Initial experience of irreversible electroporation ablation in Brazil: a retrospective analysis

New ablative therapies have been developed in the past decades with an increasing interest in local treatment of malignant neoplasms. One of the reasons these are the high rate of unresectable tumors at the time of diagnosis, for instance, 50-70% of pancreatic cancer and 80% of colorectal liver metastasis are unsuitable for surgery, mainly because of poor clinical conditions, limited liver function or involvement of vital structures. The ablatives therapies such as radiofrequency, microwave, and cryoablation have overcome these problems by enabling smaller surgical resections and better organ function preservation, increasing the number of patients suitable for curative treatments. However, these thermal ablative technologies rely on temperature variation on the surrounding tissues, bringing some limitations concerning the size and location of the tumor. Studies demonstrate that complete ablation rate is reduced below 50% when in contact with a vessel larger than 3mm due to the heat sink effect and also when close to vital structures (i.e., central bile ducts, portal vein, and bowel loops).

Irreversible electroporation (IRE) emerges as a promising technology that achieves similar ablative treatment areas with the advantage of not using thermal effects. Instead, it uses high-voltage electrical pulses (up to 3kV/cm) that open nanopores in the cell membrane's lipid bilayer, disrupting cellular homeostasis and leading to apoptosis,1 2 3 keeping the extracellular matrix unimpaired.4 5 Due to the non-thermal ablative technology, IRE has the potential to overcome the problem of difficult tumor location faced by the other ablative methods.

Clinical applications have been increasingly expanded in cases where the neoplastic tissue is in intimate contact with heat-sensitive structures that are at high risk of morbidity and mortality, such as locally advancer pancreatic cancer (LAPC) in stage III or centrally liver neoplasms (CLM), allowing changes in the patient's oncological status and thus outcomes such as overall survival and recurrence-free time.6 7 8 9 10 Another promising field of action is the association of the method with immunotherapy. Due to the mechanism of cell death by apoptosis, it has been proven that a window of tumor immune susceptibility is opened after ablative treatment, thus allowing the use of medications that strengthen the patient's immune system.11 12 13

The purpose of the present study is to present the first Brazilian IRE experience in the treatment of solid cancer.

The development of new minimally invasive technologies, such as ablative therapies for local control, is gaining an increasing role in cancer treatment. However, one of the limitations of these therapies is the thermal injury of tumor's adjacent structures. Unlike thermal ablation techniques, which induce tissue necrosis by deposition of high or low thermal energy, the electroporation phenomenon induces non-thermal tissue necrosis through high-voltage electrical pulses that determine nanopores in the cell membrane temporarily (reversible) or permanently (irreversible) in tumor cells.5 Therefore, the extracellular matrix structures are preserved and vulnerable adjacent tissues, such as vessels, nerves, and bile ducts, should remain intact.14

Centrally located liver lesions can be unsuitable for resection or thermal ablation due to their proximity to the main bile ducts and portal vein. As IRE enables the preservation of these structures, it may prove particularly useful for this indication.15 Narayanan et al. (2015)16 investigated the effect of IRE on vessels close to the ablation zone in 101 patients. Abnormal vascular changes were seen in 7 of 158 vessels (4.4%). All changes were venous in origin, with the portal vein most affected, probably due to flow dynamics within the portal venous system. Occlusion of bile ducts has been reported after hepatic IRE, which may have been caused by thermal coagulation of the bile duct that was in direct contact with one of the needle electrodes.17 Kingham et al. (2012)15 treated 65 liver malignancies that were within 1cm from a major hepatic vein or portal pedicle, showing patency of all hepatic veins and pedicles on postoperative imaging, except for 1 major vessel occlusion. Therefore, to avoid damage to thermally sensitive structures, it is recommended to place the needle electrodes at least 5mm from bile ducts or large blood vessels.18 In our 3 cases of central liver lesions, among which two were metastasis of colorectal neoplasia and the other one a cholangiocarcinoma, there was no damage to the adjacent structures with minimum distance of the probe to bile duct or major vessel >5mm.

Many studies have investigated the outcomes of percutaneous IRE under CT guidance in patients with LAPC.16 19 20 Narayanan et al. (2017)10 reported a median OS of 14.5 months from the date of IRE with no procedure-related deaths for percutaneous IRE, which is similar to those reported for intraoperative IRE.21 22 Potential benefits of percutaneous IRE include a relatively short recovery time, fewer potential complications related to surgery, and better spatial guidance for needle placement. In our experience, 3 patients have successfully undergone percutaneous IRE and 7 received the surgical approach. The choice for the surgical approach was mainly because all patients had borderline surgical LAPC and IRE was planned to treat only in the region where the surgeon could not withdraw the tumor with safe margins (i.e., near the great vessel).

