Abstract

Background: The aim of this study was to evaluate the degree and size of pleural invasion in non-small cell lung cancer patients and to compare its relationship with the survival time.

Methods: Between January 2008 and June 2019, a total of 164 patients (143 males, 21 females; median age: 64.65 years; range 39 to 92 years) who underwent surgical resection with a diagnosis of non-small cell lung cancer and who were found to have pleural invasion histopathologically were retrospectively analyzed. The control group consisted of 105 patients (95 males, 10 females; median age: 61.7 years; range, 32 to 82 years) who underwent surgical resection but who were not found to have pleural invasion histopathologically during the same time period. Survival time was compared between the groups.

Results: Median survival was 52 months in the group with pleural invasion, while it was 70.6 months in the group without pleural invasion. In the pleural invasion group, the patients who underwent sublobar resection had shorter survival. The degree of pleural invasion (p=0.028), advanced age (p=0.022), and lymph node involvement (p=0.011) were found to be poor prognostic factors for survival.

Conclusion: In non-small cell lung cancer patients, the increase in the degree and size of pleural invasion is negatively correlated with the survival time and this is thought to be associated with advanced disease stage.

In patients diagnosed with non-small cell lung cancer (NSCLC), identifying prognostic factors is critical to improve survival, understand tumor biology, and predict the outcome. The main determinant factors of survival are tumor size, lymph node metastasis, distant metastasis, and a complete resection in which the tumor is completely removed with regional and mediastinal lymph nodes.[1]

In 1970s, Brewer[2] found that prognosis in subpleural tumors was much worse than those in lung parenchyma. Shimizu et al.[3] reported that tumors which invaded elastic layer of visceral pleura showed worse prognosis and these tumors had strong growth and aggressive invasion characteristics.

In NSCLC, pleural invasion (PI) is considered as a significant negative prognostic factor and the degree of invasion should be correctly evaluated histopathologically. The degree of PI is classified histopathologically as PI0, PI1, PI2 and PI3.

In the present study, we aimed to evaluate the degree and size of PI in NSCLC and to compare its relationship with histological subtype, lymph node metastasis, surgical resection type, and survival times.

Methods

This single-center, retrospective study was conducted at Ondokuz Mayıs University, Faculty of Medicine, Department of Thoracic Surgery between January 2008 and June 2019. A total of 164 patients (95 males, 10 females; median age: 61.7 years; range, 32 to 82 years) who underwent surgical resection with a diagnosis of NSCLC and who were found to have PI histopathologically were included in the study. A total of 105 patients (95 males, 10 females; median age: 61.7 years; range, 32 to 82 years) who underwent surgical resection but who were not found to have PI histopathologically during the same time period were included as control group to compare survival through random selection from clinical archive.

Positron emission tomography/computed tomography (PET/CT), brain magnetic resonance imaging (MRI), bronchoscopic biopsy, and transthoracic needle aspiration biopsy were held as a preoperative assessment. Routine blood, laboratory tests, and pulmonary function tests (PFTs) were performed for surgery preparation. For selected patients, ventilation/perfusion scintigraphy was taken. Surgical treatment was applied after routine anesthesia examination. Patients with NSCLC prediagnosis and pathology in the postoperative process are diagnosed with primary NSCLC were included. Patients with secondary tumors were not included in the study.

All patients included in the study underwent lung resection and mediastinal lymph node dissection. Pathology preparations of the patients were taken from Ondokuz Mayıs University Faculty of Medicine, Faculty of Medicine, Department of Pathology archive and reassessed. Pathology results were classified in terms of their histological types according to the current World Health Organization (WHO) classification and restaged according to 8th Tumor, Node, Metastasis (TNM) staging.[4]

Pathology preparations of the patients who were found to have PI were evaluated in terms of the degree and size of PI. Hematoxylin and eosin (H&E)-stained specimens were evaluated under a light microscope by a single pathologist (Nikon, Eclipse, Ni-U; Nikon Corporation, Tokyo, Japan). Pleural invasion degree was grouped as PI1, PI2, and PI3. The size of PI was calculated by measuring the area of the tumor invaded pleura in millimeters under the microscope and grouped as 1-10 mm, 11-19 mm, and 20 mm and above (Figure 1).

