Utilization of transbronchial cryobiopsy in bronchoscopic procedures: a clinical practice review of procedural considerations

Introduction

Cryosurgery involves the rapid freezing, slow thawing, and repetition of this freeze–thaw cycle to perform both diagnostic and therapeutic procedures (1). Endobronchial cryosurgery was first reported in 1968 with the use of a rigid application device (2). Subsequent work showed benefits in palliation of symptoms in patients with malignant central airway obstructions (3,4) and in treatment of early-stage carcinoma in situ (5). A position paper was subsequently published in 2002 jointly by the European Respiratory Society and American Thoracic Society Task Force to better define the evolving field of interventional pulmonology (IP), with an emphasis on the tools and technologies used in this new field (6). Indications for cryotherapy were listed as “palliation of noncritical endobronchial exophytic obstructive lesions and to remove foreign bodies and clots. Other indications may include treatment of low-grade malignant lesions (e.g., adenoid cystic carcinoma) and early cancer (e.g., carcinoma in situ)” (6).

Development of a flexible cryoprobe would lead to expansion of the cryosurgery technique beyond visible endobronchial lesions. A promising initial report in 1996 using a novel flexible cryoprobe for endobronchial obstructions showed complete removal in 90% (n=18/20) of treated cases (7). With this flexible cryoprobe now available, new applications for its use were sought with transbronchial lung cryobiopsy (TBLC) first reported in 2004 (8). The initial study compared seven patients with suspected interstitial lung disease (ILD) and seven with suspected malignancy undergoing conventional transbronchial forceps biopsies to TBLC. No complications occurred with the procedure and the TBLC samples were larger in volume and without crush artifact. Safety and diagnostic value of TBLC was first reported in 2009 with an evaluation of 41 patients with ILD with the report that “in a significant number of cases, transbronchial cryobiopsy contributed substantially to the definitive diagnosis” (9). Since this initial study, TBLC has become an increasingly important diagnostic tool in the evaluation of both ILD and non-ILD lung diseases. In this review, we will highlight the available literature related to the use of TBLC for the evaluation of various lung parenchymal abnormalities and discuss considerations for performing this procedure.

Procedural mechanism

To understand the role of cryobiopsy in the diagnosis of lung diseases, it is important to understand how this process works. The mechanism of action is based on the Joule-Thompson effect, which states that when a gas’s pressure changes, its temperature will also change. To take advantage of this process, compressed gas, usually nitrous oxide or carbon dioxide, is released at a high flow rate through a small orifice at the tip of the cryoprobe, leading to rapid gas expansion and a very low temperature at the tip (1,6,8).

During the bronchoscopic procedure, the cryoprobe is inserted through the working channel of the flexible fiberoptic bronchoscope. The cryoprobe console is activated with a foot pedal to allow the flow of liquified gas to the cryoprobe tip. The instrument is then placed in direct contact with the tissue area of interest to allow for cryoadhesion. In TBLC, the large size of the specimen usually requires that the sample be subsequently removed while still adhered to the cryoprobe tip, requiring that the bronchoscope, cryoprobe, and specimen are removed en bloc to allow sample collection.

Benefits of cryobiopsy specimens

Initial studies and subsequent analyses have shown significant benefits of the TBLC specimen compared to other means of sample acquisition. The overall size of the collected TBLC specimens is significantly larger compared to conventional forceps biopsies (8-11). Additionally, TBLC specimens show either a significant decrease or complete absence of crush artifact, which is commonly encountered with conventional forceps biopsies (8,9,11). The amount of alveolar tissue present is also increased (11).

These improvements in the size and quality of the specimen obtained by TBLC has facilitated an improved diagnostic yield. Although surgical lung biopsy traditionally has been considered the gold standard for the evaluation of ILD, TBLC performed at experienced centers is a recently updated guideline recommended alternative (12). Meta-analyses have found a pooled diagnostic yield of ~80% (13-18) for TBLC, which is comparable to SLB, but with an improved safety profile. In contrast to SLB, in which the in-hospital mortality rate for elective procedures is 1.7% and for non-elective procedures may be as high as 16% (19), the procedure related mortality rate of TBLC ranges from 0.3–0.5% in meta-analyses (20,21). Additionally, a direct comparison of patients undergoing either procedure at one institution showed a higher mortality rate (2.7% vs. 0.3%) and longer length of stay (6.1 vs. 2.6 days) with SLB compared to TBLC (13).

