The rising role of cryobiopsy in diagnosis of pulmonary disorders: a narrative review

Introduction

Transbronchial lung biopsy (Tbbx) is an integral part of the diagnostic pathway for a variety of different lung pathologies. The mainstay of use lies in three main patterns of undiagnosed lung disease: pulmonary nodule/mass, interstitial lung disease (ILD), and post-lung transplant surveillance. Traditionally, these biopsies have been completed with small forceps, but these are prone to modest samples and crush artifacts that make pathological determination more difficult (1). As a result, traditional diagnostic yields have remained low, with average estimates for diffuse lung disease (34–58%) and pulmonary nodules (37–70%) being suboptimal (1-6). In 2009, the transbronchial cryoprobe, which had largely been used as a therapeutic instrument in the airway for removal of clots, foreign bodies, and treatment of central airway obstruction, was utilized in the parenchyma to help define ILD to obviate the need for surgical lung biopsy (SLB) (7). This breakthrough in application of cryobiopsy afforded the proceduralist the ability to obtain a more intact specimen, while not subjecting the patient to a surgical biopsy technique and its associated morbidity and mortality, which can be as high as 4.3% at 30 days (8).

History and technology of transbronchial cryobiopsy (TbCb)

We have come a long way since the first known direct visualization of the vocal cords was performed by Alfred Kirstein in 1895, who described the experience as “I convinced myself that one can pass the vocal cords intentionally with a middle-sized esophagoscope into the cocainized trachea and right down to the bifurcation” (9). Meanwhile, the therapeutic use of cold temperature has been described since at least 3000 BC, when cold compresses were used to treat compound skull fractures and infected wounds in Ancient Egypt (10). The use of a cryoprobe as a specific means of therapy was initially introduced in the 1960s when neurosurgeons used it as a method to localize injury to the brain before resection (11). The cryoprobe made its formal introduction to the lung in the late 1960s for destruction of abnormal tissue visualized in the airways through use of repeated freeze thaw cycles that cause tissue sloughing that is then removed on subsequent days (12,13). Cryoprobe use was limited to this application in the lungs until the early 2000s, when a more flexible instrument was introduced. This technology allowed not only for therapeutic measures, but also for the inception of TbCb (14). The technique for the initial cryobiopsy was first described in 2009, where the probe was advanced through a large working channel of a flexible bronchoscope, then frozen to the tissue for several seconds, culminating in removal of the scope, probe, and sample en-block to preserve the tissue with use of a balloon inflated after removal of the scope to tamponade any potential bleeding from the site while the scope is not in the airway (14). The growth of TbCb further flourished when a 1.1 mm flexible cryoprobe was introduced, first showing improved efficacy in a porcine model, followed by the initial safety trial in humans published in 2022 (15,16).

From a physics standpoint, cryobiopsy relies upon the utilization of rapid expansion of compressed nitrous oxide or carbon dioxide gas (17). This gas is excreted in a targeted manner at high flow rates at the tip of the probe, freezing the surrounding tissue in a matter of seconds. In the lung, this results in the probe ‘freezing’ or adhering to the lung and the tissue is then extricated via a quick pullback of the probe. Given the relatively recent application of this biopsy technique, there remains ongoing need to define the diagnostic performance, assess safety profile, and feasibility of its application. The aim of this paper is to provide a comprehensive literature update on the utilization of TbCb and its increasing role as a means to diagnose lung pathology in a minimally invasive manner. We present this article in accordance with the Narrative Review reporting checklist (available at https://ccts.amegroups.com/article/view/10.21037/ccts-24-12/rc).

Methods

A literature search of PubMed and Cochrane Library was conducted for studies with search terms “cryobiopsy”, “transbronchial biopsy”, “interstitial lung disease”, “peripheral pulmonary nodules”, and/or “lymph node”. The search was restricted to publications written in English and included all available publications without limitation of date of publication up to March 30, 2024 (Table 1). Given the current limitations of available data, particularly for application to peripheral lung lesions and lymph node evaluations, all study designs including retrospective and systematic reviews were reviewed.

