Malignant Airway Obstruction and Endobronchial Stent Placement: A Systematic Review on the Efficacy and Safety

The systematic review aims to evaluate the efficacy and safety of endobronchial stent placement for malignant airway obstruction. A comprehensive search was conducted across multiple databases to identify relevant studies. Cohort studies, randomized controlled trials, and case-control studies examining the outcomes of endobronchial stent placement in patients with malignant airway obstruction were included. Data on pre-treatment evaluation, such as pulmonary function testing, dyspnea severity scoring systems, arterial blood gas parameters, imaging, and degree of obstruction, were also collected. Primary outcomes of interest included post-procedure stenosis, pulmonary function testing evaluation, blood gas parameters, and survival outcomes. Secondary outcomes encompassed improvements in clinical status, dyspnea grade, and procedure-related complications. A total of 27 studies met the inclusion criteria and were included in the systematic review. The included studies demonstrated promising outcomes of endobronchial stent placement in managing malignant airway obstruction. Post-procedure airway diameters, pulmonary function testing, and blood gas parameters improved significantly. Survival outcomes varied among studies. Furthermore, endobronchial stent placement was associated with improvements in clinical status and dyspnea grade. Procedure-related complications ranged from pain, hemoptysis and mucus plugging to stent obstruction, migration and pneumothorax. This systematic review suggests that endobronchial stent placement is an effective and safe intervention for managing malignant airway obstruction. It offers significant improvements in post-procedure stenosis, pulmonary function testing, blood gas parameters, and clinical outcomes. However, further studies with larger sample sizes and standardized reporting are warranted to better evaluate the long-term efficacy and safety of endobronchial stent placement for malignant airway obstruction.


Introduction And Background
Central airway obstruction (CAO) is a complex problem mainly secondary to malignant lesions and to some extent, due to benign lesions. Central airway obstruction can be caused by extrinsic mass compression, intrinsic exophytic tumor, or dynamic collapse. Malignant CAO may be caused by primary lung or esophageal cancer but also by metastatic cancer leading to mass in the thoracic cavity [1]. Bronchogenic carcinoma is the most common cause of malignant CAO. CAO increases the risk of post-obstructive pneumonia and respiratory failure. Around 30% of lung cancer patients develop CAO. Unfortunately, the development of CAO decreases the survival rate remarkably; if CAO is untreated, survival is usually two to three months, but with interventional treatment survival rate improves to six to eight months [2,3].
Endoscopic management can be an essential addition to existing treatment options for symptomatic tracheobronchial complications in unresectable benign or malignant airway obstruction cases. Various endoscopic interventions are available to treat malignant CAO, including endobronchial dilation, laser therapy, and airway stents. These procedures provide symptomatic relief and improve quality of life [1,4]. The symptoms-free survival rate has increased significantly over the last few decades because of technical advances in interventional bronchoscopy procedures [5].
The risk of bias in the included studies was evaluated by one independent reviewer. A risk-of-bias assessment tool developed by the National Institutes of Health (NIH) was used to assess the quality of the included studies [7].

Search Results
In our systematic review, the initial search across various databases yielded a total of 382 records. Following the removal of duplicate records, 100 were eliminated, resulting in 282 records for further screening. From this pool, 173 records were excluded based on a preliminary assessment of the title and abstract, specifically targeting case reports. The remaining 109 records were sought for full retrieval. Fortunately, all 109 reports were successfully obtained and assessed for eligibility. Among these reports, 82 were excluded due to their classification as case series, abstracts, or full-length papers deemed irrelevant to the study. Ultimately, our systematic review included a total of 27 studies, which met the eligibility criteria and were included in the final analysis. Figure 1 below provides a diagrammatic representation of the search results.  Table 2 provides a summary of the baseline patient characteristics. 2023  Non-small cell lung cancer (18), Squamous cell cancer (19), Small cell lung cancer (7).
Not mentioned Not mentioned 9.
