In vivo results of a new focal tissue ablation technique: irreversible electroporation

The current role of minimally invasive therapies in the management of liver tumors

This is a review of minimally invasive therapy options for liver tumors, such as highly focused ultrasound, microwave ablation, and irreversible electroporation, as well as new aspects of radiofrequency ablation. Radiofrequency ablation is recommended for patients with early-stage HCC with up to 3 lesions with a tumor diameter within 3 cm and for patients with non-resectable liver metastasis. Indications and contraindications to treatment are designated, and different modalities for image-based therapy guidance are compared. Options for therapy monitoring and controlling are reviewed, namely intraprocedural tools, imaging and functional parameters and their evolution during therapy. Prevention and control of local recurrences is discussed. We also present a short review of current clinical results in treating liver metastasis and primary liver tumors.

Characterization of irreversible electroporation ablation in in vivo porcine liver

OBJECTIVE

The purpose of this study was to prospectively characterize and optimize irreversible electroporation ablation to determine the best parameters to achieve the largest target zones of coagulation for two electrodes.

MATERIALS AND METHODS

Ultrasound-guided irreversible electroporation ablation (n=110) was performed in vivo in 25 pig livers using two 18-gauge electroporation electrodes and an irreversible electroporation generator. Five variables for energy deposition and electrode configuration were sequentially studied: number of electrical pulses (n=20-90), length of pulses (20-100 microseconds), generator voltage (2250-3000 V), interelectrode spacing (1.5-2.5 cm), and length of active electrode exposure (1.0-3.0 cm). Zones of ablation were determined at gross pathology and histopathology 2-3 hours after irreversible electroporation. Dimensions were compared and subjected to statistical analysis.

RESULTS

For 1.5-cm spacing and 2-cm electrode exposure at 2250 V, there was no statistical difference in the size of coagulation when varying the number or length of pulses from 50 to 90 repetitions or 50-100 microseconds, respectively, with each parameter combination yielding 3.0±0.4×1.7±0.4×3.0±0.6 cm (width, depth, and height, respectively). Yet, increasing the pulse width or number over 70 caused increased hyperechogenic or gas and coagulation around the electrode. Increasing the voltage from 2250-3000 V for 70 pulses of 70 microseconds increased coagulation to 3.1±0.4×2.0±0.2 cm (p<0.01 for depth). Greater coagulation width of 3.9±0.5 cm (p<0.01) was achieved at 2-cm interelectrode spacing (with similar depth of 1.9±0.4 cm). However, consistent results required 90 repetitions and a 100-microsecond pulse width; 2.5-cm spacing resulted in two separate zones of ablation. Although electrode exposure did not influence width or depth, a linear correlation (r2=0.77) was noted for height, which ranged from 2.0±0.2-5.0±0.8 cm (for 1- and 3-cm exposures, respectively).

CONCLUSION

Predictable zones of tissue destruction can be achieved for irreversible electroporation. Ablation dimensions are sensitive to multiple parameters, suggesting that precise technique and attention to detail will be particularly important when using this modality.

Real-time ultrasound imaging of irreversible electroporation in a porcine liver model adequately characterizes the zone of cellular necrosis

BACKGROUND

  Irreversible electroporation (IRE) is a largely non-thermal method for the ablation of solid tumours. The ability of ultrasound (US) to measure the size of the IRE ablation zone was studied in a porcine liver model.

METHODS

  Three normal pig livers were treated in vivo with a total of 22 ablations using IRE. Ultrasound was used within minutes after ablation and just prior to liver harvest at either 6 h or 24 h after the procedure. The area of cellular necrosis was measured after staining with nitroblue tetrazolium and the percentage of cell death determined by histomorphometry.

RESULTS

  Visible changes in the hepatic parenchyma were apparent by US after all 22 ablations using IRE. The mean maximum diameter of the ablation zone measured by US during the procedure was 20.1 ± 2.7 mm. This compared with a mean cellular necrosis zone maximum diameter of 20.3 ± 2.9 mm as measured histologically. The mean percentage of dead cells within the ablation zone was 77% at 6 h and 98% at 24 h after ablation.

CONCLUSIONS

  Ultrasound is a useful modality for measuring the ablation zone within minutes of applying IRE to normal liver tissue. The area of parenchymal change measured by US correlates with the area of cellular necrosis.

Irreversible electroporation on the small intestine

BACKGROUND

Non-thermal irreversible electroporation (NTIRE) has recently been conceived as a new minimally invasive ablation method, using microsecond electric fields to produce nanoscale defects in the cell membrane bilayer and induce cell death while keeping all other molecules, including the extracellular matrix, intact. Here, we present the first in vivo study that examines the effects of NTIRE on the small intestine, an organ whose collateral damage is of particular concern in the anticipated use of NTIRE for treatment of abdominal cancers.