ILD and TbCb

Traditional modality of ILD evaluation

While the pathophysiology uniformly involves a process of inflammation leading to fibrosis, ILD comprises a large range of heterogeneous disease groups. Given this wide spectrum of disease presentation with variable severity and progression, classification and determination of cause is critical to the management of ILD. Endorsed by multiple international societies, the creation of a multidisciplinary discussion (MDD) group that contextualizes all components of the patient’s history, physical, and workup has become a standard of care for this evolving disease process (18).

One subtype of fibrosing ILD, idiopathic pulmonary fibrosis (IPF), can be defined from radiologic pattern of usual interstitial pneumonia (UIP) and does not warrant tissue diagnosis prior to initiation of treatment according to multiple societal guidelines (18). However, in cases with high-resolution computed tomography (HRCT) findings of probable UIP, indeterminate UIP, or an alternate diagnosis, further investigation is often warranted. Non-invasive biomarkers can be suggestive towards a diagnosis in conjunction with other data, but often lack adequate sensitivity and/or specificity, warranting consideration of tissue biopsy (19).

Traditionally, the gold standard modality of biopsy for evaluation of ILD has been SLB. Multiple biopsies are obtained from two to three lobes in SLB for correlation, as there may be different pathologies involved. In a systematic literature search of pooled data leading to current guidelines, adequate tissue and a specific diagnosis was obtained in 88.2% of cases via SLB (18). While therapies are able to be appropriately tailored based on histopathologic diagnosis, SLB is not without significant risk. In-hospital mortality following SLB for investigation of ILD in elective cases is around 1.7%, and is significantly higher at 16.0% for patients who undergo SLB during non-elective admissions (20,21). The risk factors of older age, non-elective nature of the procedure, and requiring long-term oxygen therapy are unsurprisingly associated with higher rate of mortality, yet this is the patient cohort in greatest need of a histopathologic diagnosis for guidance of diagnosis. There are additional risks of respiratory infection, bleeding, prolonged air leak and delayed wound healing.

The lack of a tissue diagnosis was the most common reason for unclassifiable ILD following MDD in an ILD cohort in a single center, with the invasive nature of SLB precluding the procedure. Roughly half of the patients with unclassifiable ILD were deemed not to be candidates for SLB, 9% had mild or stable disease where risks of SLB were thought to outweigh the benefit, and 8% patients declined SLB (22). At one year follow-up, disease progression was found in nearly 50% of the patients without a diagnosis, highlighting the need for a less morbid procedure.

As such, less invasive modalities of sampling via traditional Tbbx have been utilized, however with suboptimal results. The adequacy of tissue sampling is 77.6%, with even lower ability to obtain a definitive diagnosis, as the sensitivity for UIP was only 30% (17,18,23,24). As such, the role of Tbbx with conventional forceps is likely limited to imaging patterns suggestive of sarcoidosis and/or organizing pneumonia, which can be evaluated pathologically on smaller samples with crush artifact (17,25).

Application of cryobiopsy for ILD evaluation

Bronchoscopic TbCb with the ability to obtain larger, more intact specimens than the traditional forceps biopsy, would seem ideally suited as a surrogate for SLB given its advantage in safety. Moreover, the larger size of specimens is particularly valuable for histologic evaluation in ILD, as distribution of pathologic patterns is an important hallmark. Indeed, in one of the first studies to illustrate the application of cryoprobe for transbronchial lung biopsy for ILD evaluation, the samples obtained via TbCb were about three times larger than that obtained via forceps (15.11 vs. 5.82 mm², P<0.01), comparable to previously reported specimen sizes in application of cryoprobe for other indications (14).

TbCb has held more promising results for diagnostic yield in comparison to Tbbx. A meta-analysis of 11 studies of TbCb for evaluation of ILD conducted in 2016 reported histopathologic diagnostic yield ranging from 75% to 98% and diagnostic yield in conjunction with MDD ranging from 51–98% (26). Another systematic review of predominantly retrospective studies conducted in 2018 of TbCb of patients with ILD using 1.9 or 2.4 mm cryoprobe reported a histopathologic diagnostic yield of 80% (27). The subgroup analysis highlighted that the number of biopsy passes was a significant factor in obtaining adequate pathology: diagnostic yield was 77% or less when fewer than three samples were obtained, increased to 88% when three or more samples were collected. These data, however, are limited by their retrospective nature and lack of comparison to gold standard SLB.