Patients inducted into the study if airway patency was <50 % after rigid bronchoscopy intervention (dilatation and/or de-obstruction), or if the recurrence risk was high. Another indication for stent application was to cover fistula when a fistula stoma was detected by bronchoscopy evaluation in between central airway and esophagus or mediastinum.
Not mentioned Not mentioned 17.
Razi SS et al [24].   [34].     [8]. In the study conducted by Lachkara et al., the details regarding pulmonary function testing or imaging were not provided. The location of the lesion and obstruction was categorized into two groups: SYS group and SEMYS group. The SYS group consisted of 22 patients with metastatic disease and 18 patients with locally advanced disease, while the SEMYS group included 24 patients with metastatic disease and 14 patients with locally advanced disease [13]. Ma et al. also did not mention specific details about pulmonary function testing or imaging. The site of lesion and obstruction involved different areas, including the middle-lower trachea (45 cases), the right main bronchus (three cases), the left main bronchus (two cases), and coexistence of tracheal and one-sided bronchial involvement (two cases). The study reported the Karnofsky Performance Status (KPS) value (68.58 ± 8.08) and blood gas parameters (PaO2: 7.74 ± 0.99, PaCO2: 5.37 ± 0.39) [14]. In the study conducted by Marchese et al., the details about pulmonary function testing or imaging were not mentioned. The study reported the American Society of Anesthesiology (ASA) Score (mean ± SD: 3 ± 0.5), Eastern Cooperative Oncology Group (ECOG) Score (mean ± SD: 1.7 ± 0.6), and Modified Medical Research Council (MMRC) dyspnea score (mean ± SD: 2.7 ± 0.8). The site of lesion and obstruction included intrinsic compression in 10 cases, extrinsic compression in 12 cases, complex involvement in 27 cases, and fistula formation in one case [15].

Pre-intervention Parameters
Similarly, in the study by Marchese et al., no details about pulmonary function testing or imaging were provided. The site of lesion and obstruction involved the left lower lobe bronchus, left upper lobe bronchus, left secondary carina, right lower lobe bronchus, right primary carina, and right secondary carina. The study also reported the ECOG Score (mean ± SD: 1.8 ± 0.7), MMRC dyspnea score (mean ± SD: 2.6 ± 0.8), oxygen saturation (mean ± SD: 95% ± 2), and Barthel index (mean ± SD: 82 ± 2.5). Intrinsic compression was observed in eight patients, extrinsic compression in 10 patients, complex involvement in 31 patients, and no fistula formation was reported. Marchioni et al. did not mention details about pulmonary function testing or imaging in their study. The location of the lesion and obstruction was categorized based on different areas, including the trachea, main right bronchus, main left bronchus, carina, and extensive involvement. The degree of obstruction (IQR) for each category was reported, while no additional information was provided regarding dyspnea grade, stenosis type, or blood gas parameters [16]. Miyazawa et al. reported pulmonary function testing results in their study, including vital capacity (VC), forced vital capacity (FVC), peak expiratory flow (PEF), and various stenosis types. The site of lesion and obstruction included intrinsic compression in 22 cases and extrinsic compression in 12 cases. The study also provided dyspnea grades for patients before stent placement [18]. In another study by Miyazawa et al., pulmonary function testing results were reported for different stenosis types: tracheal stenosis, carinal stenosis, bronchial stenosis, and extensive stenosis. The site of lesion and obstruction involved tracheal stenosis in 20 patients, carinal stenosis in 16 patients [19].
