Ann Thorac Surg 2008;85:1026-1031. doi:10.1016/j.athoracsur.2007.10.076
© 2008 The Society of Thoracic Surgeons
Original Articles: General Thoracic
Superior and Basal Segment Lung Cancers in the Lower Lobe Have Different Lymph Node Metastatic Pathways and Prognosis
Shun-ichi Watanabe, MD*,
Kenji Suzuki, MD,
Hisao Asamura, MD
Division of Thoracic Surgery, National Cancer Center Hospital, Tokyo, Japan
Accepted for publication October 23, 2007.
* Address correspondence to Dr Watanabe, Division of Thoracic Surgery, National Cancer Center Hospital, Tsukiji 5-1-1, Tokyo 104 0045, Japan (Email: syuwatan{at}ncc.go.jp).
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Abstract
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Background: Although the lower lobe is a large entity that occupies half of the hemithorax, all tumors located within the lower lobe have been treated uniformly regardless of tumor location. The aim of this study was to reveal differences in the metastatic pathway to the mediastinum and in prognosis of N2 disease between lung cancers originating from superior and basal segment of the lower lobe.
Methods: Data on 139 patients who underwent pulmonary resection with systematic nodal dissection for pN2 non-small cell lung cancer (NSCLC) originating from the lower lobe between 1980 and 2001 were retrospectively reviewed. Those lower lobe N2 tumors were divided into two groups by origin: 51 were superior segment, and 88 were basal segment.
Results: The superior segment group showed a significantly higher incidence of superior mediastinal metastasis than the basal segment group (64% vs 36%, p = 0.0012). When superior mediastinal metastasis existed, the basal segment group showed a significantly higher incidence of synchronous subcarinal metastasis than the superior segment group (81% vs 39%, p = 0.0006). Pneumonectomy was required significantly more often in the superior segment group than in the basal segment group (45% vs 17%, p = 0.0003). The basal segment origin tumors with only subcarinal metastasis showed significantly better prognosis than other lower lobe N2 tumors (5-year survival, 43% vs 18%; p = 0.0155).
Conclusions: Basal segment tumor metastasizes to the superior mediastinum mostly through the subcarinal node, whereas superior segment tumors often metastasize directly to the superior mediastinum without concomitant metastasis to the subcarinal node. Superior mediastinal dissection will be mandatory for accurate staging of superior segment tumors even when the subcarinal node is negative on frozen section. As for the prognosis among lower lobe N2 tumors, only in cases with basal segment tumor without superior mediastinal metastasis may long-term survival be expected.
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Introduction
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The lower lobe is a large entity that occupies half of the hemithorax in each side. Its bottom rests just on the diaphragm and the apex reaches above the hilum. However, all tumors located within the lower lobe have been treated uniformly, regardless of location in the lobe. With respect to the anatomic structure, the lower lobe can be primarily divided into two segments, the superior and the basal. We hypothesized that tumors arising in those two lower lobe segments were not alike owing to differences in lymphatic drainage pathways or other clinical behaviors and thus may require different treatment strategies. We therefore investigated segment-specific patterns of nodal spread and prognosis of pN2 disease in each segment. This report describes the differences in the clinical features and prognosis between superior and basal segment tumor of the lower lobe.
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Patients and Methods
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Patients
Approval for this retrospective study was obtained and the need for individual patient consent was waived by the Institutional Review Board. From January 1981 to December 2001, 3638 patients underwent pulmonary resection for primary lung cancer at the National Cancer Center Hospital. Basically, we operate on the lung cancer patient who is considered to be cN0 to 1 on computed tomography (CT) scan. Our criterion for lymph node enlargement is more than 1.0 cm in the short axis of each nodal station on CT. Mediastinoscopy, mediastinotomy, or positron emission tomography scan were not routinely used preoperatively.
We retrospectively reviewed 139 patients (3.8%) who underwent at least lobectomy and systematic nodal dissection (SND) for lower lobe tumor in either the right or left lung and who had histologic evidence of non-small cell lung cancer (NSCLC) with mediastinal lymph node metastasis (pN2). We excluded the patients who underwent only sampling or selective nodal dissection. The study excluded tumors that crossed the fissure and invaded multiple lobes or other organs and huge tumors more than 5 cm in size. All patients underwent at least lobectomy with hilar and mediastinal lymphadenectomy.
Patients were subdivided into two groups according to origin: superior segment (n = 51) and basal segment (n = 88). The correlation between the segment of the tumor location and the involved hilar/mediastinal nodes were investigated in each case. The location of the tumor was identified by the involved bronchus in the resected specimen. When the tumor involved both the superior and the basal segments, the patient was placed in the superior segment group.