Figure 1. Microscopic measurement of pleural invasion size with millimetric ruler (×40).

Statistical analysis
Statistical analysis was performed using the IBM SPSS version 25.0 software (IBM Corp., Armonk, NY, USA). Descriptive data were expressed in mean ± standard deviation (SD), median (min-max) or number and frequency, where applicable. Comparison according to the degree and size of PI was made using the Pearson chi-square test.

Independent group comparisons for more than two were tested using the Kruskal-Wallis H test. The significance of survival times in terms of groups was tested using the Kaplan-Meier test and the statistical comparisons between the curves were conducted by using the log-rank test. The prognostic factors influencing survival time were determined using the Cox proportional hazard model. In parameters which were found to have a significant difference, the Bonferroni-corrected Mann-Whitney U test was conducted to find out the groups which caused the difference and corrected significance value was taken as p<0.017 (0.05/3 = 0.017). A p value of <0.05 was considered statistically significant.

Results

In the group with PI, there were 104 patients diagnosed with adenocarcinoma (ADC) and 60 patients with squamous cell carcinoma (SCC), while there were 46 patients diagnosed with ADC and 59 patients diagnosed with SCC in the group with no PI. Demographic data of the patients included in the study are shown in (Table 1). Lobectomy was the most common procedure in the patients in both groups (Table 2).

Table 1. Demographic data of the patients

Table 2. Pleural invasion distribution by the resection type performed

The median follow-up was 59.1 (range, 6 to 144) months. The mean survival was found as 52±10.5 months in the group with PI, while the mean survival was calculated as 70.6±5 months in the group with no PI. There was a statistically significant relationship between the two groups in terms of survival (p=0.013 and p<0.05) (Figure 2). The patients" survival according to the degree of PI (PI1, PI2, PI3) and their comparison with PI0 patients are given in Table 3 and Figure 2. Patients with PI0 were found to have higher survival rates, indicating a statistically significant difference (p=0.013).

Figure 2. Survival chart in terms of the degree of pleural invasion. PI: Pleural invasion.

Table 3. Comparison of survival in terms of pleural invasion

Pre- and postoperative treatments of patients with PI are summarized in Table 4. In cases diagnosed with ADC histopathologically, the mean survival was calculated as 63±7.6 months in the group with PI and as 66.5±7.4 months in the group with no PI. This difference was not statistically significant (p=0.488). In patients diagnosed with SCC histopathologically, mean survival was calculated as 45.2±5.6 months in the group with PI and as 72.8±5.9 months in the group with no PI. This difference was statistically significant (p=0.002). When the degree of PI was evaluated according to histopathological types, median survival was calculated as 96 months in PI1, 64 months in PI2, and 37 months in PI3 in ADC cases with PI and as 25 months in PI1, 23 months in PI2, and 14 months in PI3 in SCC patients (Figure 3 and Figure 4).

Figure 3. Survival chart of patient groups diagnosed with adenocarcinoma; red chart is PI0 in adenocarcinoma, blue chart is pleural invasion in adenocarcinoma.
PI: Pleural invasion.

Figure 4. Survival chart of patient groups diagnosed with squamous carcinoma; red chart is PI0, blue chart is pleural invasion in squamous carcinoma.
PI: Pleural invasion.

Table 4. Preoperative and postoperative oncological treatments in pleural invasion group

According to the survival analysis in terms of PI size, survival time became shorter, as the PI size increased. However, no statistically significant relationship was found between the invasion sizes (p=0.842) (Table 5 and Figure 5).

Table 5. Survival in terms of pleural invasion size

Figure 5. Pleural invasion size and survival chart.

In this study, a statistically significant relationship was found between type of surgical resection and survival in patients with PI. There was also a statistically significant difference between wedge resection and lobectomy (p=0.015 and p<0.05), and segmentectomy and pneumonectomy (p=0.033 and p<0.05) in terms of survival. In the patient group with PI, survival was found to be shorter in patients who underwent sublobar resection (Table 6 and Figure 6). No statistically significant relationship was found between type of surgical resection and survival in the group with no PI (Table 6).