Subsequently, there have been ongoing efforts to characterize diagnostic accuracy of TbCb with diagnoses obtained by SLB. To date, there have been three prospective studies featuring the same patients undergoing sequential TbCb and SLB, followed by blinded review of specimens by pathologists for designation of histopathologic diagnosis and a final diagnosis determined following MDD. The first prospective study, Cryo-PID, reported a poor concordance at 38% (Table 2) (28). This was followed by the COLD-ICE trial, that reported a contrary finding of decent concordance with a 70.8% correlation between the TbCb and SLB of samples obtained from two ipsilateral lobes, increased to 76.9% following MDD (29). Nonetheless, diagnostic confidence reported by pathologists was frequently lower for cases of TbCb compared to SLB (12.3% vs. 46.2%, P=0.007), attributed to the smaller specimen size. The CHILL study, which compared diagnostic accuracy of Tbbx with forceps, TbCb, and SLB, reported more comparable diagnostic accuracy of TbCb with COLD-ICE at 68.75% (30). No randomized controlled trial (RCT) has thus far been conducted for comparison of TbCb and SLB for their impact on clinical outcome. What these trials do highlight is the need for a multidisciplinary approach for the patient, which can hopefully arrive at a diagnosis without needing a biopsy in certain instances.

Current limitations of cryobiopsy for ILD evaluation

When compared to SLB, smaller sample sizes and more central location from which samples are obtained are recognized inherent limitations of TbCb.

In the CAN-ICE trial, TbCb and SLB correlation was assessed along with in-center and between-center agreements of the diagnosis (31). Of twenty patients studied, 61.7% achieved in-center diagnostic agreement between TbCb and SLB. The most common discordant cases were IPF diagnoses with TbCb being classified as fibrotic hypersensitivity pneumonitis with SLB, both following MDD—which can have significant treatment implications. On the other hand, between-center agreement of diagnoses was notably lower for diagnoses obtained via TbCb over SLB with correlation coefficients 0.29 and 0.71, respectively. While there are several limitations that may have contributed, it is notable that the proportion of samples deemed unclassifiable or a low-confidence diagnosis by pathologists was higher for TbCb than SLB (68.3% vs. 36.7%).

The secondary study of the COLD-ICE trial suggests that the diagnostic accuracy of TbCb may be improved by consideration of other histopathologic features predictive of UIP (33). The sensitivity of TbCb for obtaining pathologic findings of “subpleural or paraseptal predominant fibrosis” is not optimal and perhaps, may not be reasonably achievable given the presence of these features in peripheral zones. However, histopathologic findings of ‘patchy fibrosis’, ‘fibroblast foci’, and ‘absence of alternative diagnostic features’ obtained via TbCb can reliably narrow the diagnosis in combination with imaging findings and MDD consensus diagnosis, precluding the need for SLB.

A second limitation to the current use of TbCb is lack of a defined optimal number of samples to be obtained for sufficient diagnostic accuracy. As noted previously with regards to diagnostic yield, obtaining more than three samples correlated with a higher yield (33). Small sample size limits generalizability, but there was nevertheless a not statistically significant trend towards increased concordance with SLB samples with the number of TbCb samples obtained in the secondary analysis of COLD-ICE cohort: 25% with three samples, 55% with four samples, 80% with five samples, 83% with six samples, and 100% with seven samples. A different study also highlighted a general increasing trend of higher diagnostic accuracy with more samples obtained with three samples having 75% concordance with SLB compared to 50% with one sample (32). In the two prospective trials that reported acceptable diagnostic accuracy (COLD-ICE and CHILL), the median number of biopsies obtained was five, from which it may be inferred that this may be the optimal baseline goal until further studies are undertaken.

Further inhibiting diagnosis via TbCb are the current histologic diagnostic criteria that requires a significant amount of tissue for definitive answer. Ongoing studies, like FROSTBITE-II, will hopefully provide guidance for easier pathway to minimally invasive diagnosis and standardization of protocols (34).

Complications of cryobiopsy

The two most notable complications of TbCb are pneumothorax and bleeding. While there is currently a lack of consensus guidelines, the safety profile of TbCb does appear to be acceptable for use.