Surgical Procedure
Pulmonary resection and systematic nodal dissection were performed through posterolateral thoracotomy. At thoracotomy, the diagnosis was confirmed by frozen-section analysis when histologic confirmation was not available preoperatively. When the hilar nodes involved the upper lobe bronchus or pulmonary artery, or both, pneumonectomy was done. Systematic nodal dissection, including the superior and inferior mediastinum, was then performed after pulmonary resection. In left thoracotomy, superior mediastinal lymph nodes indicated the 5, 6, and 4L nodes. In right thoracotomy, superior mediastinal lymph nodes indicated the 1, 2R, and 4R nodes. Inferior mediastinal lymph nodes indicated the 7, 8, and 9 nodes in both side thoracotomies. Histologic analysis of lymph node metastasis was made by hematoxylin and eosin stain.
Statistical Analysis
Survival was calculated by the Kaplan-Meier method, and differences in survival were determined by the log-rank test. Zero time was the date of surgery, and the terminal events were death due to cancer, noncancer, or unknown causes. A multivariable analysis of independent prognostic factors was done by using Cox’s proportional hazards regression model. Relative risk and 95% confidence intervals were calculated. Proportions were compared by means of 
2 analysis. Values of p < 0.05 were considered to be statistically significant.
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Results
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Patient Characteristics
Patient characteristics are summarized in Table 1. The tumor cell types were adenocarcinoma in 94 (68%), squamous cell carcinoma in 37 (27%), and others in 8 (5%). The segments of origin were the superior segment in 51 (37%), in 35 of whom the tumor was on the right side, and basal segment in 88 (63%), in 51 of whom the tumor was on the right side. The size of the primary tumor was less than 3 cm in 65 patients (47%).
Patterns of Nodal Spread
Significant differences in patterns of lymphatic pathways on both sides were found when the superior and basal segment groups were compared (Table 2). The basal segment group showed significantly higher incidence of subcarinal metastasis than the superior segment group (80% vs 57%, p = 0.0044). The superior segment group showed significantly higher incidence of superior mediastinal metastasis than the basal segment group (64% vs 36%, p = 0.0012; Table 2). When superior mediastinal metastasis existed, the basal segment group showed a significantly higher incidence of synchronous subcarinal metastasis than did the superior segment group (81% vs 39%, p = 0.0006; Table 3).
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Table 2 Location of the Primary Tumor in the Lower Lobe and Incidence of Subcarinal and Superior Mediastinal Node Involvement
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Table 3 Location of the Primary Tumor in the Lower Lobe and Incidence of Synchronous Metastasis to the Superior Mediastinal and Subcarinal Nodes
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Differences in Surgical Procedure
The superior segment group more frequently required pneumonectomy than the basal segment group, with a significant difference (45.1% vs 17.0%, p = 0.0003), but there was no significant difference in the ratio of T1/T2 between the groups (24 of 21 vs 41 of 47, p = 0.9021; Table 4).
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Table 4 Types of Surgical Procedure for Lower Lobe pN2 Disease According to the Segment of the Primary Tumor in the Lower Lobe
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Group Differences in Prognosis of N2 Disease
Overall 5-year survival of patients with lower lobe N2 tumor was 27.9%. The 5-year survival of the basal segment group was better than for the superior segment group (32.9% vs 19.9%); however, the difference was not significant (p = 0.1308; Fig 1).
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Fig 1. Survival of patients with pN2 tumors located in the basal and superior segment of the lower lobe. The 5-year survival was 32.9% for the basal segment group (black line) and 19.9% for the superior segment group (gray line), but the difference was not significant (p = 0.1308).
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Among the basal segment group, the patients without superior mediastinal metastasis showed significantly better prognosis than did those with it, with a 5-year survival of 42.7% vs 15.6% (p = 0.0453; Fig 2A).
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Fig 2. Survival of patients with lower lobe pN2 tumors with and without superior mediastinal metastasis in tumors with (A) basal segment origin and (B) superior segment origin. (A) In the basal segment group, the patients without superior mediastinal metastasis (black line) showed significantly better prognosis than did those with (grey line) superior mediastinal metastasis (5-year survival, 42.7% vs 15.6%, p = 0.0453). (B) Survival of patients with superior segment pN2 tumors grouped by the extent of lymph node metastasis. No significant differences in survival were detected between patients without superior mediastinal metastasis (black line; 5-year survival, 25.4%) and with superior mediastinal metastasis (5-year survival, 16.5%: with only superior mediastinal node metastasis (dark gray line, 20.0%; with superior mediastinal and subcarinal metastasis, light gray line, 10.3%; p = 0.1623).
