Irreversible electroporation: local, non-thermal, ablation therapy of malignant tumours

Summary

Basic information:
Irreversible electroporation (IRE) is a new method of local therapy of malign tumours based on bioelectric effect of electrical current. Short electric pulses with high voltage create nano-pores in tumour cell membranes resulting in apoptosis of the exposed cells. The purpose of our study was to verify the IRE technique performed percutaneously under CT navigation and to assess effects of application of this method in early stages of primary and secondary hepatic, pancreatic, renal and pulmonary tumours.

Methods and results:
From November 2011 to October 2012 IRE was performed with Nano-Knife (by AngioDynamics) in the population of 15 patients – 6 males and 9 females. IRE was performed under total anaesthesia with 2-5 needle electrodes introduced under CT navigation in the tumour base. The results of the treatment were assessed on the basis of modified RECIST criteria applied in 1-, 3- and 6-month intervals. A control CT or MRI examination 6 months post IRE was undertaken by 10 patients, one patient died one month post IRE of pulmonary embolism, two refused to visit for the control examination and another two are still to undergo the examination after 6 months. Out of the 10 examined patients success of IRE was demonstrated in 7 cases (70.0 %) and IRE failure in 3 patients (30.0 %).

Conclusion:
IRE is a new, mini-invasive therapeutic method applicable to local treatment of malign tumours in cases where surgical approach is technically unfeasible or excessively risky. On the basis of first experience in a small population of patients IRE performed under CT navigation appears to be an effective and safe ablation method with a large therapeutic potential. Its results will however need to be assessed within a longer time horizon and in a larger patient population.

Keywords:
malign tumours, irreversible electroporation, Nano-Knife, thermo-ablation therapeutic methods

Ablation methods of treatment of tumour lesions represent an alternative for patients not indicated for surgical removal of the tumour tissue, because of technical unfeasibility or excessive risk. Ablation methods include interstitial laser coagulation, cryonic ablation, catheter-based chemotherapy with embolisation, RF ablation (RFA), microwave ablation (MA) and newly also irreversible electroporation (IRE). IRE, based on bioelectric effects of electrical current, represents a new potential to ablation treatment of tumours (1). Its principle is based on application of millisecond electrical pulses of low intensity and high voltage in the tumour tissue area with Nano-Knife technology (2). These high-voltage electrical pulses are applied by needle electrodes introduced percutaneously to the tumour core with CT or ultrasonic navigation or also in the course of an open surgery (2,3). The electric pulses create nano pores in the cell membranes. Where the electric field is applied through a corresponding number of pulses for a sufficiently long period of time the result is permanent – irreversible – opening of these pores in cell membranes. The damaged membrane permeability results in homeostatic disorder of the exposed cells and cellular apoptosis (4). The mechanism of the cell necrosis is not completely understood yet, but studies on animals have shown that the cells of the IRE-exposed tissues lost viability, while the connective and elastic structures containing collagen (such as blood vessels and bile ducts) remained intact (5,6). As the electroporation phenomenon is not based on thermal effect, this method is characterised as non-thermal “cold“ ablation treatment of malign tumours.