Procedural factors that increase diagnostic yield and/or accuracy are unfortunately also factors that contribute towards increased risk of pneumothorax, primarily sampling at two different sites and proximity to the pleura (17). While the rate of pneumothorax due to TbCb reported from observational studies appear variable as high as 20.2%, amalgamating studies suggests an incidence closer to 10% (Table 3) (17,26,27,35-38). Delving deeper, only half of patients with a pneumothorax required tube thoracostomy drainage. The use of fluoroscopy guidance for cryoprobe positioning and use of small (1.9 mm) rather than larger (2.4 mm) probe have been suggested to be associated with lower pneumothorax risk (39,40). Notably, pneumothorax rates utilizing the novel, disposable 1.1 mm cryoprobe are not statistically different than forceps biopsies in a recent safety study (16).

Bleeding is a commonly noted complication of TbCb. Due to the larger size of the specimens obtained via TbCb when using a 2.4 mm probe, these samples frequently are retrieved en-bloc with the bronchoscope as they cannot be removed through the working channel (17). As such, the bronchoscope cannot be maintained wedged in position to minimize potential bleeding as is possible with Tbbx with forceps. Furthermore, there may be patient-associated risk factors that increase risk for incidence and duration of hemorrhage, particularly with background of ILD due to traction bronchiectasis and location of medium-large vessels (41). As illustrated by Table 3, the bleeding rates and the severity therein is quite variable, ranging from about 10% to a wider range of 20–40% of moderate to severe bleeding (26,27,35,37,38). The degree of bleeding is not graded in all studies that report the complication rate and moreover, amongst those reported, the definition of severity of bleeding is rather variable across studies, making the clinical implication and the safety profile more difficult to elucidate. Given the inconsistencies in reporting, we would advocate for researchers to grade bleeding using an established scale (42).

Within these confines, in general, most studies used the guide of moderate bleeding as that requiring wedging of bronchoscope or endotracheal tube and use of iced saline and severe bleeding as that requiring further interventions such as use of endobronchial blocker and/or other resuscitative measures. One observational study noted significantly lower incidence of bleeding of TbCb with prophylactic endobronchial blocker placement compared to none [1.8% vs. 35.7%, adjusted odds ratio (AOR) 0.02] (39). Routine placement of an endobronchial blocker has been recommended if using large cryoprobes (2.4 mm), however newer safety data on smaller probes is promising that bleeding risk if comparable to traditional forceps biopsy (16,17).

Furthermore, the application of advanced imaging in conjunction with TbCb may further aid with improving safety profile. Preliminary studies of TbCb with CT-fluoroscopy and cone beam CT guidance have reported improved accuracy with probe-to-pleura positioning to areas of interest, which is particularly of importance in ILD evaluation for the balance of optimization of diagnostic accuracy with procedure-associated complications (43-46). In a cohort study with use of cone beam CT guidance with TbCb for evaluation of ILD, the cone bean CT guidance warranted repositioning in 42.6% of the patients and overall yielded favorable diagnostic yield (86.5% pathologic diagnosis, improved further to 90.3% with MDD) with comparatively low complication rates (1.9% pneumothorax, 12.3% moderate bleeding, no severe bleeding, and no 30-day mortality) (44).

Direct comparison of complication rates between SLB and TbCb are difficult due to variable definition of complications and the populations that are selected for the corresponding biopsies (47). For instance, pneumothorax is not considered a complication in studies with SLB as chest tube drainage is a part of the post-operative care. Nonetheless, severe complications are generally estimated to be lower with TbCB, especially with mortality; mortality of TbCb and SLB have been reported to be 0.3% and 2.3% respectively.

Current consensus and practice

Given the high variability of reported diagnostic yield and accuracy, along with lack of procedural standardization, controversies regarding recommendations for or against the use of TbCb remain (48). Nonetheless, TbCb appears to be associated with shorter hospital length of stay and mortality associated with adverse event in comparison to SLB (49). Additionally, two available studies of cost analysis comparing TbCb and SLB have shown favorable cost reduction with TbCb of about EUR 60,000 over a 3-year period and estimated reduction of about GBP 647 per patient per year (35,50).

