Baseline data of 358 patients included in the retrospective analysis

This retrospective study included 358 patients (N1: 9.5%, N2: 32.1%, and N3: 58.4%) with confirmed NSCLC. The majority of patients were men (67.1%), and the average age was 63.2 years (range, 27–96 years). Of the total patients, 230 (64.2%) were aged ≥ 60 years. Primary lesions developed in the left lung in 45% of the patients (upper lobe, 28.8%; lower lobe, 16.2%) and the right lung in 55% of the patients (upper lobe, 26.3%; middle lobe, 5.8%; lower lobe 22.9%). The most common types of lung cancer were adenocarcinoma (62%) and squamous cell carcinoma (33.3%). The average maximum tumor diameter was 4.59 cm (range, 0.9–16 cm). Local invasion was observed in 233 patients (65.1%), with visceral pleura being the most common site. Distant metastases occurred in 55.6% of patients, most commonly affecting the bones and non-regional lymph nodes. The proportion of patients with T stage was as follows: T1a (0.3%), T1b (8.4%), T1c (8.1%), T2a (19.5%), T2b (9.5%), T3 (20.4%), and T4 (33.5%). One patient had an indeterminate T stage due to the challenges in measuring tumor size. The proportion of patients according to TNM staging was as follows: IIB (2.8%), IIIA (12%), IIIB (16.2%), IIIB (13.4%), IVA (31%), and IVB (24.6%). The patients’ baseline data are presented in Table 1.

Table 1 Characteristics of the patients with NSCLC (N = 358)Variations in the distribution of metastatic lymph nodes in different regions

In the present study, we investigated the distribution of LNM in patients with primary lung tumors in different regions. The findings showed variations in the levels of lymph node involvement based on tumor location.

In patients with tumors in the right upper lung, the most common LNM sites were the 10R, 4R, 2R, 7, and 1R. In patients with tumors in the right middle lung, the top five LNM sites were 10R, 4R, 2R, 7, and 1R. In patients with tumors in the right lower lung, the most frequent sites of LNM were 10R, 7, 4R, 2R, and 8. In patients with tumors in the left upper lung, the top five LNM sites were 10L, 4L, 5, 4R, and 6. Lastly, in patients with tumors in the left lower lung, the most common sites of LNM were 10L, 7, 4L, 4R, 5, and 8. Overall, the location of the lung tumor influenced the LNM patterns. The left lung had a higher likelihood of contralateral hilar and contralateral supraclavicular LNM compared with the right lung. Additionally, metastases to the opposite mediastinal lymph nodes were more common in the left than in the right lung. However, no significant difference was found in the prevalence of ipsilateral hilar LNM between the two groups. All P-values were < 0.05. All data are presented in Fig. 1 and Supplementary Tables 1 A, 1B and 2.

Fig. 1

Distribution patterns of metastases in different lymph node levels by lung lobe. The lymph node levels classified according to the IASLC Staging Project in 2009: 1L, 1R, 2L, 2R, 3a, 3p, 4L, 4R, 5, 6, 7, 8, 9, 10L, and 10R

Correlation analysis of metastatic lymph nodes in different levels (left lung, LL)

To further investigate the association between different levels of metastatic lymph nodes in patients with left lung tumors, we initially performed univariate analysis followed by subsequent multivariate analysis based on the findings from the univariate analysis. (Table 2 and Supplementary Tables 3 A and B). The average p-value of Hosmer–Lemeshow test in the logistic regression was 0.635.

Table 2 Correlation of LNM among different levels in the left lung cancer patients (N = 161). The lymph node levels were classified according to the IASLC Staging Project in 2009: 1L, 1R, 2L, 2R, 3a, 3p, 4L, 4R, 5, 6, 7, 8, 9, 10L, and 10R. The most common LNM sites were 10L, 4L, 5, 4R, and 7. + LNM positive level, + + LNM in both two levels were considered positive

The correlation analysis confirmed significant correlations between 1L and 1R, 2L, and 4L; as well as associations between 1R and 1L, 2R, and 4L (all P < 0.05). Similarly, the analysis revealed significant correlations between 2L and 1L and 1R, while also showing correlations between 2R and 1R, 4L, and 8 (all P < 0.05). In addition to this, the correlation analysis demonstrated a specific correlation of level 3a with invasion in level 6 (P = 0.001), while no correlation was found for level 3p with any other lymph node levels. Furthermore, the presence of LNM in level 4L was significantly correlated with metastasis in 1L, 1R, and 4R; whereas metastasis in 4L, 7, and 10R was significantly correlated with metastasis in 4R (all P < 0.05). The further study revealed that level 6 was correlated with level 5 (P = 0.007), and also showed that level 6 had a correlation with 3a and 5 (all P < 0.05). Additionally, it was confirmed that 4L, 4R, 8, 9, and 10L were correlated with level 7 (all P < 0.05); level 8 was correlated with 2R and 7 (all P < 0.05).; and for level 9, the analysis revealed a significant correlation with level 7 (P = 0.032). Finally, the correlation analysis indicated a relationship between level 7 (P = 0.033) and 10L, and a particular correlation between 4R (P = 0.0001) and 10R.

Correlation analysis of metastatic lymph nodes in different levels (right lung, RL)

We applied the same univariate and multivariate analysis techniques used for left lung primary tumors to examine the correlations between different levels of metastatic lymph nodes in patients with primary tumors in the right lung. (Table 3 and Supplementary Table 4 A and B). The mean p-value of the Hosmer-Lemeshow test in logistic regression was 0.690.

Table 3 Correlation of LNM among different levels in patients with right lung cancer (N = 197). The lymph nodes levels classified according to the IASLC Staging Project in 2009: 1L, 1R, 2L, 2R, 3a, 3p, 4L, 4R, 5, 6, 7, 8, 9, 10L, and 10R. The most common LNM sites were 10R, 4R, 7, 2R, and 1R. + LNM positive level, + + LNM in two levels were considered positive

Correlation analysis confirmed that the presence of 1L LNM was associated with 1R, 3a, 4L, and 6 (all P < 0.05). Additionally, 1R showed a significant relationship with 2R (P = 0.0001). Subsequent analysis revealed that the presence of 2L LNM was correlated with 5 and 6, while 1R, 3a, and 4R were found to be significantly associated with 2R (all P < 0.05). The study further indicated that the only significantly related level to 3a was 2R (P = 0.020); meanwhile, level 6 (P = 0.026) demonstrated a correlation with level 3p. Furthermore, it was observed that the presence of LNM in level 4L was linked to 1L, 4R, 5, and 7 (all P < 0.05); similarly, LNM in 4R showed a significant association with 2R and 4L (all P < 0.05). Further analysis revealed that the presence of level 5 LNM was related to 2L, 4L, and 10L, while level 6 was related to 1L, 2L, and 3p (all P < 0.05). Additionally, it was found that level 7 LNM was significantly associated with 4L and 8, while 5 and 9 were found to be related to level 8 (all P < 0.05). Only level 8 demonstrated a significant relationship with level 9 LNM (P = 0.004). Finally, it was discovered that there was a specific correlation between 10L and 5 (P = 0.002), while no significant relationship was found for 10R with any other lymph node levels in either univariate or multivariate analysis.