Table 6. Survival in terms of resection types in patients with and without pleural invasion

Figure 6. Type of resection and survival chart in patients with pleural invasion; blue chart is wedge resection, green chart is segmentectomy, black chart is lobectomy, purple chart is bilobectomy, red chart is pneumonectomy.

In 36 (22%) of 164 patients with PI, lymph node metastasis was found. A statistically significant relationship was observed between the pathological diagnosis and lymph node metastasis (p=0.007 and p<0.05). In addition, SCC patients (30%) were found to have a higher lymph node metastasis rate compared to ADC patients (17.3%). Table 7 shows the relationship between the degree of PI and lymph node factor. No statistically significant relationship was found between the degree of invasion and lymph node metastasis (p=0.385 and p>0.05). However, in PI3 patients, lymph node metastasis was found to be significantly higher compared to the others (31.4%).

Table 7. Relationship between degree of pleural invasion and lymph node metastasis

When the PI size and lymph node factor were compared, it was found that lymph node metastasis rate also increased, as the invasion size increased; there was a higher involvement rate particularly in N1 lymph nodes in the area close to the tumor and this was more prominent in cases diagnosed with SCC (Table 8).

Table 8. Relationship between pleural invasion and lymph node metastasis

In cases with PI, stage-survival relationship was statistically significant. When survival was evaluated among the factors of tumor stage, T factor and N factor were found to be statistically significant (Table 9).

Table 9. Tumor, Node, Metastasis and survival statistics

Discussion

The pleura covers the lung parenchyma and maintains intrapleural fluid balance. In addition, pleura is an oncologically important structure that should be well-known due to both its exposure to tumor metastasis and its role in distant organ spread. In the visceral pleura, there is a very rich lymphatic network in connective tissue under the mesothelial covering and this network connects to lung lymph circulation.[5] Lymphatic drainage of the pleura is critical for the management of intrathoracic malignancy. In their study, Miura et al.[6] showed mesothelial cells in two structures as microvilli and stoma and they reported that the passage of malignant cells was through the same mechanism through mesothelial cells and that distant organ spread could be explained this way.

The cadaveric study conducted by Fourdrain et al.[7] is the first study to evaluate visceral pleura tract specifically and fully for lymphatic drainage of the lung. In this study, 21 cadavers were dissected and 2 mL of triphenylmethane blue dye solution was injected subpleurally to each lung segment. The lymphatic drainage of the visceral pleura was found to be segmental in 44.2% of the injections and intersegmental in 55.8%. The authors suggested that the presence of missing fissures or accessory fissures might affect lymphatic drainage in visceral pleura and that further researches should be conducted to evaluate the effect of lymphatic drainage of visceral pleura in lung cancer surgery, particularly in sublobar resections such as segmentectomy.

In another study, Jiang et al.[8] found that visceral pleural invasion (VPI) and tumor size together had a synergetic effect on survival in negative lymph node with NSCLC. The effect of VPI on survival increased with the size of the tumor. As the size of the tumor increases, the negative effect of VPI on survival also increased. In our study, when T stage was evaluated with the degree of PI, its effect on survival was found to be more prominent. The T factor (p=0.045 and p<0.05) and N factor (p=0.011 and p<0.05) were found to be statistically significant in terms of survival, and T factor was found to increase the mortality risk 1.38 times, while N factor was found to increase the mortality risk 1.68 times.

In a study investigating poor prognostic factors in cases with an SCC diagnosis, Zhao et al.[9] reported that, of 132 cases, 40% had PI and, in these cases, the five-year-survival was shorter compared to the cases that did not have PI (30% vs. 76%). In our study, while the mean survival time was 28±4.9 months in PI1, PI2, PI3 groups in cases diagnosed with SCC, the mean survival time was 63±5.9 months in PI0 and this rate was found to be statistically different.

In a meta-analysis of 16 studies, Wang et al.[10] found that the overall survival rate was lower in PI1 or PI2 patients compared to PI0 patients and that PI2 patients showed a lower overall survival than PI1 patients. When the five-year survival rates of PI1 or PI2 patients were examined, they had lower rates than PI0 patients. Similarly, in our study, the survival time decreased, as the degree of invasion increased. A statistically significant relationship was found between PI0 and PI1-PI2-PI3, between PI1 and PI3 and between PI2 and PI3 in terms of survival. The five-year survival was higher in patients with PI0 tumor (60%) and PI1 tumor (45%), compared to the patients with PI3 tumor (36%). Sakakura et al.[11] also reported that PI0-PI1 tumor results were different from PI2 tumors. Likewise, Osaki et al.[12] reported that PI0 had a better prognostic value than PI1-PI2, regardless of the N state and tumor size.

