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Cone Beam CT-Guided Bronchoscopy

Here to Stay?

Casal, Roberto F. MD

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Journal of Bronchology & Interventional Pulmonology: October 2018 - Volume 25 - Issue 4 - p 255-256
doi: 10.1097/LBR.0000000000000546
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Despite recent advances in peripheral bronchoscopy, its diagnostic yield for lung nodules remains suboptimal and varies considerably among different publications.1–5 The explanation for this wide range in the reported diagnostic yield is multifactorial. Different populations, operators, techniques, and study designs are some of the contributing factors. Unlike our gratifying experience with the advent of endobronchial ulrasound-guided transbronchial needle aspiration and its steep learning curve, the wide range in diagnostic yield of peripheral bronchoscopy highlights the lack of a reliable and reproducible technique. From the technical standpoint, the diagnostic yield of peripheral bronchoscopy can be influenced at 3 major levels: “navigation,” “confirmation,” and “acquisition.” “Navigation” is the ability to reach a target. This is dependent, among others, on target-related factors (ie, size, location, “bronchus sign”), the operator’s ability to interpret chest computed tomography (CT) findings/anatomy and apply them during bronchoscopy, and the ability to follow the desired pathway. Navigation software, electromagnetic-guided navigation, ultrathin scopes, steerable catheters, and robotic bronchoscopy may have an impact at this level. “Confirmation” is the ability to demonstrate that our navigation or sampling tools are in contact with the target. Leaving chest CT aside, the only technique that can potentially confirm contact with the target in real time is radial-probe endobronchial ultrasound. The pseudo-confirmation provided by navigational software is based on prebronchoscopy CT scans, it is not real time, and it is bound to a margin of error. Cone beam CT (CBCT), as we will discuss later on, may play a key role at this level. “Acquisition” is our ability to obtain diagnostic samples. This can be influenced by factors associated with the target (ie, malignant vs. benign histology), with our sampling tools, and also with the type of contact that we achieve with the target (ie, center, periphery, or adjacent to it). This last step—acquisition—is responsible for the gap between navigational yield and diagnostic yield. We may successfully reach a lesion, and yet, not be able to obtain diagnosis.

CBCT is a newer CT modality that has been adopted widely by interventional radiologists. Unlike standard (fan beam) CT, the system is compact enough to allow mounting it on a moving C-arm. CBCT performs volumetric data acquisition in a single rotation of the source and detector, with the patient remaining stationary during the examination.5 This latter characteristic and its ability to provide standard fluoroscopy images make CBCT more suitable than standard CT as an aid in peripheral bronchoscopy.

In the current issue of this journal Pritchett and coworkers retrospectively describe their experience on the use of CBCT during peripheral bronchoscopy.6 The authors performed a CBCT scan once patients were intubated for bronchoscopy before navigation. They utilized this scan to identify the target and, with a dedicated software, to create an overlay of the target on live fluoroscopy images (a modality that they termed “augmented fluoroscopy”). They then navigated to the lesions utilizing electromagnetic navigation equipment and tools—without using RP-EBUS for confirmation—and they utilized on-site cytology. They report that a second CBCT scan was only performed when deemed necessary. This technique was utilized for 93 lesions in 75 consecutive patients. The median size of these lesions was 16 mm (range, 7 to 55 mm). The overall diagnostic yield was 83.7% (95% confidence interval, 74.8%-89.9%) and pneumothorax occurred in 3 patients (4%). Radiation exposure data originated in a small subset of 9 patients and the mean effective dose (E) was 2 mSv per CBCT scan. The average number of CBCT scans per patient was 1.5. The first scan was performed before bronchoscopic navigation to generate augmented fluoroscopy images. Hence, the impact of CBCT was mainly at the level of navigation because CBCT was not uniformly utilized for confirmation before tissue acquisition. The high performance of this technique may then be dependent on the use of augmented fluoroscopy rather than the actual CT images from CBCT. Nonetheless, given its retrospective nature, single-arm design, and the fact that bronchoscopy was performed by a single operator, these promising results may not be generalizable, a limitation clearly acknowledge by the authors. Data on CBCT-guided bronchoscopy are rather scant. Studies from Hohenforst-Schmidt and coworkers (prospective, n=33) and Park and coworkers (retrospective, n=59) report on the use of conventional bronchoscopy associated with CBCT and describe a diagnostic yield of 70% and 71.2%, respectively.7,8 Bowling and coworkers described a small case series (n=14) with the combination of electromagnetic-guided navigation, a transbronchial access tool, and CBCT for lesions without a “bronchus sign,” reporting an overall diagnostic yield of 71%.9 There is still paucity of good-quality evidence (for both radiation exposure and diagnostic yield) to justify the combination of CBCT with diagnostic peripheral bronchoscopy.

Leaving aside the impact of CBCT in diagnostic peripheral bronchoscopy, this technique may still play a relevant role in the developing field of bronchoscopic ablation of peripheral lung tumors. CBCT can accurately and in “real time” show the location of the ablative probe with respect to the target, vital structures, and the pleura. Different bronchoscopic ablative probes produce dissimilar zones of ablation (size and shape), making their position with respect to the tumors key for a complete tumor ablation. Bronchoscopic ablation studies without CBCT confirmation can potentially introduce a negative bias in their results. Unsuccessful ablations may not be because of failure of the ablative technique itself, but because of suboptimal positioning of the ablative probe. Because incomplete ablation translates into untreated cancer cells and potential tumor progression, we should utilize all available tools to prevent it, and CBCT is bound to be part of our new armamentarium.

Despite the promising results from Pritchett and coworkers, data to corroborate that CBCT images can enhance navigational and diagnostic yield for peripheral lung tumors with acceptable radiation exposure are still needed. Randomized studies or prospective studies that evaluate navigational and diagnostic yield before and after obtaining CBCT images are needed to achieve these important goals. However, irrespective of its impact on diagnosis, CBCT may soon play a vital role in the developing field of bronchoscopic ablation of peripheral lung tumors and it may indeed be here to stay.

REFERENCES

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