Özdemir et al. did not provide specific information regarding pulmonary function testing, imaging, site of lesion/location of obstruction, or type of stenosis. The degree of obstruction and blood gas parameters were also not mentioned. The ASA patient score prior to intervention was reported to have a mean value of 2.64 ± 0.74, ranging from 1 to 4 [23]. Razi et al. did not mention the specific details of pulmonary function testing, imaging, or site of lesion/location of obstruction. The extent of luminal obstruction was reported, with 10 patients (20%) having an obstruction ranging from 75% to 90% and 40 patients (80%) having an obstruction of more than 90%. The mean value of the MRC dyspnea scale score was 4.40, indicating significant dyspnea. The study also reported the mean preoperative ASA score of 3.31 and the mean preoperative ECOG performance status score of 3.36. The types of stenosis observed in this study included intrinsic compression in five patients, extrinsic compression in 10 patients, complex obstruction in 32 patients, and fistula formation in three patients. The blood gas parameters were not mentioned [24]. Sajia et al. did not provide specific information regarding pulmonary function testing, imaging, or site of lesion/location of obstruction. The degree of obstruction, dyspnea grade, type of stenosis, and blood gas parameters were also not mentioned. However, they reported the performance status of the patients in the study. Out of the total patients, 44 had a performance status of 0-2, indicating good functional ability. Four patients had a performance status of 3-4, indicating limited functional ability. The performance status of 11 patients was unknown. No further information was provided regarding the blood gas parameters or other specific details of the study [26].
The study conducted by Wilson et al. focused on patients with airway obstruction. Pulmonary function testing revealed reduced lung function, with forced expiratory volume in 1 second (FEV1) (mean ± SD) at 1.13 ± 0.41, FVC (mean ± SD) at 1.96 ± 0.7, and peak expiratory flow rate (PEFR) (mean ± SD) at 134 ± 12. The location of obstruction varied, involving the trachea, right and left bronchial trees, and both main bronchi. A significant proportion of patients (53%) experienced severe obstruction (>90%) in a main bronchus, while the remaining patients had partial obstruction in the trachea or a main bronchus. Dyspnea scores indicated significant respiratory impairment, with a mean MRC score of 5 ± 0, KPS of 29.1 ± 11.4, and visual analogue scores for breathing and walking at 40 ± 23 and 51 ± 23, respectively. Blood gas parameters showed compromised oxygenation and ventilation, with a mean PaO2 of 8.81 ± 2.7, PaCO2 of 5.33 ± 1.0, and pH of 7.45 ± 0.03. These findings highlight the severity and impact of airway obstruction on respiratory function in the studied population [29].
One study by Zwischenberger et al. did not provide specific details about pulmonary function testing or imaging. However, Karnofsky scores were reported, with patients scoring between 50 and 70, some scoring above 70, and others scoring below 50 [31]. Akram et al. reported bronchial stenosis, tracheal stenosis, tracheo-esophageal fistula, tracheal stenosis with fistula, bronchial stenosis with fistula, and bronchial fistula as the site of lesion/location of obstruction. They also provided the ECOG score (mean ± SD) of 3.65 ± 0.6. Additionally, they mentioned intrinsic compression, extrinsic compression, mixed compression, and fistula formation in their findings [32]. Bolliger et al. reported the mean FEV1 and FVC values for their 26 patients. The site of lesion/location of obstruction included the right main bronchus, left main bronchus, trachea, and tracheo-bronchial. They also reported a dyspnea index (mean ± SD) of 3.3 ± 0.7 [33]. Chhajed et al. provided spirometry results, including the median FEV1 and FVC values. They reported various locations for procedures, such as the trachea, trachea with one or both main bronchi, left bronchial system, right bronchial system, and both left and right bronchial systems. They also mentioned airway obstruction exceeding 50% of the lumen [34]. Ma G et al [14].

27.
Chhajed PN et al [34].  included patients with lung and esophageal cancer who underwent stent placement using tube and Yshaped stents of varying lengths and locations [11]. Iyoda et al. enrolled patients with central airway obstruction due to thoracic malignancy who underwent either silicone stent or metallic stent placement. Additional chemotherapy was required for 14 patients after stent placement (eight patients with silicone stents and six patients with metallic stents) [12].