There is also a component of learning curve of the use of TbCb from a procedural perspective. One retrospective study from a single center reviewing their experience of the first 100 TbCb performed showed improved diagnostic yield and sample size in the second group of 50 patients compared to first, and while not statistically significant, a decreased rate of pneumothorax rate (12% vs. 24%) (51). As overall application and training with cryoprobe become more widespread, the risk-benefit ratio of pursuing TbCb with optimal approaches for diagnostic yield may become more favorable.

In the most recent European Respiratory Society (ERS) guidelines specifically addressing the use of TbCb in diagnosis of ILD, conditional recommendations are made for (I) TbCb as a reasonable alternative modality for patients with uncategorized ILD otherwise eligible for SLB, (II) TbCb as evaluation in undiagnosed ILD, if patient is not eligible for SLB and histopathological data is indicated, (III) pursuing SLB in setting of non-diagnostic TbCb rather than second TbCb, and (IV) TbCb to be performed by trained proceduralists, though the optimal training is not defined (47). With recognition that diagnostic yield is higher for SLB, the CHEST guideline of 2020 also suggests the role of TbCb as comparable to SLB with MDD (17). While lacking adequate evidence for strong guidelines, there are general recommendations for sampling with cryoprobe positioning 1 cm from the pleura, at two different sites, use of fluoroscopy, endobronchial blocker and small cryoprobe. As more understanding of the safety profile and role of TbCb in diagnosis of ILD grows, future studies may reveal not just equivalence or superiority of TbCb over SLB, but also better selection of cases that may warrant one diagnostic approach over the other (48).

Pulmonary nodules and TbCb

Rising need for evaluation of pulmonary nodules

Perhaps the fastest growing area of use for TbCb is in the lung periphery for the diagnosis of peripheral pulmonary lesions (PPLs). Prompt diagnosis and treatment of lung cancer is critical, as the stage of malignancy has significant impact on modality of treatment and survival rate (52). There has been an increasing incidence of pulmonary nodule detection owing to the implementation of lung cancer screening (LCS) programs in high-risk populations and more ubiquitous use of computed tomography (CT) in other practices (i.e., coronary calcium scoring) (53-55). In one large population-based study between 2006 and 2012, the annual frequency of chest CT imaging increased from 1.3% to 1.9%, leading to an increased rate of detection of pulmonary nodules from 3.9 to 6.6 per 1,000 person-years (54). While most of these incidental pulmonary nodules are benign, appropriate surveillance and identification of those that require further evaluation are crucial as this population does not otherwise receive the dedicated scrutiny of LCS as those identified as having high risk factors (55).

Methods of peripheral pulmonary nodule evaluation

CT or ultrasound-guided transthoracic biopsy (TTBx) and transthoracic needle aspiration (TTNA) have been traditionally the most common approach for PPLs, with overall consistently high diagnostic yield and accuracy around 90% or higher (56). Albeit considered minor risks and not always requiring intervention, TTBx and TTNA carry a high risk of pneumothorax, with an occurrence rate of up to 20–25% (57). This differs from the bronchoscopic transbronchial approach, with a much lower pneumothorax rate related to typically not violating the parietal pleura (6,58). Additionally, bronchoscopy affords the additive benefit of lymph node staging with endobronchial ultrasound (EBUS) as a single procedure. This ability is favorable as it exposes patients to less burden of frequency of procedures, but also importantly facilitates more prompt treatment (59). Yet, diagnostic yield of PPL via bronchoscopy suffers from a wide range of diagnostic yield varying from 40% to 90% (6,60). Despite the implementation of different guided bronchoscopy technologies, including the most recent robotic-assisted bronchoscopy (RAB), there remains a lag in consistent optimization of diagnostic yield (60,61).

To this end, TbCb is being explored as a potential tool that may finesse diagnostic yield further than the traditional sampling tools of brush, forceps, and needle aspiration (62). An additional unique advantage of cryobiopsy is its ability to obtain tissue circumferentially from the probe (63). Even with the aid of guided localization for PPL, the orientation of PPL is important for optimal diagnostic yield via conventional sampling tools. Radial EBUS probe positioning relative to the PPL has been associated with significant impact on diagnostic yield; diagnostic yield of samples when probe was positioned within the lesion (with concentric view) has been shown to be about double that if positioned adjacent to the lesion (with eccentric view) at 84% and 48% respectively (64). With forceps and needle aspirations, sampling can only be directed in a forward movement and thereby limits yield when the lesion is adjacently located with access to edge of the lesion. On the other hand, given tissue is able to be obtained in a 360-degree orientation relative to the probe, TbCb is thought to optimize sampling of tissue that is lateral to the probe.