Materials and Methods

IRE was performed with a Nano-Knife (by AngioDynamics), designed for ablation of soft tissues. The Nano-Knife includes a mobile direct current generator, a display monitor, a synchronised ECG unit and a foot pedal. Each individual electroporation involved direct current (25-45 A) with 1,500 – 3,000 V voltage applied in 90 pulses (i.e. nine sequences by ten pulses). The length of one pulse ranged between 30 and 100 μsec. These high-voltage pulses were applied by unipolar 18 G needle electrodes (AngioDynamics) 25 cm long, introduced in the tumour core with the help of CT navigation. Accurate values of the achieved high voltage needed for electroporation of the tumour core are specified by a test with a computer model based on the distances between the individual needle electrodes in relation to the size of the tumour core. The distance between the positive and the negative electrode in any pair of electrodes used for the electroporation ranged between 2 and 2.4 cm. Depending on the tumour size from 2 to 5 electrodes were used for the tumour core electroporation. The number of electroporation interventions differed for the individual tumour sizes and the electric field values achieved by each series of pulses. IRE used direct current voltage ranging between 1,200 V and 3,000 V depending on the tissue resistance and the level of cell membrane permeability achieved. All patients of our trial population were indicated for IRE on the basis of their CT or MRI find of malign tumour lesion (in the liver, the pancreas, the lungs or the kidneys) not treatable by another therapeutic methods (surgical or oncology therapies). Laboratory blood examinations (including coagulation) were performed in each patient before IRE together with examination of urea, electrolyte and hepatic and renal functions. As total anaesthesia and deep muscle relaxation are necessary preconditions of IRE, all patients were also examined by an anaesthesiologist before the intervention. Application of IRE at the FNKV hospital was commenced after grant of approval by the ethical committee. The patients were informed about all aspects of the therapeutic method including potential complications beforehand and confirmed their consent with signature of an informed consent form. The irreversible electroporation itself was performed in all patients of the trial population under total anaesthesia and absolute muscle relaxation in synchronisation with the Nano-Knife generator and ECG apparatus. First native and post-contrast CT examination was performed for optimum planning of puncture direction and specification of the number of needle electrodes in relation to the tumour size. Unipolar needle electrodes were introduced in the tumour under sterile conditions and under CT navigation in parallel, always in apnoea, with the distance between the positive and the negative electrode of each pair always ranging between 2 and 2.5 cm. Where the subsequently performed conductivity test shows on the Nano-Knife display that the electric current intensity and voltage values were not sufficient for IRE, position of one of the electrodes was changed and the test was repeated. The number and layout of the needle electrodes were planned with the help of Nano-Knife software for the ablation zone to cover the whole tumour in all three dimensions. Two electrodes were usually sufficient for tumours of up to 2 cm in size, while three electrodes were used for tumour sizes up to 3 cm and up to five electrodes were applied in the case of larger tumours. Where two electrodes were used, the positive one was introduced to the upper peripheral zone of the tumour and the negative one to its bottom edge. The length of the active sections at the ends of both needle electrodes was 1.5 cm. Then the first electroporation was performed. At its end the electrodes were extracted by 1 – 2 cm and the second electroporation followed under the same electrical parameters. The created ablation zone was rectagonal or cylindrical, as confirmed by the subsequent post-contrast CT examination (Fig. 1).

The values of high voltage and intensity of the applied electric current and the distances between the electrodes were computerised and recorded on the display following every IRE. The length of one IRE ranged between 2 – 3 minutes, and the length of the whole intervention under total anaesthesia was 135±10 min. (median 125 min). After IRE the patients were transferred to ICU for 24 hours and then to standard ward. The length of hospitalisation ranged between 2 and 7 days depending on complications occurring after IRE (such as PNO). Clinical and laboratory examinations were performed before release of each patient from hospital (blood, urea and electrolyte) together with hepatic and renal function and CT examinations. The success of the treatment was assessed on the basis of modified RECIST criteria (7) applied to the control CT or MRI examination in 1-, 3- and 6-month interval. Group A included patients with successful IRE. Their control CT or MR image showed tumour necrosis without signs of post-contrast enhancement. Group B was represented by patients with partially successful IRE, i.e. with signs of residual enhancement at the periphery of the tumour necrosis. Group C then showed failure of IRE with incomplete necrosis of the central part of the tumour and strong signs of post-contrast enhancement in the place of the original tumour.     

Results

In the period between November 2011 and October 2012 IRE was performed in 15 patients including 6 males and 9 females aged 51 to 82  (median 66 years) with primary or secondary malign tumours of the liver, the lungs, the kidneys and the pancreas. All IRE interventions were performed percutaneously under CT navigation. In 11 patients one malign lesion was treated and in 4 patients there were two or more lesions to be treated. In the case of 11 patients liver metastases were treated, including 9 cases of rectum carcinoma metastases, 1 case of breast carcinoma metastasis and 1 case of prostate carcinoma metastasis. In 2 patients pulmonary tumour lesions were treated, one linear flexure carcinoma metastasis and one pulmonary carcinoma. Renal IRE was performed in a patient with adeno-carcinoma in the solitary kidney (Fig. 2). One IRE case involved carcinoma of the pancreas.