In the study by Lachkara et al., a total of 78 patients underwent stent placement, with 40 patients receiving Silicone Y stent placement and 38 patients receiving SEM Y stent placement. Post-stent placement, 21 autoexpansive esophageal stents were also placed, with nine in the silicone Y group and 12 in the SEM Y group. Radiation therapy was administered to 12 patients in each group, and mechanical and/or electrocoagulation debulking was performed in 19 patients in the Silicone Y group and 19 patients in the SEM Y group. After stenting, 20 patients in the Silicone Y group and 26 patients in the SEM Y group received oncological treatment, including chemotherapy [13]. In the study conducted by Ma et al., stent placement for malignant airway obstruction was performed using a bronchoscope, and all 52 cases were successfully implanted with Ultraflex self-expandable metallic stents. Among the patients, 33 from the lung cancer and esophageal carcinoma group received postoperative radiotherapy/chemotherapy, while six patients from the lymphoma group received postoperative chemotherapy [14]. Marchese et al. reported the use of fully covered SEMS Silamet stents in 52 patients with malignant airway obstruction, inserted in the trachea, main bronchi, and peripheral bronchi. Laser therapy was employed for endoluminal lesions. In the postoperative period, three patients required mechanical ventilation for less than eight hours [15]. In another study by Marchese et al., stenting procedures were performed using a rigid bronchoscope under general anesthesia and jet ventilation. Chemotherapy was administered in 13 patients, while three patients received chemoradiotherapy. Both metallic and silicone stents were used, with a total of 52 stents placed. Postprocedural treatment included chemotherapy in 33 patients, radiotherapy in two patients, and surgery in four patients [16]. Marchioni et al. conducted a study where stenting procedures were performed using a Dumon rigid bronchoscope under general anesthesia. Patients were divided into two groups: integrated treatment (endoscopic treatment plus chemotherapy/radiotherapy) and standard treatment (chemotherapy/radiotherapy alone). A total of 54 stenting procedures were performed, with Y-shaped stents used in 24 cases and single stents in 34 cases. Various treatment modalities such as bougies, laser therapy, and mechanical debulking were employed, but specific post-procedural treatments were not mentioned [17].

Miyazawa et al. utilized both flexible and rigid bronchoscopes for stent insertion, with a total of 54 Ultraflex
Nitinol stents placed in cases of malignant airway obstruction. Debunking procedures were performed in 11 patients using Nd-YAG laser and/or mechanical debulking, and additional radiotherapy/chemotherapy was administered to eight patients after stent implantation [18]. In another study by Miyazawa  Radiotherapy was administered to some patients as an additional treatment modality [31]. Akram et al. employed flexible bronchoscopy for various treatments such as chemotherapy, radiotherapy, and mechanical debulking. Self-expanding metallic stents were used for airway stenting, and some patients received both pre-and post-procedural chemotherapy and radiotherapy [32].
Bolliger et al. utilized rigid bronchoscopy under general anesthesia for the placement of studded Polyflex stents in neoplastic obstruction of the central airways. Follow-up bronchoscopies were performed within a specific timeframe after stent placement [33]. Chhajed et al. compared stent placement versus laser therapy for malignant airway stenosis. Different types of stents were used depending on the location of the lesion, and laser therapy was used in most procedures [34]. Table 5 gives a summary of the outcomes and complications of endobronchial valve placement.

Not mentioned
Infection: 1.  In a study conducted by Dalar et al., the median follow-up period was 5.3 months. The study aimed to investigate the impact of the type of malignancy causing central airway obstruction and the site of lesion on survival. Furthermore, the study examined the influence of different treatment modalities on survival outcomes, including laser only, argon plasma coagulation only, cryotherapy only, stent only, laser and stent, and argon plasma coagulation and stent. The reported survival outcomes included mean months with a 95% confidence interval, as well as survival rates at three and six months for each treatment modality. Additionally, the study highlighted various complications, such as stent obstruction due to tumor overgrowth, complications following endobronchial treatment, arrhythmias during treatment, hypertensive attacks, oxygen desaturation, and restenosis due to tumor progression [8]. In a study conducted by Dutau et al., the follow-up period revealed that out of 40 patients in the stent arm, 11 were alive, while in the no stent arm, 10 out of 38 patients were alive. The study examined the impact of stent placement on survival, revealing non-significant improvements in survival times. The causes of death included progressive cachexia, progressive bronchial obstruction, metastases, and other factors, such as hemoptysis. One unknown cause of death was reported in the stent arm [9]. Grosu et al. reported that in their study, 30% of patients experienced post-procedure airway obstruction ranging from 0-49%, while 8% had an obstruction of 50-100%. The study also documented various complications, including lower respiratory tract infections, mortality, granulation tissue, stent obstruction due to tumor overgrowth, interventions performed for restenosis, new stent placements for restenosis, stent removal, and migration [10].