Although it remains challenging to accurately predict the diagnostic accuracy and safety of any biopsy, studies have shown some guidance in both procedural approach and patient selection when considering TBLC. Both the number of biopsies performed in a procedure and the number of procedures performed at an institution appear to impact the results of TBLC. Diagnostic accuracy is improved when performing three or more biopsies (85%) vs. two or one biopsies (77%) (14). Additionally, there is an increase diagnostic yield when TBLC is performed at centers with at least 70 cases per year (80.7%) compared to those performing less than 70 (76.8%) (18). Recommendations for patient selection vary regarding pulmonary function test (PFT) criteria. Minimal threshold values have been suggested by some governing societies for PFTs due to the potential for increased risk in these patients (22,23), although this has not been uniformly addressed (21) and remains an area of future research. Similarly, the presence of pulmonary hypertension, defined as a systolic pulmonary artery pressure of >50 mmHg, is considered a relative contraindication (22), but safety data in this population is also lacking. Further studies will also be needed to clarify the risk of performing TBLC in this patient population.

In addition to its role in ILD, TBLC also plays an important role in facilitating a diagnosis of non-ILD diseases. Although forceps biopsies (FB) have traditionally been considered a first line diagnostic test for the evaluation of suspected granulomatous disease or organizing pneumonia (24,25), a recent observational retrospective multicenter study shown an increased diagnostic yield for TBLC compared to FB for patients with both fibrotic and non-fibrotic ILD (26), the later representing an area that has been lacking evidence in the literature. For patients with lung nodules concerning for malignancy, there is also an increased diagnostic yield when performing TBLC compared to FB (10,27). Similar increased diagnostic results for TBLC are also seen when there is imaging concerning for malignancy or infection, including in immunosuppressed individuals, compared to performing bronchoalveolar lavage (28). The identification of acute cellular rejection after lung transplantation is also improved when TBLC is performed compared to FB (29).

Safety considerations

After TBLC was first performed for the evaluation of patients with ILD, numerous different approaches to performing the procedure were reported. Many of these efforts were designed to improve the safety profile and diagnostic accuracy of the procedure. Regarding safety, the major potential complications of TBLC are pneumothorax and bleeding. Recent meta-analyses report pneumothorax rates from 8–12% (14,16-18), but as high as ~20% (13) and moderate to severe bleeding (grade 2 or higher) in 9.9% (18) with mild bleeding in up to 29.9% (18). Rates of both complications decreased, however, when TBLC was performed at high volume centers, defined as institutions performing 70 or more procedures per year (18). The incidence of pneumothorax fell to 5.3% and moderate to severe bleeding decreased to 6.9% (18).

Initial efforts to mitigate these risks included the use of fluoroscopic imaging to guide positioning of the cryoprobe in relation to the pleural surface to reduce the occurrence of pneumothorax (9). While fluoroscopy guidance is helpful for biopsies performed in lateral lung segments with the cryoprobe perpendicular to the pleura (30), it remains challenging to accurately estimate probe-to-pleura distance for anteriorly or posteriorly directed lung segments. However, utilization of fluoroscopy still reduces the risk of complications compared to performing the procedure without this guidance (31,32).

Efforts to minimize bleeding focus both on decreasing the incidence of it occurring and managing it effectively once it occurs. The use of radial endobronchial ultrasound (EBUS) prior to performing TBLC may decrease unintentional sampling of the pulmonary vasculature (33). A multicenter, prospective study showed that the visualization of a dense radial EBUS sign, which implies lung consolidation, may decrease both bleeding risk and total procedure time (34). While this information may be informative, reliance on a radial EBUS signal is limited by the need to remove the radial EBUS probe to allow for cryoprobe insertion. Placement of the cryoprobe in an appropriately identified area by radial EBUS, or similar assurance that the cryoprobe has avoided a higher risk area, is challenging.

Thus, an additional consideration to mitigate the risk of bleeding is the prophylactic use of vasoconstricting medications. Although prophylactic instillation of medications for bronchoscopy is not well studied (35), the British Thoracic Society recommends considering this practice if bleeding may be likely (36). Initial studies instilling dilute phenylephrine (1:100,000 from a stock 100 mcg/mL concentration) and allowing it to dwell for ~3 min prior to TBLC have shown promising results as no bleeding above grade 1 (37) that required suctioning alone developed (38,39). Further studies will be needed to confirm the potential benefit of this practice before it is widely adapted.