METHODS

A typical NTIRE electrical protocol was applied directly to the rat small intestine and histological analysis was used to examine the effect of NTIRE over time.

RESULTS

The application of NTIRE led to complete cell ablation in the targeted tissue, but the animal did not show any physiological effects of the procedure and the intestine showed signs of recovery, developing an epithelial layer 3 days post treatment and regenerating its distinct layers within a week.

CONCLUSION

Our results indicate that this novel procedure can be used for abdominal cancer treatment while minimising collateral damage to adjacent tissues because of the unique ability of the NTIRE ablation method to target the cell membrane.

Experimental characterization and numerical modeling of tissue electrical conductivity during pulsed electric fields for irreversible electroporation treatment planning

Irreversible electroporation is a new technique to kill cells in targeted tissue, such as tumors, through a nonthermal mechanism using electric pulses to irrecoverably disrupt the cell membrane. Treatment effects relate to the tissue electric field distribution, which can be predicted with numerical modeling for therapy planning. Pulse effects will change the cell and tissue properties through thermal and electroporation (EP)-based processes. This investigation characterizes these changes by measuring the electrical conductivity and temperature of ex vivo renal porcine tissue within a single pulse and for a 200 pulse protocol. These changes are incorporated into an equivalent circuit model for cells and tissue with a variable EP-based resistance, providing a potential method to estimate conductivity as a function of electric field and pulse length for other tissues. Finally, a numerical model using a human kidney volumetric mesh evaluated how treatment predictions vary when EP- and temperature-based electrical conductivity changes are incorporated. We conclude that significant changes in predicted outcomes will occur when the experimental results are applied to the numerical model, where the direction and degree of change varies with the electric field considered.

The improvement of irreversible electroporation therapy using saline-irrigated electrodes: a theoretical study

Irreversible electroporation (IRE) is a novel therapy used to ablate tumors with high-field electric pulses applied in short durations. It is important to reduce the generation of heat in IRE to avoid the harmful effects of thermal damage. The objective of this simulation study was to examine the effects of saline irrigation in the reduction of heat upon electrodes used in IRE treatment of hepatocellular carcinoma. We used a two dimensional Finite Element Model of a tumor in a liver with electrodes placed at the center of the tumor. We simulated a typical electroporation protocol with varying thicknesses and conductivities of the saline layer, and we observed the maximum temperature and the distribution of the electric field and temperature in the tissue. Our results showed that the maximum temperature in the tissue decreases with the use of saline, but the surface area of the tumor that could potentially be thermally damaged may increase with the thickness and conductivity of the saline. With the use of saline, one can achieve upwards of a 17% reduction of the maximum temperature at the electrodes. Also, the distribution of temperature and the electric field becomes more homogenous between the electrodes as the conductivity of the saline layer increases for all thicknesses of saline. We conclude that irrigating electrodes with saline may be an effective measure to enhance the efficacy of irreversible electroporation by reducing the maximum temperature at the electrodes and also improving the extent and distribution of the electric field in the tissue. However, the properties of the saline should be adjusted so as to limit the increase of thermal damage propagated in the tissue.

US findings after irreversible electroporation ablation: radiologic-pathologic correlation

PURPOSE

To characterize ultrasonographic (US) findings after irreversible electroporation (IRE) to determine the utility of these findings in the accurate assessment of ablation margins.

MATERIALS AND METHODS

The institutional animal care and use committee approved the study. IRE ablation (n = 58) was performed in vivo in 16 pig livers by using two 18-gauge electroporation electrodes with 2-cm tip exposure, 1.5- or 2.0-cm interelectrode spacing, and an electroporation generator. Energy deposition was applied at 2250-3000 V (pulse length, 50-100 μsec; pulse repetition, 50-100). Ablations were performed under US guidance. Images were obtained during ablation and at defined intervals from 1 minute to 2 hours after the procedure. Zones of ablation were determined at gross and histopathologic examination of samples obtained from animals sacrificed 2-3 hours after IRE. Dimensions of the histologic necrosis zone and US findings were compared and subjected to statistical analysis, including a Student t test and multiple linear regression.