In a study by Huang et  .220 months after stenting. The one-year and two-year survival rates were also reported for both groups. SS patients exhibited significantly better prognoses than MS patients. The study identified complications, including infections, 30-day mortality, granulation tissue, stent obstruction due to tumor overgrowth, migration, retained secretions/mucoid impaction, bleeding, and halitosis [12]. In a study conducted by Lachkara et al., two types of stents were compared, and the Silicone Y group exhibited a median survival of 171 days (IQR 53-379 days), while the SEM Y group had a median survival of 104 days (IQR 53-230 days). Symptom relief was observed in patients from both groups, and the mean duration of stent placement differed between the two groups. The study also reported early and late complications for each stent type [13]. Ma et al. conducted a study that reported the KPS value and blood gas parameters for the study participants. The three-year survival rates and average survival periods were provided for the lung cancer, esophageal carcinoma, and lymphoma groups. The study found a significantly higher three-year survival rate in the lymphoma group compared to the lung cancer and esophageal cancer groups. Additionally, the study reported complications, such as infections, stent obstruction due to tumor overgrowth, chest pain, and mild fever [14].
In the study conducted by Marchese et al., they found statistically significant improvements in functional outcomes following the procedure. The Barthel Index, which measures activities of daily living, showed a significant increase from a median of 69 immediately after the procedure to a median of 90 after 24 hours (p < 0.001). Similarly, the MRC score, indicating the severity of dyspnea, significantly improved from a median of 3 to a median of 1 (p < 0.001) within the same time frame. The follow-up period ranged from 22 to 549 days, with a mean of 119 ± 120 days and a median of 74 days. Radiographic improvement was observed in 48% of the patients. Several complications were reported, including infections (5.7%), granulation tissue (3.8%), stent obstruction due to tumor overgrowth (15%), and stent migration (13.4%). Some patients experienced postoperative mechanical ventilation for a short duration or atrial fibrillation that was successfully treated with medication [15]. In another study by Marchese et al., the researchers assessed the impact of bronchoscopy intervention on dyspnea and oxygen saturation. They found a significant improvement in the MMRC dyspnea score (2.6 ± 0.8 vs. 1.2 ± 0.5; p < 0.01) and oxygen saturation (95 ± 2 vs. 96 ± 2.4; p < 0.01) immediately after the procedure and at the one-month follow-up. The average duration of hospital stay was 2 ± 3 days, and the mean follow-up period was 123 days ± 157. The median overall survival was 118 ± 21 days, with patients having a double airway stent showing worse survival outcomes compared to those with a single stent (p < 0.01). Early complications included atrial fibrillation (3.9%), respiratory distress requiring non-invasive ventilation (5.8%), pneumonia (1.9%), and obstruction due to tenacious secretions (1.9%). Additionally, infections (13%), granulation tissue (7.6%), stent obstruction due to tumor overgrowth (9.8%), and stent migration (3.9%) were reported [16].
