Background
Lung cancer remains the leading cause of cancer-related death in the western population [
1,
2]. In addition to primary tumours, the lung is the most common site for metastatic spread of other malignancies. Curative treatment of lung tumours or metastases requires surgical resection, which results in the loss of functional lung parenchyma. Unfortunately, fewer than half of patients are eligible for curative surgery due to limited lung function [
3]. Radiation or chemotherapy alone rarely cures patients. Minimally invasive local ablation (for example, percutaneous laser or radiofrequency) results in major complications in 6% of patients and procedure-specific 30-day mortality rates of 2.6% [
4]. Furthermore, the local progression rate is 35% [
5]. Therefore, these procedures are palliative strategies.
High-intensity focused ultrasound (HIFU) is a non-invasive, highly precise procedure to locally destroy tissue through ablation. Current clinical trials are evaluating the effectiveness of HIFU for the treatment of cancers of the brain, breast, liver, bone, and prostate [
6‐
9]. However, lung cancers have never been treated with this approach, because the ventilated lung is a total acoustic absorber and reflector [
10]. This problem was solved by lung flooding. Lung flooding enables efficient lung sonography and tumour imaging in
ex vivo human and
in vivo porcine lung cancer models [
11]. The conditions for applying HIFU to lung tumours are provided.
The aim of this study was to evaluate the effectiveness of sonography-guided HIFU with lung flooding for lung tumour ablation in ex vivo human and in vivo animal models.
Discussion
Currently, the best curative treatment for early-stage non-small cell lung cancer and lung metastases is surgical resection. However, surgery is not appropriate for patients with reduced pulmonary function due to the loss of lung parenchyma. Minimally invasive technologies for local tumour control have been developed as alternatives to surgery. The most common method is percutaneous radiofrequency ablation. However, this kind of thermal ablation is limited because the extent to which tissue has been successfully ablated cannot be accurately assessed. This could be a reason for the observed rate of local recurrence. Another risk factor for local tumour progression is the adherence of viable tumour tissue to needle applicators and subsequent spreading of tumour cells [
12]. In addition, pulmonary percutaneous radiofrequency ablation is invasive and associated with major complications and mortality.
HIFU has been examined in animal experiments and clinical trials as a technique for localized tissue ablation for more than 60 years. HIFU is superior to other radiation beams, because it penetrates deep and selectively destroys tumour tissue. In addition, HIFU can be applied as many times as needed [
13]. HIFU is currently used as a therapy for cancers of the abdominal organs, breast, and brain [
14‐
18].
Non-invasive local therapies with curative intention are not available for lung tumours. HIFU has never been applied to lung tumours, because the air content in ventilated lungs reflects acoustic intensities. We solved this problem with lung flooding. Lung flooding enables efficient lung sonography and tumour imaging in
ex vivo human and
in vivo porcine lung cancer models [
11]. The cancerous tissue was visualized by replacing the alveolar gas with fluid, facilitating ultrasound-guided HIFU. The aim of the present study was to explore the ability of therapeutic ultrasound to penetrate the overlying lung parenchyma without damage and to increase the temperature of the tumour tissue.
A tissue temperature greater than 60°C will usually cause instantaneous and irreversible cell death within one second in most extrapulmonary tissues due to coagulation necrosis. Coagulation necrosis is the primary mechanism by which HIFU destroys tumour cells [
6]. Our
ex vivo studies showed that lung cancer tissue absorbed acoustic energy, leading to an increase in tumour temperature of 52.1 K. Based on the temperature of a cooled lung lobe (15°C), a peak temperature of 67°C was created. The heat induced by HIFU was sufficient to generate coagulation necrosis in the lung cancer tissue. In contrast, the same acoustic HIFU energy caused a minimal temperature increase (7.1 K) in flooded parenchyma. NADPH-diaphorase staining showed that HIFU was highly selective for tumour tissue in flooded lungs. The lung parenchyma directly adjacent to the ablated cancer tissue was undamaged and viable. This can be explained by the very low attenuation of saline, which is the major mass component in flooded lungs [
19]. Thus, the risk of damaging the surrounding tissue or adjacent organs can be minimised as previously described in a study of HIFU in abdominal tumours [
20]. A normal saline solution was injected into the abdominal cavity to reduce complications from HIFU therapy for abdominal tumours. This method does not have adverse effects on the efficacy of HIFU ablation [
21].
