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Original Research |
1 Center for Airway Imaging, Department of Radiology, Beth Israel Deaconess
Medical Center and Harvard Medical School, 330 Brookline Ave., Boston, MA
02215.
2 Department of Interventional Pulmonary Medicine, Beth Israel Deaconess Medical
Center and Harvard Medical School, Boston, MA.
Received March 26, 2008;
accepted after revision May 19, 2008.
Address correspondence to V. Dialani
(vdialani{at}bidmc.harvard.edu).
Abstract
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MATERIALS AND METHODS. A review was performed of all consecutive patients undergoing MDCT and bronchoscopy for suspected complications of airway stents during an 18-month period. MDCT images were interpreted in a blinded fashion by an experienced thoracic radiologist before bronchoscopy was performed, and the accuracy of MDCT was determined using bronchoscopy as the gold standard. MDCT images were specifically assessed for the presence of the following complications: narrowing of stent lumen due to granulation tissue or secretions (or both), stent fracture, stent invasion by adjacent neoplasm, stent migration, and perforation of adjacent airways.
RESULTS. The study population was composed of 21 patients, with mean age of 48 years (range, 16–79 years), who underwent tracheal (n = 3), tracheobronchial (n = 7), or bronchial (n = 11) stent placement for benign (n = 13) or malignant (n = 8) airway disorders. Eleven of 21 stents were metallic and the remaining 10 were silicone. Thirty complications were detected in 21 patients, including stent luminal narrowing due to granulation tissue or secretions (or both) (n = 13), stent migration (n = 9), stent fracture (n = 4), stent invasion by adjacent neoplasm (n = 3), and tracheal perforation (n = 1). MDCT accurately detected 29 (97%) of the 30 complications diagnosed by bronchoscopy. There was one false-negative case in which MDCT failed to detect a stent fracture. There were no false-positive diagnoses of stent complications.
CONCLUSION. MDCT is highly accurate for detecting airway stent complications.
Keywords: airway stents • bronchoscopy • interventional radiology • MDCT • stent complications
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Bronchoscopy is currently the reference standard for the detection and treatment of stent complications and usually involves a two-step procedure [4]. Initially, diagnostic bronchoscopy is performed with a flexible bronchoscope to detect and characterize a stent complication; if a treatable complication is detected, rigid bronchoscopy is usually required for therapeutic intervention.
MDCT is a noninvasive imaging alternative to diagnostic bronchoscopy for the detection of stent complications. However, to date, only limited data regarding the accuracy of single-detector helical CT for detecting complications of metallic stents are available [9]. Importantly, clinical practice has shifted to the greater use of silicone stents because of their ease of removal and more favorable complication rate compared with metallic stents [4, 8]. To our knowledge, no prior studies have assessed the accuracy of MDCT for the detection of silicone stent complications.
To determine the role of MDCT in the assessment of airway stents, there is a need to assess its accuracy in a larger series of patients with complications from both silicone and metallic stents. Thus, the purpose of this study was to determine the MDCT detection rate of airway stent complications using bronchoscopy as the reference standard.
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The study cohort included 11 males and 10 females, with a mean age of 48 years (range, 16–79 years). Of the 21 airway stents, three were tracheal, seven were tracheobronchial, and 11 were bronchial in location. Stents were placed for benign airway disorders in 13 patients and for malignant airway disorders in eight patients. Eleven of the 21 stents were metallic (placed for malignancy, n = 7; tracheobronchomalacia, n = 3; postintubation stenosis, n = 1), and the remaining 10 were silicone (placed for malignancy, n = 1; tracheobronchomalacia, n = 7; sarcoidosis n = 1; severe kyphoscoliosis causing tracheal stenosis, n = 1).
MDCT Protocol
All patients underwent the same MDCT protocol on an 8-MDCT scanner
(LightSpeed, GE Healthcare) with a gantry rotation time of 0.5 second. The
MDCT protocol parameters were as follows: 170 mAs, 120 kVp, 2.5-mm
collimation, high-speed mode, and pitch equivalent of 1.5. The CT acquisition
focused on the central airways and included the entirety of the trachea and
main bronchi. Coronal and sagittal multiplanar volume reformation images, as
well as 3D internal and external renderings of the airways, were routinely
obtained in all cases.
Analysis of CT and Bronchoscopic Studies
All MDCT images were prospectively reviewed and the results were reported
by an experienced thoracic radiologist before bronchoscopy. MDCT images were
reviewed on a PACS workstation (Centricity version 2.0, GE Healthcare). The
interpreting radiologist was aware of the indication for stent placement and
the intended location of the stent but was unaware of the presence of or
concern for a specific stent complication.
For each patient's CT scan, the radiologist assessed for stent complications using MDCT criteria previously described in the literature [8]. Stent fracture was defined as discontinuity of the stent wall with or without identification of stent fragments protruding into the airways. Stent migration was defined as a stent position that differed from its intended location. Narrowing of the stent lumen was defined as the presence of a circumferential or polypoid intraluminal soft-tissue density within the stent lumen resulting in > 25% reduction in cross-sectional area. Neoplastic stent invasion was defined as contiguous spread of neoplasm into the affected airway through the stent wall. Airway perforation was defined as disruption of airway wall adjacent to the stent with an associated extraluminal air collection. Stent erosion into adjacent structures was defined as extension of the stent into extratracheobronchial structures.