Marchioni et al. conducted a study comparing interventional bronchoscopy and standard treatment with chemo-radiotherapy (ST). They found that the overall survival was longer in the interventional bronchoscopy group, although not statistically significant. However, the interventional bronchoscopy group showed a significantly higher survival gain over ST in patients with KRAS mutation, lumen occlusion >65%, and no involvement of the left bronchus. They also observed statistically significant favorable differences in terms of overall new hospitalizations, symptom-free interval, and onset of atelectasis in the interventional bronchoscopy group. Complications reported in their study included granulation tissue (eight cases), postobstructive pneumonia (five cases), dislocation (eight cases), and stent occlusion (six cases) at one-year follow-up [17]. In the study by Miyazawa et al., significant improvements were observed in the obstruction of the airway diameter following stent implantation. The airway diameter showed a remarkable improvement from 81.6% before the procedure to 14.6% on day 1, 12.6% on day 30, and 22.6% on day 60 (p < 0.001). Spirometry measurements also demonstrated improvements in lung function parameters such as VC, FEV1, and PEF after stent implantation. Additionally, flow volume loops showed immediate enhancement of flow limitation. The dyspnea index also showed a significant improvement after the procedure. However, despite these positive outcomes, the median survival time for patients was only three months, with a one-year survival rate of 25.4%. Symptom improvement was observed in 82% of the patients immediately after the procedure. The most common causes of mortality were cachexia, bleeding, and respiratory insufficiency. Complications such as granulation tissue, stent obstruction due to tumor overgrowth, and retained secretions/mucoid impaction were also reported [18]. In another study conducted by Miyazawa  Patients with an intraluminal obstruction that reduced the lumen by less than 50% of the endoluminal diameter also benefited from stenting. However, in patients with more severe intraluminal obstruction, or in whom the tumor reduced the lumen by more than 50% of the endoluminal diameter, only slight improvement was observed. Stent obstruction due to tumor overgrowth and bleeding were reported as complications in the study [27]. Verma et al. conducted a study to assess the survival outcomes and complications associated with laser therapy and ultraflex stenting. The study reported different survival rates depending on the treatment modality. Laser therapy alone had a median survival of 12.4 months, while ultraflex stenting alone had a median survival of 4.6 months. Combined laser therapy and ultraflex stenting had a median survival of 5.9 months. Complication rates varied between the treatment groups, with laser therapy-only and ultraflex stenting-only groups experiencing higher mortality rates and complications compared to the combined treatment group. Complications observed in the study included bleeding, unexpected respiratory failure, and escalation of the level of care [28].
In a study conducted by Wilson et al., the pulmonary function and clinical outcomes of patients who underwent stent insertion were assessed. The study findings revealed a mean forced expiratory volume in 1 second of 1.38 with standard deviation (SD), and a mean forced vital capacity of 2.15 with SD. The peak expiratory flow rate showed a significant improvement with a mean of 158 and SD. The Medical Research Council score had a mean of 4 with SD, while the Karnofsky score had a mean of 51.8 with SD. Visual analogue scores for breathing and walking were reported as 63 (mean ± SD) and 65 (mean ± SD), respectively. The pH and PaCO2 levels did not show significant changes, but the PaO2 level significantly improved with a mean of 10.24 (mean ± SD). The median length of hospital stay was five days, and the study reported one infection and four deaths among the patients [29]. Yerushalmi et al. conducted a study analyzing six patients who underwent stent insertion. The study evaluated their pulmonary function before and after the procedure. The results indicated an improvement in the forced expiratory volume in 1 second ranging from 5% to 35% and an improvement in forced vital capacity ranging from 5% to 15% for all patients. Additionally, the degree of dyspnea showed improvement, and the median survival was six months, ranging from 0.25 to 105 months. Three cases of stent obstruction due to tumor overgrowth were observed [30]. In a study conducted by Zwischenberger et al., all patients underwent successful stent deployment, resulting in initial relief of airway stenosis. Among the patients, 50% showed improvement in dyspnea scores, while the remaining patients experienced unchanged symptoms. Some patients reported significant improvement in functional status. Four patients showed improvement in the Karnofsky score, while it remained unchanged in five patients who survived longer than two months. The total length of stay ranged from three to 22 days, with an average of 10.2 days. At the eight-month follow-up, 10 patients had died, and complications identified before death included tracheoesophageal fistula, pneumothorax, tracheostomy, and atrial fibrillation [31].
