized LNs in the hilar and lung, leading to stage migration; considered to be the main role in the improvement of OS and DFS in patients with large number of N1 LNs examined.
Inadequate LNs examination may result in some metastatic lymph nodes not being detected and patients would be wrongly staged as IA or IB. Actually, this part of patients should be staged as IIB and received adjuvant therapy. Resecting micrometastases and the effect of immunologic microenvironment may be also related with survival advantage brought by the larger number of N1 LN examined. (16)(17)(18)(19) In this study, patients received sublobectomy were excluded which means all intrapulmonary lymph nodes are removed along with the lobes. Resecting micrometastases did not have significant impact on survival advantage brought by the larger number of N1 LN examined.  Therefore, we recommend at least 6 N1 LNs examined in surgical and pathological management.
However, the examination of N1 lymph node has not received enough attention. One of the manifestations was that that the quality of LN examination exists noteworthy variability during surgical and pathological management. (25,26) Another manifestations is that incomplete intrapulmonary lymph node retrieval in pathological examination. One previous study revealed that a median of six additional LNs were discovered after rechecking remnant lung specimens and the median number of N1 LNs examined was only 3 in the community-based Memphis Metropolitan Area Quality of Surgical Resection cohort. (27) Although with superb quality of N2 LNs examination, the median number of N1 LNs examined was 5 in the ACOSOG Z0030 trial. (21) In this study, the median number of resected N1 LNs was eight. The pattern of LN examination that N1 LNs were dissected by the surgeons and reconfirmed by pathologists contributed to this result.
There are some limitations exist in this study. First, this was a single-center retrospective study and associated biases may have been inevitable. Second, external validation was not performed to validate the findings. In addition, data of this study did not find the survival advantage from the increase in the number of N2 LNs examined and cannot answer how many N2 LNs should be examined in surgical and pathological management. Therefore, further validation from multicenter database is needed and meanwhile, the findings from this study should be cautiously interpreted.

Conclusions
Increasing the number of N1 LNs examined could improve the OS and DFS of T1-2N0 NSCLC patients.

Ethics approval and consent to participate
This study was approved by the Institutional Review Board of Sun Yat-sen University Cancer Center.

Consent for publication
collected and interpreted the data. WYZ carried out the statistical analysis. FFD and YZZ critically revised the manuscript. All authors read and approved the final manuscript.