Application of cryobiopsy on PPL evaluation

In the first RCT of TbCb use for PPL, CT-guided TTBx was compared with radial EBUS (r-EBUS) guided TbCb, showing a comparative diagnostic yield (CT-guided TTBx 93.8% vs. r-EBUS guided TbCb 85%, P=0.61), as well as comparative performance in ability for genomic testing for epidermal growth factor receptor (EGFR) (65). While maintaining comparable diagnostic yield, the rate of pneumothorax was remarkably lower by about 10-fold (CT-guided TTBx 44% vs. R-EBUS guided TbCb 4.2%, P=0.004). There have since been additional studies highlighting similarly high diagnostic yields with low complication rates, particularly with smaller cryoprobes (Table 4) (65-69). While these early studies still require further confirmation as other studies do not as robustly replicate the superior diagnostic yield, they show TbCb had higher diagnostic yield for eccentrically or adjacently oriented lesions and for PPLs that are less than 2 cm in diameter (63,70-72). The assistance of platforms such as electromagnetic navigation guided and RAB appear to further strengthen diagnostic yield in combination (73-75). A small prospective study of TbCb with electromagnetic navigation bronchoscopy compared to Tbbx of PPL less than 2 cm showed improved diagnostic yield (69% vs. 38%, P=0.017) (76). In a study with RAB with three different sampling modalities of transbronchial needle aspiration (TBNA), forceps Tbbx, and TbCb, TbCb was the only sampling method that yielded a diagnosis in 17.6% and noted to have improved cellularity for analysis (75).

Moreover, molecular genetic tumor characterization is an important component of diagnostics with the now standard role of immunotherapy and ever-growing development of more targeted therapy in thoracic oncology, particularly for non-small cell lung cancer (77,78). The larger specimen sizes obtained via TbCb are thought to be beneficial for molecular genetic analyses and immunohistochemical staining (79,80). Early studies suggested that EGFR mutations may be detected with increased rate with TbCb and subsequently, similar suggestions have been made regarding next generation sequencing (65,81).

The introduction of a smaller 1.1 mm cryoprobe has allowed for hopes of mitigation of some previously noted limitations of TbCb. In one of the initial studies to evaluate for the safety profile and feasibility of its use, findings were suggestive of low pneumothorax rate comparable to that of Tbbx with forceps at 4%, with none requiring intervention (16). While with higher reported incidence of some degree of bleeding, only 4% required wedging and/or iced saline and none required more advanced intervention such as endobronchial blocker inflation. TbCb has not yet been routinely adopted for use for PPL evaluations, however if further data is reassuring against some of the complication rates that have previously been raised as concern and the ability to perform similar in style with forceps biopsies without need for prophylactic endobronchial blocker placement or en bloc removal, this technique may become generalizable and accessible to more practice settings. Furthermore, the flexibility of the smaller cryoprobe affords compatibility with advanced navigation techniques and maneuverability with more peripheral airways that may be more angulated, raising further potential for growing application of this sampling tool (82). With an ongoing FROSTBITE-2 trial continuing to evaluate the performance of 1.1 mm cryoprobe in comparison to forceps biopsies, there may be further data to support guidance for more widespread application of TbCb for PPL in the future.

Transthoracic lymph node cryobiopsy

Another area in which bronchoscopy is valuable in diagnostics is evaluation of mediastinal lymphadenopathy via EBUS, which can be implicated in both malignant or non-malignant disease processes. The role of EBUS-guided TBNA has been well-established as the initial preferred modality of evaluation over historically used mediastinoscopy, particularly in cases with suspicion of malignancy (83-85). Three most common indications and etiologies of lymphadenopathy that warrant diagnostic evaluation include staging for suspected or confirmed malignancy, lymphoma, and granulomatous processes. Despite its widespread use, the limitations of EBUS-TBNA include instances in which histopathology rather than cytology may be valuable, particularly in the case of benign disease to more confidently rule out malignancy or malignancy that require evaluation of background architecture (i.e., lymphoma). Transbronchial mediastinal cryobiopsy (TMC) is hence being increasingly applied for optimization of bronchoscopic lymph node evaluation. Table 5 summarises some of the diagnostic yields and complication rates of TMC for indications discussed as follows.