RESULTS

Within 20-50 pulse repetitions of IRE energy, the ablation zone appeared as a hypoechoic area with well-demarcated margins. During the next 8-15 minutes, this zone decreased in size from 3.4 cm ± 0.5 to 2.5 cm ± 0.4 and became progressively more isoechoic. Subsequently, a peripheral hyperechoic rim measuring 2-7 mm (mean, 4 mm ± 1) surrounding the isoechoic zone developed 25-90 minutes (mean, 41 minutes ± 19) after IRE. The final length of the treatment zone, including the rim, increased to 3.3 cm ± 0.6. The final dimensions of the outer margin of this rim provided greatest accuracy (1.7 mm ± 0.2) and tightest correlation (r(2) = 0.89) with gross pathologic findings. Histologic examination demonstrated widened sinusoidal spaces that progressively filled with spatially distributed hemorrhagic infiltrate on a bed of hepatocytes with pyknotic nuclei throughout the treatment zone.

CONCLUSION

US findings in the acute period after IRE are dynamic and evolve. The ablation zone can be best predicted by measuring the external hyperechoic rim that forms 90-120 minutes after ablation. This rim is possibly attributable to evolving hemorrhagic infiltration via widened sinusoids.

High-frequency irreversible electroporation (H-FIRE) for non-thermal ablation without muscle contraction

BACKGROUND

Therapeutic irreversible electroporation (IRE) is an emerging technology for the non-thermal ablation of tumors. The technique involves delivering a series of unipolar electric pulses to permanently destabilize the plasma membrane of cancer cells through an increase in transmembrane potential, which leads to the development of a tissue lesion. Clinically, IRE requires the administration of paralytic agents to prevent muscle contractions during treatment that are associated with the delivery of electric pulses. This study shows that by applying high-frequency, bipolar bursts, muscle contractions can be eliminated during IRE without compromising the non-thermal mechanism of cell death.

METHODS

A combination of analytical, numerical, and experimental techniques were performed to investigate high-frequency irreversible electroporation (H-FIRE). A theoretical model for determining transmembrane potential in response to arbitrary electric fields was used to identify optimal burst frequencies and amplitudes for in vivo treatments. A finite element model for predicting thermal damage based on the electric field distribution was used to design non-thermal protocols for in vivo experiments. H-FIRE was applied to the brain of rats, and muscle contractions were quantified via accelerometers placed at the cervicothoracic junction. MRI and histological evaluation was performed post-operatively to assess ablation.

RESULTS

No visual or tactile evidence of muscle contraction was seen during H-FIRE at 250 kHz or 500 kHz, while all IRE protocols resulted in detectable muscle contractions at the cervicothoracic junction. H-FIRE produced ablative lesions in brain tissue that were characteristic in cellular morphology of non-thermal IRE treatments. Specifically, there was complete uniformity of tissue death within targeted areas, and a sharp transition zone was present between lesioned and normal brain.

CONCLUSIONS

H-FIRE is a feasible technique for non-thermal tissue ablation that eliminates muscle contractions seen in IRE treatments performed with unipolar electric pulses. Therefore, it has the potential to be performed clinically without the administration of paralytic agents.

Cryosurgery with pulsed electric fields

This study explores the hypothesis that combining the minimally invasive surgical techniques of cryosurgery and pulsed electric fields will eliminate some of the major disadvantages of these techniques while retaining their advantages. Cryosurgery, tissue ablation by freezing, is a well-established minimally invasive surgical technique. One disadvantage of cryosurgery concerns the mechanism of cell death; cells at high subzero temperature on the outer rim of the frozen lesion can survive. Pulsed electric fields (PEF) are another minimally invasive surgical technique in which high strength and very rapid electric pulses are delivered across cells to permeabilize the cell membrane for applications such as gene delivery, electrochemotherapy and irreversible electroporation. The very short time scale of the electric pulses is disadvantageous because it does not facilitate real time control over the procedure. We hypothesize that applying the electric pulses during the cryosurgical procedure in such a way that the electric field vector is parallel to the heat flux vector will have the effect of confining the electric fields to the frozen/cold region of tissue, thereby ablating the cells that survive freezing while facilitating controlled use of the PEF in the cold confined region. A finite element analysis of the electric field and heat conduction equations during simultaneous tissue treatment with cryosurgery and PEF (cryosurgery/PEF) was used to study the effect of tissue freezing on electric fields. The study yielded motivating results. Because of decreased electrical conductivity in the frozen/cooled tissue, it experienced temperature induced magnified electric fields in comparison to PEF delivered to the unfrozen tissue control. This suggests that freezing/cooling confines and magnifies the electric fields to those regions; a targeting capability unattainable in traditional PEF. This analysis shows how temperature induced magnified and focused PEFs could be used to ablate cells in the high subzero freezing region of a cryosurgical lesion.

Irreversible electroporation is a surgical ablation technique that enhances gene transfer

BACKGROUND

Reversible electroporation has long been used to transfer macromolecules into target cells in the laboratory by using an electric field to induce transient membrane permeability. Recently, the electric field has been modulated to produce permanent membrane permeability and cell death. This novel technique, irreversible electroporation (IRE), is being developed for nonthermal cancer ablation. We hypothesize that outside the central zone of IRE exists a peripheral zone of reversible electroporation where gene transfer may occur.