IRE was performed with 2 to 5 needles, depending on the tumour size. The mean size of the treated tumours was 4.5 cm3 (ranging from 2.5 to 88.5 cm3). The success of the treatment was assessed on the basis of modified RECIST criteria (7) applied to the control CT or MRI examination in 1-, 3- and 6-month interval. Out of the population of 15 patients 14 patients were evaluated 1 month post IRE, with one patient not turning up for the examination. According to the modified RECIST criteria IRE was successful in 13 patients (92.8%) and failed in 1 case (7.2%) of a patient with quickly progressing liver metastases of thyroid gland carcinoma. Control CT or MRI examinations 3 months post IRE involved 13 patients, with one female patient missing for the reason of death of pulmonary embolism and one not turning up for the examination. IRE was successful in 11 patients (84.6%), one female patient (7.7%) with progressing metastatic liver disease caused by mamma carcinoma metastasis was assessed as patina success, while another female patient (7.7%) with thyroid gland carcinoma metastasis was classified as IRE failure. The control CT or MRI examination after 6 months has so far been performed in 10 patients. One female patient has not been examined for the reason of death of pulmonary embolism, one male patient has refused the control examination, another has not turned up for the examination despite repeated invitations and two patients are still to be examined. IRE has so far been found successful in 7 patients (70.0 %) (Fig. 3). IRE failure has been found in 3 patients (30.0%) – cases of hepatic metastases of colorectal carcinoma, thyroid gland carcinoma and mamma carcinoma, respectively. In addition control CT or MRI examinations of 6 patients have revealed new metastases. Three patients with original rectum carcinoma have developed new metastases in the liver, one patient with pulmonary carcinoma has shown a new metastasis in the contra-lateral lung wing and another two patients (with thyroid gland and mamma carcinomas) suffer from new liver metastases (Tab.1.).

The earliest new metastases were revealed by the examination 1 month post IRE in a patient with primary thyroid gland carcinoma. In addition to post-contrast enhancement in the original tumour area several other meta bases were found and therefore further IRE was not indicated an the patient was transferred to biological therapy (Fig. 4.).

Post-procedural complications were observed in 10 patients. These included three cases of coating PNO 1 to 3 cm wide, resulting from the introduction of the needle electrodes. There was also one case of PNO in a patient treated with IRE for pulmonary meta, and two cases in patients treated for hepatic metastases. In all three cases the PNO retreated spontaneously without the necessity of specific therapy in 3 - 5 days. Seven control CT scans post IRE revealed minimum bilateral fluidothorax, which also completely retreated spontaneously. Post-procedural pain, signs of atrium fibrillation, neuropraxia of brachial plexus and other serious complications were not reported.

Biochemical follow-up of our population showed no remarkable signs in the control mineralogram performed on the first post-operative day, with only a minimal trend towards hypocalaemia. Elevation of hepatic counts (ALT and AST up to 10 ucat/l) and bilirubin to around 40 µmol/l, (in one case to 350 µmol/l.) appeared in patients with treated liver. The pathological laboratory values either returned to normal at the next examination on the third post-operative day or continuous progress towards the normal was observed (the extreme bilirubin level was reduced to one half after a week). Renal function was unchanged or showed clinically insignificant decreases of urea and creatinine levels. Two patients with pulmonary metastases showed no significant change in their post-operative laboratory results.

Discussion

IRE is a new ablation therapy based on research on effects of electric field on cellular and sub-cellular structures of living organisms (5,6). Research by Davalos has shown that application of low-energy electrical current in micro-second high-voltage pulses opens nano pores in cell membranes, which increases their permeability (4). These electric pulses can be applied by needle electrodes introduced to the tumour either under visual control in the course of open surgeries or under CT or ultrasonic navigation (2,6). On condition of sufficient intensity and length of the pulses the electroporation phenomenon becomes irreversible and results in apoptosis of the exposed cells. The resulting tumour necrosis is absorbed by the lymphatic system and heals with a scar (1). The electroporation effect is used for treatment of malign soft tissue tumours of the liver, the pancreas, the kidneys, the prostate, the lungs, the skin, the breast and certain connective tissues (2).