Akram et al. conducted a study to examine the effect of stent insertion on arterial oxygen levels. The study results showed a statistically significant improvement in partial pressure arterial oxygen after the procedure. The median survival time was 16 weeks, with the highest survival observed in cases of intrinsic compression of the airway. Patients who received pre-and post-procedure chemotherapy and radiotherapy had better survival rates. Improvements were also observed in oxygen saturation, white blood cell count, performance status, serum albumin, and hemoglobin. Symptomatic improvement was observed in 56.9% of patients. Complications included stent obstruction, stent migration, and acute respiratory distress [32]. Bolliger et al. conducted a study to assess the pulmonary function and clinical outcomes of patients after stent placement. One month after the procedure, the forced expiratory volume in 1 second and forced vital capacity were evaluated and found to be 1.9 ± 0.6 and 2.8 ± 0.7, respectively. At three months, the values were 1.5 ± 0.5 and 2.5 ± 1.0, showing slight variations. The study included patients with a mean follow-up of 4.3 months, and out of 26 evaluable patients, 23 had died. Complications observed in the study included migration and tenacious secretions leading to stent obstruction [33]. Chhajed et al. conducted a study that involved 87 patients who underwent spirometry assessment. The median values for forced expiratory volume in 1 second and forced vital capacity were 62% and 69%, respectively. The median survival times varied based on the treatment modality, with stent-only patients having a median survival of 2.7 months, laser and stent combined patients with three months, and laser-only patients with 10.4 months. The survival rates at three and six months differed among the treatment groups. The study identified various complications such as infection, granulation tissue, mortality, migration, mucus plugging, stent restenosis, pericardial effusion, respiratory failure, esophago-tracheal fistula, ventricular arrhythmias, severe cough, and acute laryngospasm [34].

Discussion
Central airway obstruction is a major therapeutic challenge for physicians dealing with pulmonary and mediastinal malignancies. Blockage of the central airways either intrinsic from bronchial tumors or extrinsic from other malignancies results in significant morbidity and contributes to mortality due to repeated episodes of post-obstructive pneumonia, respiratory failures, and atelectasis [25].
Most of the malignant pulmonary masses are identified at advanced stages, where conventional treatment with chemotherapy and surgery does not provide much benefit. In these advanced cases, radiotherapy does not provide immediate relief. Significant mortality from these advanced endobronchial obstructions results from loco-regional pathology and its complications [35]. Median survival in cases of endo-bronchial tumors worsens with involvement of trachea than major bronchi, with a reported median survival of 1.8, 4.8, and 4.7 months with involvement of trachea, right main bronchus, and left main bronchus [34].
Several endobronchial treatments have been designed to date including stenting, laser ablation, bronchoscopic guided brachytherapy, and photocoagulation. Each of these is variably applied depending on the ease of access, tumor location, and availability of technology [30]. Intraluminal stenting was initially designed for intravascular application. Over time, the advancement delivery system and the development of endoluminal self-expanding stents made them a feasible treatment option for endobronchial strictures, especially in benign cases where they have a definite therapeutic advantage [31].
Airway stents retain their position due to radial traction against the airway walls. It is important for the stents to be size appropriate for the airway. Excessive force from oversized stents can lead to bronchial ischemia, irritation, and granulation tissue formation, while undersized stents have an increased risk of migration. Endobronchial stents were designed to be inert in place, causing minimal granulomatous tissue formation, being resistant to obstructive force, and allowing for manipulation in case of obstruction by secretions or tumor overgrowth. With these facts, silicon and metallic self-expanding stents were designed for endobronchial tumors. Expandable metallic stents (EMS) were initially designed for both intra and extrabronchial obstructions. However, It was observed that intra-bronchial tumors tend to grow between the gaps in the stent resulting in re-occlusion of the bronchus, causing treatment failure and making second stent placement difficult. To deal with that difficulty, covered metallic stents of Dumon tubes were introduced [36,37]. On the other hand, silicon stents were associated with lesser granulation tissue formation and were easy to insert and remove than EMS. Sawada et al. reported that EMS can get impregnated with the bronchial epithelium as quickly as three weeks, and histological assessment shows that stents penetrate up to cartilages, having more favorable outcomes in extrinsic endobronchial obstructions [27,38].