METHODS

IRE of the liver was performed in a Yorkshire pig model with administration of a plasmid expressing the marker gene green fluorescent protein (GFP) by bolus or primed infusion through the hepatic artery or portal vein. After 6 hours, livers were harvested for fluorescent microscopy and histologic examination.

RESULTS

Of 36 liver specimens treated with IRE and the GFP plasmid, 31 demonstrated strong green fluorescence. Liver ablation by IRE was demarcated clearly on histology.

CONCLUSION

IRE is a promising technique not only for operative tissue ablation but also for gene therapy. Because IRE ablation may leave behind intact tumor antigens, these findings encourage clinical studies of tumor ablation with delivery of immunostimulatory plasmids for combined local eradication and systemic immunotherapy.

Drug delivery by electropulsation: Recent developments in oncology

Electro-permeabilisation allows the free access of polar compounds to the cytoplasm by a reversible alteration of the cell membrane. It is now used in clinics for the eradication of cutaneous solid tumors. New developments predict its future applications for other anti-cancer treatments.

Hepatic tumor ablation: application in a community hospital setting

Primary liver tumors are a common clinical problem in the United States and worldwide. Resection has historically been used to treat liver lesions. Commonly used liver-directed therapies include transarterial chemoembolization, selective internal radiation therapy, and ablative therapy. Only ablative therapy can cause direct destruction of the targeted tissue. The commercially available modalities in the United States are all based on thermoablative technology. This article examines the various ablative technologies and their application, as well as how these procedures can be performed safely and with optimal outcomes, in a community cancer center.

Microsecond and nanosecond electric pulses in cancer treatments

New local treatments based on electromagnetic fields have been developed as non-surgical and minimally invasive treatments of tumors. In particular, short electric pulses can induce important non-thermal changes in cell physiology, especially the permeabilization of the cell membrane. The aim of this review is to summarize the present data on the electroporation-based techniques: electrochemotherapy (ECT), nanosecond pulsed electric fields (nsPEFs), and irreversible electroporation (IRE). ECT is a safe, easy, and efficient technique for the treatment of solid tumors that uses cell-permeabilizing electrical pulses to enhance the activity of a non-permeant (bleomycin) or low permeant (cisplatin) anticancer drug with a very high intrinsic cytotoxicity. The most interesting feature of ECT is its unique ability to selectively kill tumor cells without harming normal surrounding tissue. ECT is already used widely in the clinics in Europe. nsPEFs could represent a drug free, purely electrical cancer therapy. They allow the inhibition of tumor growth, and interestingly, nsPEF can target intracellular organelles. However, many questions remain on the mechanism of action of these pulses. Finally, IRE is a new ablation procedure using pulses that provoke the permanent permeabilization of the cells resulting in their death. This technique does not result in any thermal effect, which is its main advantage in current physical ablation technologies. For both the nsPEF and the IRE, the preservation of the normal tissue, which is characteristic of ECT, has not yet been shown and their safety and efficacy still have to be investigated thoroughly in vivo and in the clinics.

Temperature modulation of electric fields in biological matter

Pulsed electric fields (PEF) have become an important minimally invasive surgical technology for various applications including genetic engineering, electrochemotherapy and tissue ablation. This study explores the hypothesis that temperature dependent electrical parameters of tissue can be used to modulate the outcome of PEF protocols, providing a new means for controlling and optimizing this minimally invasive surgical procedure. This study investigates two different applications of cooling temperatures applied during PEF. The first case utilizes an electrode which simultaneously delivers pulsed electric fields and cooling temperatures. The subsequent results demonstrate that changes in electrical properties due to temperature produced by this configuration can substantially magnify and confine the electric fields in the cooled regions while almost eliminating electric fields in surrounding regions. This method can be used to increase precision in the PEF procedure, and eliminate muscle contractions and damage to adjacent tissues. The second configuration considered introduces a third probe that is not electrically active and only applies cooling boundary conditions. This second study demonstrates that in this probe configuration the temperature induced changes in electrical properties of tissue substantially reduce the electric fields in the cooled regions. This novel treatment can potentially be used to protect sensitive tissues from the effect of the PEF. Perhaps the most important conclusion of this investigation is that temperature is a powerful and accessible mechanism to modulate and control electric fields in biological tissues and can therefore be used to optimize and control PEF treatments.