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In the superior segment group, no significant differences in survival were detected between patients with and without superior mediastinal metastasis: at 5 years, the survival was 25.4% for those without and 16.5% for those with, survival with only superior mediastinal node metastasis was 20.0%; and with superior mediastinal and subcarinal metastasis, 10.3% (p = 0.1623; Fig 2B).
Collectively, the basal segment origin tumors with only subcarinal metastasis showed significantly better prognosis than other lower lobe N2 tumors (5-year survival 43% vs 18%, p = 0.0155).
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Comment
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The lower lobe has a large volume of lung parenchyma, including 5 segments in the right and 4 segments in the left, and occupies half of the hemithorax in each side. Despite the extensive size of the lower lobe, all tumors located there have been treated similarly, regardless of whether the tumor originated in the superior or the basal segments. Owing to a lack of information on the variations in clinicopathologic features between tumors located in the superior and basal segments, we conducted the present study to investigate the differences in patterns of lymph node metastasis and prognosis of each segment.
Our results on the metastatic pathway showed a possibility that basal segment tumors metastasizing to the superior mediastinum mostly went through the subcarinal node, whereas SS tumors often metastasized directly to the superior mediastinum without concomitant metastasis to the subcarinal node (Tables 2 and 3). Furthermore, the patterns of metastatic pathway in the right and left side were identical (Tables 2 and 3). The schemes demonstrating a possibility of the main stream of lymphatic spread in each segment on the basis of these results are shown in Figure 3. Perhaps superior segment tumors tend to metastasize directly to the upper mediastinum owing to the anatomically shorter distance between these sites compared with the longer distance between the basal segment and the upper mediastinum. Alternatively, for basal segment tumors, the subcarinal node could be a barrier on its metastatic way to the upper mediastinum.
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Fig 3. The scheme of the main stream of lymphatic spread of the tumor in each segment. (A) Basal segment tumor metastasizes to the superior mediastinum mostly through the subcarinal node. (B) Superior segment tumor often metastasizes directly to the superior mediastinum, without concomitant metastasis to the subcarinal node.
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These factors might contribute to the higher incidence of pneumonectomy in superior segment tumors than that in basal segment tumors. Probably in many patients in the superior segment group, interlobar nodes that were located on the way from the primary site to superior mediastinum were involved. That is, superior segment tumor seems to metastasize to the superior mediastinum by involving the nodes adjacent to the bronchus and pulmonary artery of the upper lobe in consideration of the anatomy of the hilum. Then, this point will lead to the high incidence of pneumonectomy in the superior segment group.
Extensive nodal dissection, including the superior and inferior mediastinum, has been universally performed in lung cancer operations [1, 2]. This technique, termed "systematic nodal dissection" remains an important component of the investigative and therapeutic process in all patients undergoing thoracotomy for lung cancer [3–5]. However, because the number of lung cancers detected early is increasing with the development of CTs scanners, a new therapeutic strategy for selective nodal dissection is required instead of systematic nodal dissection [6, 7]. The extent of nodal dissection could be tailored according to the tumor location–specific patterns of nodal spread [8].
Riquet and associates [9] reported that lung cancer metastasizes so easily to the mediastinum that selection of the patients for limited surgical intervention should be discussed carefully. Some previous reports have described the appropriateness of selective nodal dissection based on the lobe-specific extent of nodal spread [7, 10, 11]. Okada and associates [11] reported that superior mediastinal dissection might be unnecessary for lower lobe tumors when the subcarinal node was negative. Our results support their conclusion for basal segment tumor; however, for the superior segment tumor, our results reveal that superior mediastinal dissection should be mandatory for accurate staging even when the subcarinal node is negative.
The prognosis for patients with superior segment tumors was worse, with a 5-year survival of 20% compared with 33% for patients with basal segment tumors, although this difference was not statistically significant. Poor survival rates may be attributed to the increased incidence of pneumonectomy in superior segment tumors (45%) compared with 17% for basal segment tumors (Table 4). Although the prognoses of patients with superior mediastinal metastasis from superior and basal segment tumors of the lower lobe were dismal, with respective 5-year survivals of 17% and 16%, the patients with basal segment N2 tumors who had only subcarinal metastasis showed significantly better 5-year survival of 43%, an acceptable result, compared with other lower lobe N2 patients. This will be mainly because they have metastasis to a single N2 station with an anatomically shorter distance from the primary site. Only in this small subgroup of lower lobe N2 patients, those with tumors of basal segment origin and having no superior mediastinal metastasis, may long-term survival be expected.
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References
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Abstract
Introduction