To examine the effects of VPI on patients" prognosis in terms of tumor size, Tian et al.[13] classified 521 patients into four groups according to the tumor size (≤2, 2-3, 3-5, and 5-7 cm). The results showed that VPI did not have a significant effect on prognosis in patients with a tumor of ?2 cm. The rate of patients with regional or distant metastases was significantly higher in those with VPI compared to those without VPI. In our study, survival was evaluated particularly in terms of the invasion size, since the invasion size would increase with the tumor size. Survival size was found to decrease, as PI size increased. The average survival was found to be 63 months in the group with a pleural size of 10 mm and below, while it was found to be 45 months in the group with a pleural size of 20 mm and above.

Lymphatic metastasis is an important prognostic factor in NSCLC. Patients with multi-station mediastinal lymph node metastasis (≥2 stations) were found to have a significantly worse prognosis compared to those with a single station mediastinal lymph node metastasis. Manac'h et al.[14] reported that tumor cells were more prone to disintegration throughout the lymphatic system, when the tumor invaded visceral pleura. Supporting these studies, they showed that the probability of mediastinal lymph node metastasis in patients with NSCLS who underwent lung resection was significantly higher in patients with VPI compared to those without VPI. In addition, they reported that these patients were more vulnerable to multi-station mediastinal lymph node metastasis.

Jiwangga et al.[15] reported that the most common forms of recurrence of VPI and Stage 1 lung ADC were pleural metastasis and bilateral lung metastasis. They, therefore, reported that patients with VPI and Stage 2B lung cancer should be followed carefully in terms of distant recurrence. In our study, lymph node metastasis rate increased, as the degree of PI increased and, consistent with the literature, a statistically significant relationship was found between metastatic lymph node number and survival (p=0.003 and p<0.01). Lymph node metastasis was found in 36 (22%) of 164 patients, and a statistically significant relationship was observed between pathological diagnosis and lymph node metastasis (p=0.007 and p<0.01). In SCC patients, the stage was more advanced, survival was shorter, and N1 stage was more prominent. As in Oyama et al.'s[16] study, we found that, as the degree of PI increased, survival time decreased and N stage increased.

The main limitation of this study is its single-center, retrospective design. In addition, evaluations were made on specimens obtained from the pathology clinic archive. Another limitation is the small sample size. Further multi-center, prospective studies in which specimens are macroscopically examined would provide more reliable conclusions with a better comparison.

In conclusion, the degree of PI was a negative prognostic factor alone or with the tumor size (T stage), lymph node involvement (N stage), and overall survival. The survival time of patients with a squamous cell carcinoma and PI was found to be shorter compared to those with an adenocarcinoma. We believe that, in non-small cell lung cancer patients with PI, anatomic resection should be performed and sublobar resection should be avoided as much as possible.

Ethics Committee Approval: The study protocol was approved by the Ondokuz Mayıs University Ethics Committee (date: 23.05.2019, no: 2019/423). The study was conducted in accordance with the principles of the Declaration of Helsinki.

Patient Consent for Publication: A written informed consent was obtained from each patient.

Data Sharing Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request.

Author Contributions: Idea/concept: V.Y., B.Ç., A.B., Y.S.; Design: V.Y., B.Ç., Y.S.; Control/supervision: V.Y.; Data collection and/or processing: V.Y., B.Ç., Y.S.; Analysis and/or interpretation: V.Y., B.Ç., A.B.; Literature review: V.Y., B.Ç., S.G.; Writing the article: V.Y., S.G., M.G.P.; Critical review: A.B.; References and fundings: B.Ç; Materials; V.Y., Y.B.K., Y.S., A.T.Ş.

Conflict of Interest: The authors declared no conflicts of interest with respect to the authorship and/or publication of this article.

Funding: The authors received no financial support for the research and/or authorship of this article.

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