Additional efforts to decrease both the incidence of pneumothorax and bleeding relies on the use of advanced imaging techniques. Multiple studies have shown a significant reduction in the rates of pneumothorax and bleeding when combining TBLC with cone beam computed tomography (CBCT) (38-41). A single-center prospective cohort study of 155 patients reported a 1.9% (3/155) pneumothorax rate and a 12.3% (19/155) incidence of moderate bleeding (41). Another smaller study of 33 patients reported no incidence of either complication (38), while a study utilizing both standard fluoroscopy alone or CBCT guidance found no evidence of either complication only in the CBCT guided group (39). Future randomized studies will be needed to confirm the potential benefit of this advanced imaging modality.

Efforts to control bleeding once it occurs have also been studied. Approaches have included the use of endobronchial blockers or balloons to control bleeding (18,31,42). Because TBLC requires removal of the scope en bloc with the cryoprobe and adhered specimen, continuous direct visualization of the biopsied airway lumen is not possible as in the case of conventional forceps biopsies. By occluding the lumen in which the TBLC was performed, the incidence of moderate to severe bleeding may be decreased as any blood from the biopsy will remain isolated in the distal airway rather than spread throughout the tracheobronchial tree before the bronchoscope is reinserted. Another effort to better control bleeding is the use of a two-scope technique in which the initial bronchoscope used for TBLC is removed from the airway and a second bronchoscope is then used to quickly re-enter the airway for visualization and bleeding control (43,44). While this method should work in theory, it has not been shown on a large scale to definitively decrease the rate of complications (43,44).

Guideline recommendations

Over time, guidelines and expert statements were developed to facilitate best practices for performing the procedure (21,45,46). Although studies have shown that TBLC may be performed under moderate sedation without an endotracheal tube or fluoroscopy (44), best practice recommendations (21,45) include the use of a large diameter flexible endotracheal tube (8.5 mm or larger) or rigid bronchoscope and utilization of general anesthesia. Fluoroscopic guidance is recommended to mitigate the higher rate of pneumothorax following TBLC without fluoroscopy (18). Similarly, prophylactic placement of an adequately sized endobronchial blocker or balloon to occlude the target lobe to be biopsied is recommended to control bleeding if it occurs.

Samples should be obtained from at least two different sites in the same lobe or two different lobes. Cryoprobes exist in both a reusable and single use model as well as in various sizes for these systems (reusable: 1.9 and 2.4 mm outer diameter vs. single use: 1.1, 1.7, and 2.4 mm outer diameter) with both carbon dioxide and nitrous oxide available to be used as the cryogen. Although no difference in diagnostic yield has been shown, a smaller cryoprobe outer diameter (1.7 or 1.9 vs. 2.4 mm) is recommended to be used to decrease risk (21,47,48). Freezing time varies depending on the condition and should be checked prior to each procedure, but a range of 3–6 seconds is generally utilized to obtain an appropriately sized biopsy specimen (49-51). Although these guidelines exist, heterogeneity in patient characteristics and operator and center experience must always be taken into account before deciding to perform TBLC and in the specific approaches of how the procedure is performed.

Conclusions

In conclusion, cryobiopsy has evolved from a procedure confined to the central airway for visible abnormalities to one that is relied upon to inform the diagnose and treatment of patients with both ILD and non-ILD diseases. Efforts over time to improve both diagnostic yield and safety have led to its inclusion in international recommendations for the evaluation of ILD and formalization of guidelines for performance of this important procedure. Future studies are needed to refine this procedure and evaluate the inclusion of new modalities and techniques that seek to further improve its safety and yield.

Acknowledgments

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, AME Medical Journal for the series “Diagnostic & Therapeutic Bronchoscopy”. The article has undergone external peer review.

Peer Review File: Available at https://amj.amegroups.com/article/view/10.21037/amj-23-136/prf

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://amj.amegroups.com/article/view/10.21037/amj-23-136/coif). The series “Diagnostic & Therapeutic Bronchoscopy” was commissioned by the editorial office without any funding or sponsorship. B.S.B. and J.S.K. served as the unpaid Guest Editors of the series. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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