For patients with intrinsic endobronchial obstruction, a combination of different endobronchial procedures including stenting, laser application, photodynamic ablation, and mechanical debridement have been shown to have more favorable outcomes over extended follow-up as compared to the single modality of stenting. Santos et al. demonstrated that cumulative one-and three-year survival in patients receiving multimodal intervention vs stenting was 51.3% vs 50% and 22% vs 2.3% respectively [39]. Saji et al. reported smaller group results revealing that even though the difference in survival after stenting was insignificant, stenting results in significant improvement in symptoms and quality of life [26]. Modern self-expanding stents are made of nickel-titanium alloy-covered silicon and have a shape memory function [23]. In the modern era of 3-D Printing and customized stents, 3D printed stents have been shown to significantly improve survival even in proximal laryngotracheal stenosis, both malignant and benign [40]. SEMS are either placed with rigid bronchoscopy under general anesthesia or with flexible bronchoscopy under local anesthesia with the help of fluoroscopy. In the placement of stents, it is important to include the choke point i.e. the point of maximal obstruction to provide the greatest symptomatic relief and avoid stent migration [41].
In the analysis being conducted, all the data collected from the studies was found to be symmetrically reported by all studies. The majority of sites used for stenting were major airways either with isolated lesions of the trachea or main bronchi, or combined complex lesions including the tracheobronchial tree. In most of the studies recruited details of pulmonary function status, ASA, or ECOG score were not mentioned. Most of the studies included reported the use of stents in conjunction with other intraluminal procedures in intrinsic bronchial obstruction, with selective reporting on extrinsic obstruction cases raising the bias in reporting. Mortality was reported in 84 cases, with the most common reason for mortality being cancerrelated cachexia, hypoxia and infections. Median survival after the procedure was 5.4 months with the maximal reported survival of 17.6 months. One study reported a remarkable improvement from 81.6% before the procedure to 14.6% on day 1, 12.6% on day 30, and 22.6% on day 60 (p < 0.001) [18]. Among the complications reported in the included studies were lower respiratory tract infections, mortality, granulation tissue, stent obstruction due to tumor overgrowth, interventions performed for restenosis, new stent placements for restenosis, stent removal, secretions/mucoid impaction, fistula, atelectasis, infections, granulation tissue, tumor overgrowth, and migration were the most common complications.

Conclusions
In conclusion, this comprehensive systematic review provides valuable insights into the management of malignant airway obstruction through endobronchial stent placement. The findings presented herein demonstrate that endobronchial stenting is an effective and minimally invasive technique for relieving symptoms, improving quality of life, and prolonging survival in patients with malignant airway obstruction.
Through a meticulous evaluation of the available literature, this review highlights the benefits of endobronchial stents, including their ability to restore airway patency, alleviate dyspnea, and facilitate the delivery of other therapeutic modalities. The analyzed studies consistently report significant improvements in respiratory parameters, functional capacity, and overall patient well-being following stent placement. Moreover, the low complication rates and high technical success rates associated with this procedure further reinforce its clinical utility.
The review also underscores the importance of appropriate patient selection, procedural expertise, and multidisciplinary collaboration in achieving optimal outcomes. Identifying suitable candidates for endobronchial stenting necessitates a comprehensive evaluation of tumor characteristics, airway anatomy, comorbidities, and patient preferences. Additionally, close collaboration between pulmonologists, interventional radiologists, thoracic surgeons, and oncologists is crucial to ensure proper patient management and follow-up care.
While endobronchial stent placement emerges as a promising therapeutic option, several areas for future research and improvement are identified. Further investigations are warranted to determine the long-term durability of stents, refine patient selection criteria, and evaluate the comparative effectiveness of different stent types. Additionally, studies exploring the optimal timing of stent insertion, the role of adjunctive therapies, and the impact on survival outcomes would enhance our understanding of this intervention.
In summary, this systematic review serves as a comprehensive synthesis of the current evidence regarding malignant airway obstruction and endobronchial stent placement. The findings provide compelling support for the use of endobronchial stents as an effective and safe approach for managing this challenging condition. Continued research and collaboration among clinicians and researchers are essential to further refine and expand the application of endobronchial stent placement, ultimately benefiting patients facing malignant airway obstruction.

Conflicts of interest:
In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.