Unlike RF ablation resulting in thermal necrosis at around 100 deg. C irreversible electroporation does not result in thermal damage of the tissues as the maximum temperature of the intervention does not exceed 50 deg. C. This fact is mainly significant in relation to the collagen-containing structures forming the basis of the tubular structures – bile ducts, renal cups, ureters, bronchial structures and arteries – which remain intact (Fig. 5).

IRE has neither shown the heat sink effect – temperature reduction in close proximity to large blood vessels by the circulating blood. This way temperature deficit affect the temperature needed for achievement of thermal destruction of the tumour tissue, which results in persistence of viable tumour structures and recurring tumours. The heat-sink effect reduces therapeutic effect of RF ablation especially in cases when the tumour is localised close to the aorta, the vena portae or the bottom vena cava (1, 2). Irreversible electroporation itself is performed under total anaesthesia and absolute muscle relaxation preventing development of generalised muscle jerks reminding of grand mal (3). The needle electrodes are introduced to the tumour in apnoeic pause for most accurate placement. A sufficiently long respiratory circuit is needed due to the repeated movement of the patient into and outside the examination gantry. Standard monitoring during IRE includes ECG, non-invasive control of blood pressure, pulse oxymetry, capnometry and depth of muscle relaxation. Careful padding and fixation of the upper extremities in necessary prevention of neuropraxia of the brachial plexus (3,8).

Literature references show that IRE as a mini-invasive alternative may be successfully used for treatment of different primary and secondary tumours, especially where conventional surgical treatment represents a considerable risk for the patient or a technically unfeasible variant and where the options of application of any other thermo ablation method are limited (9, 10, 11). This mainly applies to treatment of malign lesions localised near bile and pancreatic ducts or hilum arteries (9) (Fig. 6). IRE used for treatment of pulmonary and mediastinum tumours does not damage critical hilum structures – bronchi and large vessels or diaphragm nerves. IRE treatment of renal and prostate tumours dies not damage critical tubular structures such as ureters, the urethra or the adjacent neurovascular bundles (10,11).

Peri- and post-operative complications related to treatment of malign tumours with IRE in comparison to conventional surgery and thermo ablation techniques are qulitatively and quantitatively reduced (1). While tissue preparation for conventional surgery represents considerable risk of haemorrhage and thermal ablation may damage vascular wall with subsequent bleeding into the ablation necrosis, IRE does not damage the vascular system as it uses 22 G needle elctrodes 1 mm wide. The risk of infection following in the case of surgical interventions from the scope of damage to the skin and other structures is minimised by IRE for the skin and other structural integrity is only broken by a couple of punctures of thin needle electrodes and the resulting tissue necrosis is non-thermal. The minimum invasiveness of IRE in comparison to surgeries and thermal ablation methods is connected with shorter hospitalisation and post-operative recovery. Potential risks of IRE include development of sub-capsular hematoma in the case of multiple punctures and potential implantaiton of tumour cells in the puncture channel (2). 

Conclusion

IRE is a new alternative mini-invasive therapeutic method applicable to local treatment of malign tumours in cases where surgical approach is technically unfeasible or excessively risky. On the basis of first experience IRE performed under CT navigation appears to be an effective and safe ablation method with a large therapeutic potential. Control examinations of a larger patient population within a longer time horizon and will however be necessary to allow for statements on effectiveness of therapeutic effects of IRE.             

Acknowledgements

This work was supported  by the PRVOUK P 27/2012 program of  the Charles University Prague.

Doc.MUDr Václav Janík Csc,

Radimova 136 A, Prague 6, post code 169 00

e-mail: janik@fnkv.cz

Phone: +420 256162812