Acute and subacute effects of irreversible electroporation on nerves: experimental study in a pig model

PURPOSE

To evaluate whether irreversible electroporation (IRE) has the potential to damage nerves in a porcine model and to compare histopathologic findings after IRE with histopathologic findings after radiofrequency ablation (RFA).

MATERIALS AND METHODS

This study was approved by the institutional animal care and use committee. Computed tomography (CT)-guided IRE of 11 porcine sciatic nerves was performed in nine pigs, and histopathologic analysis was performed on the day of ablation or 3, 6, or 14 days after ablation. In addition, acute RFA of six porcine sciatic nerves was performed in six pigs that were harvested on the day of ablation. All nerves and associated muscles and tissues were assessed for histopathologic findings consistent with athermal or thermal injury, respectively, such as axonal swelling, axonal fragmentation and loss, Wallerian degeneration, inflammatory infiltrates, Schwann cell proliferation, and coagulative necrosis. The percentage of fascicles affected was recorded.

RESULTS

All nerves had an axonal injury. The percentage of affected nerve fascicles after IRE was 50%-100%. Axonal swelling and perineural inflammatory infiltrates were detectable at every time point after ablation. Axonal fragmentation and loss, macrophage infiltration, and Schwann cell proliferation were found 6 and 14 days after ablation. Distal Wallerian axonal degeneration was observed 14 days after ablation. The endoneurium and perineurium architecture remained intact in all cases. RFA specimens at the day of ablation revealed acute coagulative necrosis associated with intense basophilic staining of extracellular matrix, including collagen of the perineurium and epineurium consistent with thermal injury.

CONCLUSION

IRE has the potential to damage nerves and may result in axonal swelling, fragmentation, and distal Wallerian degeneration. However, preservation of endoneurium architecture and proliferation of Schwann cells may suggest the potential for axonal regeneration. In contrast, RFA leads to thermal nerve damage, causing protein denaturation, and suggests a much lower potential for regeneration.

A parametric study delineating irreversible electroporation from thermal damage based on a minimally invasive intracranial procedure

BACKGROUND

Irreversible electroporation (IRE) is a new minimally invasive technique to kill undesirable tissue in a non-thermal manner. In order to maximize the benefits from an IRE procedure, the pulse parameters and electrode configuration must be optimized to achieve complete coverage of the targeted tissue while preventing thermal damage due to excessive Joule heating.

METHODS

We developed numerical simulations of typical protocols based on a previously published computed tomographic (CT) guided in vivo procedure. These models were adapted to assess the effects of temperature, electroporation, pulse duration, and repetition rate on the volumes of tissue undergoing IRE alone or in superposition with thermal damage.

RESULTS

Nine different combinations of voltage and pulse frequency were investigated, five of which resulted in IRE alone while four produced IRE in superposition with thermal damage.

CONCLUSIONS

The parametric study evaluated the influence of pulse frequency and applied voltage on treatment volumes, and refined a proposed method to delineate IRE from thermal damage. We confirm that determining an IRE treatment protocol requires incorporating all the physical effects of electroporation, and that these effects may have significant implications in treatment planning and outcome assessment. The goal of the manuscript is to provide the reader with the numerical methods to assess multiple-pulse electroporation treatment protocols in order to isolate IRE from thermal damage and capitalize on the benefits of a non-thermal mode of tissue ablation.

Percutaneous irreversible electroporation lung ablation: preliminary results in a porcine model

OBJECTIVE

Irreversible electroporation (IRE) uses direct electrical pulses to create permanent "pores" in cell membranes to cause cell death. In contrast to conventional modalities, IRE has a nonthermal mechanism of action. Our objective was to study the histopathological and imaging features of IRE in normal swine lung.

MATERIALS AND METHODS

Eleven female swine were studied for hyperacute (8 h), acute (24 h), subacute (96 h), and chronic (3 week) effects of IRE ablation in lung. Paired unipolar IRE applicators were placed under computed tomography (CT) guidance. Some applicators were deliberately positioned near bronchovascular structures. IRE pulse delivery was synchronized with the cardiac rhythm only when ablation was performed within 2 cm of the heart. Contrast-enhanced CT scan was performed immediately before and after IRE and at 1 and 3 weeks after IRE ablation. Representative tissue was stained with hematoxylin and eosin for histopathology.

RESULTS

Twenty-five ablations were created: ten hyperacute, four acute, and three subacute ablations showed alveolar edema and necrosis with necrosis of bronchial, bronchiolar, and vascular epithelium. Bronchovascular architecture was maintained. Chronic ablations showed bronchiolitis obliterans and alveolar interstitial fibrosis. Immediate post-procedure CT images showed linear or patchy density along the applicator tract. At 1 week, there was consolidation that resolved partially or completely by 3 weeks. Pneumothorax requiring chest tube developed in two animals; no significant cardiac arrhythmias were noted.