All patients underwent flexible bronchoscopy performed by an experienced interventional pulmonologist using standard technique [3]. After completion of both MDCT and bronchoscopy, MDCT and bronchoscopic findings were reviewed jointly by the radiologist and bronchoscopist and evaluated for any discrepancies. In this review process, bronchoscopy was considered the reference standard for the diagnosis of airway complications.
Statistical Analysis
The CT detection rate of stent complications was calculated using
bronchoscopy as the reference standard. Detection rates were calculated for
each individual type of complication rather than on a per-patient basis
because some patients had more than one type of airway complication
present.
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Table 2 lists the frequency of specific complications for the subgroups of patients with metallic and silicone stents. As shown in Table 2, certain complications, such as stent fracture, were seen more frequently with metallic stents, whereas other complications, such as stent migration, were seen more commonly with silicone stents.
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There was a single false-negative diagnosis of metallic stent fracture. Although no abnormality was detected on MDCT in this case, a focal disruption of the stent was observed on bronchoscopic inspection, and the stent was subsequently noted to be fragmented at the time of bronchoscopic removal. After completion of the study, the MDCT images from this case were rereviewed by two radiologists with knowledge of the bronchoscopy findings to determine whether this false-negative case was an error in detection or was a technical limitation of MDCT. The stent disruption was not visible on retrospective review of the axial and multiplanar reformation images. Thus, the false-negative case was defined as a technical limitation of MDCT rather than an observer error. There were no false-positive diagnoses of stent complications.
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The most common complications of airway stents reported in the literature are stent migration, luminal narrowing (secondary to granulation tissue formation or adherent secretions around the stent), malignant disease recurrence within the stent, and stent fracture [8, 12–14]. Rare complications include airway perforation and stent erosion into adjacent structures [8]. Our results show that CT can accurately detect a broad spectrum of stent complications.
To our knowledge, this study is the first to assess the detection rate of CT for both major types of stents, silicone and metallic (Figs. 6A, 6B and 7A, 7B). The main advantages of silicone stents are that they are easily adjusted and removed, whereas metallic stents are quite difficult to adjust and remove once placed [15]. Metallic stents also have a higher rate of several complications, including fracture, erosion, tumor ingrowth, and granulation tissue response, than silicone stents [15]. Silicone stents are more predisposed to migration than metallic stents [15]. The complication rates for silicone and metallic stents in our study (Table 2) are consistent with these profiles [15].
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Ferretti et al. [9] previously compared single-detector helical CT and bronchoscopy in the assessment of 16 metallic stent complications among a series of 28 stents that underwent surveillance by CT. In that series, CT detected all clinically significant abnormalities, but two cases of moderate luminal narrowing due to granulation tissue at the stent origin were missed. These authors concluded that CT should serve as a primary surveillance technique for patients with stents and as a first-line test for suspected stent complications. They emphasized that bronchoscopy is not necessary when CT shows no evidence of stent complication. Importantly, our results confirm the accuracy of MDCT in a larger series (n = 30) of stent complications and also show that MDCT can be used to detect complications of silicone stents, which were not included in their series.
A major impact of our study is that patients with positive MDCT findings of a stent complication can avoid diagnostic flexible bronchoscopy and may go directly to therapeutic rigid bronchoscopy. There are two additional potential benefits to performing MDCT in patients with stent complications: First, MDCT findings can be used to plan the bronchoscopic intervention, thus potentially decreasing the time of the procedure; and, second, if the stent needs to be replaced due to fracture, a custom stent can be designed using measurements obtained from MDCT [8] (Fig. 5A, 5B, 5C). A future study is necessary to quantify these potential benefits.
Although our study was not designed to compare the roles of axial and multiplanar reformation CT images in the assessment of stent complications, we found that reformation images are particularly helpful for aiding in the detection and characterization of certain complications such as stent fracture and migration. We emphasize that axial and reformation CT images should be considered complementary tools in the assessment of airway stents.
Because MDCT is often performed in patients with airway stents for follow-up of an underlying disease process [9], it is important for radiologists to be aware of the accuracy of CT findings for detecting various stent complications. Thus, our results also have a practical application for radiologists interpreting routine MDCT scans of patients with airway stents.
Although to our knowledge our study is the largest series of stent complications imaged by MDCT, we acknowledge that there are relatively few rare complications. In addition, our study was limited to patients with proven stent complications. Because our study was limited to patients with bronchoscopically proven complications, we acknowledge that there may be a bias toward more severe complications. A future surveillance study comparing MDCT and bronchoscopy in a larger cohort of patients with and without stent complications would be helpful to determine the negative predictive value of MDCT and to ensure that the entire range of stent complications is assessed.
In summary, the results of this study show that MDCT is highly accurate for detecting metallic and silicone airway stent complications in comparison with the reference standard of bronchoscopy. MDCT has the potential to replace bronchoscopy for the routine surveillance of patients with airway stents.
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