Technical aspects of nodal biopsy with cryoprobe

A major challenge of biopsy via forceps and cryoprobe of lymph nodes is the ability to penetrate the tracheobronchial wall and lymph node capsule. There have been two main modalities utilized for cryobiopsy of lymph nodes in reported studies comparing EBUS-TBNA and TMC. In the earlier randomized trial by Zhang et al., high-frequency needle-knife was used to first make a small incision in the tracheobronchial wall adjacent to the mediastinal lesion, followed by placement of the cryoprobe within this lesion (86). The limitation of this method is the resultant damage of the airway due to the incision along with increased risk of bleeding. This method also requires the bronchoscopist to be familiar with electrocautery (90). Without making an incision, some of the previously encountered challenges with attempts of forceps biopsies through sites of needle aspiration was the inability to pass the instrument through the TBNA puncture site (91). In a later study, it was reported that TbCb was obtained with 1.1 mm cryoprobe introduced into the puncture site made by the 22-gauge needle without additional incisions (88). There has since been a report of further optimization to create a channel for the cryoprobe entry by advancing the TBNA needle sheath into the lymph node (90). The complication rates of both the studies of TbCb by needle-knife incisions and through TBNA needle puncture sites were minimal. Our current practice has evolved for use of the 1.1mm cryoprobe after an initial puncture with a 21- or 22-gauge TBNA needle to create a channel to pass the probe given the reduction in additional steps and required skills and tools.

Cryobiopsy application to lymph node staging

Cytology of lymph nodes obtained by EBUS-TBNA has been an increasingly used and reliable method for molecular analysis and immunohistochemical staining (92). There is a paucity of data regarding cryobiopsy sampling of lymph nodes, with at least a component of the current limitation likely due to the lack of familiarity of technical aspects required. Nonetheless, some early case series of EBUS guided cryobiopsy of lymph node show promising results of obtaining samples that are adequate, and likely superior, for histopathologic examination, immunohistochemistry staining, and molecular analysis compared to EBUS-TBNA (93-96).

Role of TbCb in evaluation of lymphoproliferative disorders

For the diagnosis of lymphoma, the pathologic subtype, grade, and assessment of transformation is particularly important for the course of treatment. Due to this, excisional biopsy has traditionally been the preferred modality of tissue diagnosis in suspected lymphoma, not only to assess the lymph node architecture but in order to obtain adequate tissue to perform immunohistochemical staining and flow cytometry (97). Nonetheless, mediastinoscopy or thoracoscopy are modalities of mediastinal biopsy that carry higher morbidity and mortality (98). While these evaluations may eventually become necessary if cytology obtained via EBUS-TBNA is not adequate, the minimally invasive nature and safety profile of bronchoscopy in comparison has led this to be a reasonable initial step of evaluation in those with suspected or recurrence of lymphoproliferative disorders (99). The largest meta-analysis that evaluated the diagnostic yield EBUS-TBNA in cases of suspected lymphoma thus far showed the sensitivity and specificity for new diagnoses of lymphoproliferative disorders to be 68.6% and 99% respectively. Sufficient samples were obtained in about 63% of the positive samples for subtyping. To note, the included studies all reported minimal risks with an overall adverse event rate of about 0.7%, including one incidence of minor bleeding and excessive cough. Most of the cases that required subsequent evaluation for confirmation were cases of new diagnoses of lymphoma that required subtyping. Given the alternative option of mediastinoscopy in an otherwise undifferentiated lymphadenopathy and the comparative associated cost and morbidity, EBUS-TBNA has become adopted as an acceptable first step in evaluation of suspected lymphoma.