CONCLUSION

Our preliminary porcine study demonstrates the nonthermal and extracellular matrix sparing mechanism of action of IRE. IRE is a potential alternative to thermal ablative modalities.

Non-thermal irreversible electroporation for deep intracranial disorders

Non-thermal irreversible electroporation (N-TIRE) is a new minimally invasive technique to kill undesirable tissue. We build on our previous intracranial studies in order to evaluate the possibility of using N-TIRE for deep intracranial disorders. In this manuscript we describe a minimally invasive computed tomography (CT) guided N-TIRE procedure in white matter. In addition, we report the electric field threshold needed for white matter ablation (630 - 875 V/cm) using four sets of twenty 50 µs pulses at a voltage-to-distance ratio of 1000 V/cm. We also confirm the non-thermal aspect of the technique with real time temperature data measured at the electrode-tissue interface.

Nonthermal irreversible electroporation for intracranial surgical applications

Object

Nonthermal irreversible electroporation (NTIRE) is a novel, minimally invasive technique to treat cancer, which is unique because of its nonthermal mechanism of tumor ablation. This paper evaluates the safety of an NTIRE procedure to lesion normal canine brain tissue.

Methods

The NTIRE procedure involved placing electrodes into a targeted area of brain in 3 dogs and delivering a series of short and intense electric pulses. The voltages of the pulses applied were varied between dogs. Another dog was used as a sham control. One additional dog was treated at an extreme voltage to determine the upper safety limits of the procedure. Ultrasonography was used at the time of the procedure to determine if the lesions could be visualized intraoperatively. The volumes of ablated tissue were then estimated on postprocedure MR imaging. Histological brain sections were then analyzed to evaluate the lesions produced.

Results

The animals tolerated the procedure with no apparent complications except for the animal that was treated at the upper voltage limit. The lesion volume appeared to decrease with decreasing voltage of applied pulses. Histological examination revealed cell death within the treated volume with a submillimeter transition zone between necrotic and normal brain.

Conclusions

The authors' results reveal that NTIRE at selected voltages can be safely administered in normal canine brain and that the volume of ablated tissue correlates with the voltage of the applied pulses. This preliminary study is the first step toward using NTIRE as a brain cancer treatment.

Irreversible electroporation of the pancreas: definitive local therapy without systemic effects

BACKGROUND

The use of thermal tumor ablative techniques in the pancreas is limited due to the risk of pancreatitis and damage to major vascular structures. Irreversible electroporation (IRE) is a non-thermal ablation technique that could allow ablation in the pancreas while preserving vital surrounding blood vessels. The aim of this study was to assess the safety and ablation volume of IRE in porcine pancreatic tissue.

METHODS

IRE of swine pancreases was performed using an open technique and ultrasound guidance and the animals were followed for 72 hr, 7 days, and 14 days post-IRE.

RESULTS

All pigs underwent IRE, with one pig was unsuccessful after three attempts using at 3,000 V due to inability to achieve a stable current. All animals recovered well and revealed only mild adhesions, no ascites, and no pancreatic necrosis. All animals had transient increases in amylase and lipase that normalized on post-IRE day 3. Pathologic analysis revealed that ablation defects (median size 3 cm × 2.8 cm) were seen in electroporated areas with significant destruction of the pancreatic tissue with patent vascular structures.

CONCLUSIONS

This animal model demonstrates that IRE of the pancreas performed at an optimal voltage is well tolerated, with rapid resolution of pancreatic inflammation and preservation of vascular structures.

Targeted tissue ablation with nanosecond pulses

In-vivo porcine studies on the effect of nanosecond high voltage pulses on liver tissue have shown that cell death can be induced in well-defined tissue volumes without damaging collagen-predominant structures. Comparison of the experimental results with the results of a three-dimensional finite element model allowed us to determine the threshold electric field for cell death. For 30, 100 nanosecond long pulses this was found to be in the range from 12 to 15 kV/cm. Modelling of the temperature distribution in the tissue using Pennes' bioheat equation showed that the lethal effect of nanosecond pulses on cells is non-thermal. Muscle contractions, generally caused by high voltage pulses, were significantly reduced for the 100 nanosecond pulses compared to microsecond long pulses. The results of these studies indicate that high voltage nanosecond pulses reliably kill normal liver cells in vivo and therefore may be useful for liver tumor treatments.

Renal tissue ablation with irreversible electroporation: preliminary results in a porcine model

OBJECTIVES

To evaluate the histopathologic and computed tomography imaging features associated with irreversible electroporation (IRE) ablation performed in normal porcine kidneys. IRE is a nonthermal ablative tool that uses direct electrical pulses to create permanent "pores" in cell membranes and cell death. It does not affect the extracellular matrix.