In vivo transthoracic HIFU application in pigs caused a mean peak temperature increase up to 53.7°C in a simulated lesion (BioGlue®) deep inside the flooded lung. HIFU energy penetrated through the pleura and flooded lung into the target lesion. The simulated lesion, which was located at a 6 cm depth below the transducer, was heated by HIFU. The temperature increase was highly variable and inconstant. The mechanism of heat generation in the BioGlue®-simulated lesion was unclear. The lesion consisted of purified BSA and glutaraldehyde. Although this simulated lesion was not a true human tumour, the acoustic and thermal properties are similar to those of human cancerous tissue due to similar densities and high protein contents. In addition, movements of the heart and mediastinum may be transmitted to the target lesion, bringing them out of the focal zone.
Lung flooding seems to be ideal for HIFU application, because sonographic imaging is possible and because of other advantages.
Compared to other human tissues, a flooded lung has an ideally suitable beam path, because water has a very low attenuation. In contrast to flooded lung tissue, tumour tissue converts acoustic energy into a therapeutic thermal dose and enables selective heating of the cancer mass. Consequently, damage to healthy lung tissue is minimised.
In addition to acoustic advantages, there are other favourable conditions for tumour ablation in a flooded lung. There is no pulmonary blood flow in a flooded lung [
22]. Perfusion reduces heat and is not desirable with locoregional thermal therapy. In addition, ischaemia-related acidosis sensitises tumour tissue to heat. In comparison to a ventilated lung, lung flooding reduces tumour movements caused by breathing.
The following limitations existed in the current study.
The risk of lung flooding in patients with limited lung function is unclear. Further studies on patients with limited lung function should be performed to examine the influence of unilateral lung flooding on haemodynamics and gas exchange. Ongoing studies in pigs showed that lung flooding of only one lobe is feasible.
Transcutaneous application was not applied for the
in vivo or
ex vivo studies. One rib had to be resected to apply the HIFU applicator, which consisted of a HIFU transducer and sector-array probe, to the chest of a pig. Technical improvements in the application system are necessary to ensure that the HIFU focal zone is aimed precisely at the target lesion with the guidance of sonography. Transthoracic HIFU application is difficult despite resection of the ribs, because the intercostal spaces in the animal model are narrow. A method and transducers have been developed to avoid the shielding of therapeutic ultrasound by the human rib cage [
23].
Different HIFU exposure schemas (ten seconds ex vivo versus ‘one second on/off’ in vivo) and transducers were applied during this study. Ultrasound imaging is disturbed if HIFU is continuously on. Therefore, the intermittent ‘one second on/off’ in vivo schema was needed to control the focal alignment with the thermocouple during the ‘one second off’ interval.
For HIFU application to human tumours, resected lung lobes were used.
The gas-free filling occurs at 71.4% of the resected lobes. This limitation is due to residual air in non-collapsed bronchi that can only solved by resorption under
in vivo conditions [
11]. Lung flooding was performed with cooled saline at 15°C for the
ex vivo HIFU application to ensure that hypoxic damage did not occur within the tumour and lung tissues. This was important, because the NADPH-diaphorase staining method is based on mitochondrial vitality, which is very sensitive to ischaemia.
A hyperechoic area was found within the tumour tissue immediately following HIFU exposure. Grayscale changes have been shown to be a useful marker for HIFU-induced tissue destruction [
8]. However, it is unclear whether the area of grayscale changes corresponds with the ablated area, such that a hyperechoic sonolesion represents irreversibly damaged tissue. B-mode ultrasound imaging is probably not the best method for monitoring tissue response. Currently, the most important problem associated with ultrasound-guided HIFU ablation is the lack of reliable thermometry and lesion production monitoring [
6]. In addition to ultrasound imaging, real-time magnetic resonance thermometry could be important for ascertaining the extent of tumour destruction [
24‐
26].
Competing interests
The authors declare that they have no financial or non-financial competing interests.
Authors’ contributions
FW was responsible for HIFU technique and temperature measurement. He collected and evaluated the data and wrote the manuscript. HS and SB performed the anaesthesia. CB performed the histopathological and enzyme histochemistry examinations. TGL co-wrote and revised the manuscript and discussed the results with the authors. All authors read and approved the manuscript.