The first systematic review of the diagnostic yield of EBUS-guided cryobiopsy in compared to EBUS-TBNA in mediastinal adenopathies suggested the utility of cryobiopsy with higher diagnostic yield in overall cases. In particular, it reported a remarkably higher diagnostic yield in cases of lymphomas (83% cryobiopsy vs. 42% EBUS-TBNA) (89). Further, the cases of samples obtained by cryobiopsy more frequently allowed for the lymphoma subtype. Similarly, a RCT of mediastinal lymph node evaluation with 1:1 randomization to combined EBUS-TBNA and TMC versus control group of EBUS-TBNA alone showed similar findings of improved diagnostic yield (93% vs. 81%, P=0.0039) (87). There has additionally been a case report of diagnosis of adult T-cell lymphoma with use of TbCb, a subtype that is typically thought to be difficult to diagnose and further limited due to small sample size of samples obtained via forceps by Tbbx or TTNA (93). This initial data holds much potential given the current recognized utility yet significant limitations of EBUS-TBNA that often necessitate subsequent invasive mediastinoscopy to obtain optimal samples for ancillary testing and delineation of subtypes.

Evaluation of sarcoidosis

As a multisystemic disorder characterized by noncaseating granulomas, the most common manifestation of sarcoidosis is hilar lymphadenopathy (100). Though lymph node sampling is not required for diagnosis in patients with high clinical suspicion due to rather specific clinical manifestation such as Löfgren’s syndrome, many require tissue sampling particular if symptomatic to confirm suspicion prior to proceeding with treatment and to exclude other pathology, particularly malignancy. Since EBUS-TBNA was first reported to have good diagnostic yield for sarcoidosis, it has been recommended as an optimal minimally invasive and safe modality of evaluation in setting of suspected sarcoidosis (101-103). The diagnostic yield of EBUS-TBNA alone is estimated to be about 80% and as such, is recommended to be combined with other sampling modalities such as endobronchial biopsy and transbronchial lung biopsy that increase diagnostic yield to closer to 95%. TMC has been either with comparable or superior sensitivity compared to EBUS-TBNA for the diagnosis of sarcoidosis in available studies thus far (86,88). While much of the need for more invasive methods to evaluate lymphadenopathy in setting of suspected sarcoidosis has been lowered by the relatively high efficacy with multiple sampling modalities, TMC adds to another option that can obviate need for mediastinoscopy.

Future applications

As technology improves, cryoprobe technology may be applied in the future for not only diagnostics but also in therapeutics in the form of cryoablation in treatment of early-stage lesions in the lung periphery.

The concept of thermal ablation is that the cold temperature induces tissue injury and generates anti-tumor immune response (104,105). Cryoablation has already been applied to other extrathoracic malignancies, particularly skin cancer, and through image-guided cryoablation in selected cases of hepatic and renal malignancies. Application of bronchoscopic cryotherapy for treatment of early stage, superficial bronchoscopic carcinoma has been demonstrated to have favorable outcomes (106,107). Further, interventional radiologists have been utilizing this technology with some success, and the application via bronchoscopy would afford the proceduralist the ability to treat some of the common complications simultaneously (i.e., bleeding) (108). While it has been demonstrated primarily in animal models, bronchoscopic cryotherapy and the proposed mechanism of synergism generated by the anti-tumor response of cryoablation suggests a potential role to be further explored as an adjunct to systemic immunotherapy in thoracic malignancies (109,110).

Conclusions

Medical applications of cryotechnology are increasing, and the lungs are no exception. With increased familiarity in technique along with findings suggestive of improved adaptability and safety profile, the use of cryoprobe technology beyond the traditional therapeutic role in airway obstruction may further expand its diagnostic and therapeutic roles. The unique benefits of cryobiopsy are in its ability to obtain larger and intact specimens in a radial manner. There are promising data in improved diagnostics and acceptable safety profiles for multiple lung disease, including ILD, PPL, and lymphoproliferative disorders. While in relatively initial stages of adaptation, cryobiopsy is a tool with encouraging potential to facilitate more prompt diagnosis and minimization of procedures that can be feasibly incorporated into current practices of bronchoscopy, with an eye toward future therapeutics.

Acknowledgments

Funding: None.

Footnote

Reporting Checklist: The authors have completed the Narrative Review reporting checklist. Available at https://ccts.amegroups.com/article/view/10.21037/ccts-24-12/rc

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Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://ccts.amegroups.com/article/view/10.21037/ccts-24-12/coif). The authors have no conflicts of interest to declare.

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