METHODS

Fifteen female swine were treated with IRE using acute (<24 hours), subacute (36 hours), and chronic (3 weeks) treatment settings. Unipolar IRE applicators were placed under CT guidance. The renal pelvis/calyx was included in 18 ablations. Imaging and histopathologic follow-up were performed.

RESULTS

A total of 29 ablations (19 acute, 4 subacute, and 6 chronic) were created. Acute/subacute ablations showed complete cortical necrosis without intervening live cells. The pelvic epithelium was necrotic with urothelial sloughing; pelvic extracellular matrix was intact. Chronic ablations showed cortical fibrosis, regenerating renal pelvic epithelium and intact pelvic extracellular matrix. No thermal injury, renal pelvic, or blood vessel injury was seen. Immediate postprocedure CT imaging demonstrated a hypodense nonenhancing lesion that persisted at 1 week. Of the 6 chronic lesions, 4 showed complete resolution at 3 weeks on imaging. No collecting system damage was seen.

CONCLUSIONS

This preliminary porcine study demonstrates the nonthermal and connective tissue sparing mechanism of action of IRE. These features may protect against collecting system damage after IRE ablation of renal tissue. IRE could therefore play an important role in ablation of centrally located renal tumors.

Ablative therapies for colorectal liver metastases

Ablative therapies remain a useful adjunct in the multidisciplinary treatment of patients with colorectal liver metastases not amenable to hepatic resection. This review summarizes the rationale, underlying mechanisms, techniques, complications, and outcomes of current and emerging ablative modalities.

Towards the creation of decellularized organ constructs using irreversible electroporation and active mechanical perfusion

BACKGROUND

Despite advances in transplant surgery and general medicine, the number of patients awaiting transplant organs continues to grow, while the supply of organs does not. This work outlines a method of organ decellularization using non-thermal irreversible electroporation (N-TIRE) which, in combination with reseeding, may help supplement the supply of organs for transplant.

METHODS

In our study, brief but intense electric pulses were applied to porcine livers while under active low temperature cardio-emulation perfusion. Histological analysis and lesion measurements were used to determine the effects of the pulses in decellularizing the livers as a first step towards the development of extracellular scaffolds that may be used with stem cell reseeding. A dynamic conductivity numerical model was developed to simulate the treatment parameters used and determine an irreversible electroporation threshold.

RESULTS

Ninety-nine individual 1000 V/cm 100-μs square pulses with repetition rates between 0.25 and 4 Hz were found to produce a lesion within 24 hours post-treatment. The livers maintained intact bile ducts and vascular structures while demonstrating hepatocytic cord disruption and cell delamination from cord basal laminae after 24 hours of perfusion. A numerical model found an electric field threshold of 423 V/cm under specific experimental conditions, which may be used in the future to plan treatments for the decellularization of entire organs. Analysis of the pulse repetition rate shows that the largest treated area and the lowest interstitial density score was achieved for a pulse frequency of 1 Hz. After 24 hours of perfusion, a maximum density score reduction of 58.5 percent had been achieved.

CONCLUSIONS

This method is the first effort towards creating decellularized tissue scaffolds that could be used for organ transplantation using N-TIRE. In addition, it provides a versatile platform to study the effects of pulse parameters such as pulse length, repetition rate, and field strength on whole organ structures.

Irreversible electroporation (IRE): a novel method for renal tissue ablation

OBJECTIVE

• To evaluate the effects of irreversible electroporation (IRE) on renal parenchyma and the renal collecting system in a porcine model.

MATERIALS AND METHODS

• Eight female Yorkshire pigs underwent a series of laparoscopic ablations using either monopolar or bipolar IRE (Angiodynamics, Queensbury, NY, USA). • The pigs were killed between 10 min and 14 days after IRE, and the kidneys were harvested for gross and histological analysis, including NADH staining for cellular viability.

RESULTS

• In all, 24 ablations were performed and all the pigs survived without complications. • Initial gross lesions were diffusely haemorrhagic, decreasing progressively in size (30-40%) to small white scars over the 14-day period. • Immediately after IRE, ablated tissue was characterized by diffuse tubular desquamation, eosinophilia, and nuclear pyknosis, with absence of cellular viability by NADH. • At 7 days after IRE, there was diffuse cellular necrosis with early peripheral granulation changes, and by 14 days there was marked tissue granulation, chronic inflammation, and dystrophic calcification with early fibrosis and cellular contraction. • Initial patchy urothelial injury and ulceration showed signs of repair and viability by 14 days after IRE.