During the probe positioning in the surgical access, the main difficult was the lack of support to maintain the probe static in the correct place, the poor ultrasound visualization of the probe while into the abdominal fat, the high flexibility of the needle turning it difficult to puncture the hard tissue of the pancreatic lesions, and the probe distribution avoiding the major vessel. As for the percutaneous approach, the access to position all needles parallel to each other proved to be quite challenging due to the costal arches and the poor visualization of the vessel anatomy in the non-enhanced CT. To overcome the latter, Timmer et al. (2020)6 described a technique where a transarterial catheter is placed into the celiac trunk with small amount of contrast injection during probe positioning for better artery visualization. As for the laparoscopic IRE, we preferred not to use this approach once because the positioning of the probe could result in a challenging task, raising the surgical time and probably leading to higher complication rates and partial tumor ablation, mainly in the initial learning curve. As for the procedure time, the percutaneous approach was longer than surgical (median of 38min. vs. 30min.), probably because of the probe repositioning, which is more time consuming in percutaneous approach.

CT and MRI are considered the best modalities to evaluate the efficacy and possible complications after IRE.14 A contrast-enhanced CT scan after IRE usually shows different results compared to RFA and MW due to the non-coagulative action and the consequent preservation of vital structures.23 In our experience, the ablation zone appeared as a hypodense area with small gas bubbles, best visualized in the portal phase, as described by Rashid et al. (2018).24 Ultrasound monitoring showed subtle hyperechogenic change with small gas formation. Several studies have demonstrated that the apparent ablation zone on CT grossly corresponds to the pathological zone of cell death.25 26 27

Several studies have reported major complications, in the range of 19%-59%,5 21 22 23 28 including hemorrhage from gastrointestinal ulceration, vessel stenosis, or bile duct injury.24 28 In the present study, there was no death within the 30-day follow-up period. In our study, the percutaneous approach had only one adverse event related to a pneumothorax due to transpulmonary access during probe positioning. Three patients in the surgical approach presented with complications, one with persistent vomiting requiring IV medication and fluid hydration; two with fluid collection in the abdominal cavity that was percutaneously drained and considered most likely to be related to the surgery. In the present study, patients treated for LAPC had transient elevation of serum amylase and lipase levels one day after IRE, returning to normal values one week after IRE. This result could be due to transient inflammation of the pancreas after inserting the electrode during the procedure.29 No clinical significative pancreatitis was observed in our patients.

To guarantee the long-term success of IRE as an established tool for the treatment of LAPC, we must strive for continuous improvement of the technique, aiming for minimal complications and maximal attainable results. Important physical factors influencing these outcome values include the adjustable parameter settings of the IRE apparatus. These individually adjustable parameters include number, length and duration of the electrical pulses, interval between the pulses, pulse delivery protocol, interelectrode distance, voltage, number of needle electrodes, and their geometry. Hence, for optimization of IRE procedures, it is essential to elucidate the exact effect of each parameter on the ablation zone in terms of geometry and homogeneity.6

The current focus of several trials is the immunogenic potential of IRE, as the mechanism through which IRE operates also results in a systemic effect. Locally generated antitumor T-cell responses could eventually provide protection against tumor outgrowth of distant metastases, a process known as the “abscopal effect”, which could positively affect survival.30 31 32 33 34 A recent preclinical study involving immunocompetent mice with PDAC that received combined treatment with IRE and an anti-PD1 checkpoint inhibitor demonstrated significant survival benefits.35 Furthermore, in a clinical study for unresectable LAPC, a combination of IRE and allogeneic natural killer cells achieved significant improvement in OS over IRE alone.36 37

The present study has some limitations. First, this study included a small number of patients and the follow-up time after IRE was relatively short. Second, there was no control group, such as patients undergoing conventional chemotherapy, to compare. Further randomized controlled studies are needed to overcome these limitations.

In conclusion, IRE is a promising treatment modality for LAPC and CLM. Although the high complication rates on this study, most of them represents minor complications with conservative treatments. Furthermore, prospective studies with control group are necessary to determine the efficacy of IRE.