CONCLUSIONS

• Renal IRE in the porcine kidney leads to predictable histological changes characteristic of cellular death within 1 h of ablation, with relative urothelial sparing. • Further animal studies are warranted to determine safety and efficacy of this novel ablation technology.

MR imaging to assess immediate response to irreversible electroporation for targeted ablation of liver tissues: preclinical feasibility studies in a rodent model

PURPOSE

To test the hypothesis that magnetic resonance (MR) imaging measurements can be used to immediately detect treated tissue regions after irreversible electroporation (IRE) ablation procedures in rodent liver tissues.

MATERIALS AND METHODS

All experiments received institutional animal care and use committee approval. In four rats for preliminary studies and 18 rats for formal assessment, MR imaging-compatible electrodes were inserted into the liver and MR imaging-monitored IRE procedures were performed at one of three electrode voltages (1000, 1500, or 2500 V), with T1- and T2-weighted images acquired before and immediately after application of the IRE pulses. MR imaging measurements were compared with both finite element modeling (FEM)-anticipated ablation zones and histologically confirmed ablation zones at necropsy. Intraclass and Spearman correlation coefficients were calculated for statistical comparisons.

RESULTS

MR imaging measurements permitted immediate depiction of IRE ablation zones that were hypointense on T1-weighted images and hyperintense on T2-weighted images. MR imaging-based measurements demonstrated excellent consistency with FEM-anticipated ablation zones (r > 0.90 and P < .001 for both T1- and T2-weighted images). MR imaging measurements were also highly correlated with histologically confirmed ablation zone measurements (rho > 0.90 and P < .001 for both T1- and T2-weighted images).

CONCLUSION

MR imaging permits immediate depiction of ablated tissue zones for monitoring of IRE ablation procedures. These measurements could potentially be used during treatment to elicit repeat application of IRE pulses or adjustments to electrode positions to ensure complete treatment of targeted lesions.

Irreversible electroporation: a novel image-guided cancer therapy

Irreversible electroporation (IRE) is a novel tumor ablation technique using a non-thermal energy to create innumerable permanent nanopores in the cell membrane to disrupt cellular homeostasis. This disruption of cellular homeostasis initiates apoptosis which leads to permanent cell death. In our translational research, we have demonstrated that IRE can be a safe, fast and powerful method of tumor treatment. In this review, we present an overview of IRE ablation including a brief history of IRE, advantages and disadvantages of IRE and clinical and research implications of IRE.

The Effect of Electroporation Type Pulsed Electric Fields on DNA in Aqueous Solution

Electroporation is a physical phenomenon in which pulsed electric fields applied across a cell produce transient (reversible) or permanent (irreversible) permeabilization of the cell membrane. Irreversible electroporation is an important method of sterilization in the food industry and it is becoming an important minimally invasive tissue ablation technique in medicine. Motivated by recent observations of apoptosis like marker stains in irreversibly electroporated cells we performed a study on the effects of electroporation type electric pulses on the integrity of naked DNA in solution. Using gel electrophoresis analyses we show that pulses of the irreversible electroporation type have the ability to affect the naked DNA in solution. It is found that some electric parameters that lead to cell death by irreversible electroporation also cause changes in the naked DNA exposed to the same procedure. Our analysis tentatively suggests that some electroporation type electric pulses cause nicks in the DNA molecule. Therefore, it is possible that the mechanisms of cell death in irreversible electroporation also include damages to the DNA. However, this work did not investigate the possible effects of electroporation induced electrode corrosion byproducts, such as Al3+ ions on DNA integrity; which should be also studied in the future. In general, since electroporation phenomena based applications are widely used in medicine and biotechnology, the current study suggests that further research into the effects of electroporation type electric pulses on the DNA are warranted.

Intracranial nonthermal irreversible electroporation: in vivo analysis

Nonthermal irreversible electroporation (NTIRE) is a new minimally invasive technique to treat cancer. It is unique because of its nonthermal mechanism of tumor ablation. Intracranial NTIRE procedures involve placing electrodes into the targeted area of the brain and delivering a series of short but intense electric pulses. The electric pulses induce irreversible structural changes in cell membranes, leading to cell death. We correlated NTIRE lesion volumes in normal brain tissue with electric field distributions from comprehensive numerical models. The electrical conductivity of brain tissue was extrapolated from the measured in vivo data and the numerical models. Using this, we present results on the electric field threshold necessary to induce NTIRE lesions (495-510 V/cm) in canine brain tissue using 90 50-mus pulses at 4 Hz. Furthermore, this preliminary study provides some of the necessary numerical tools for using NTIRE as a brain cancer treatment. We also computed the electrical conductivity of brain tissue from the in vivo data (0.12-0.30 S/m) and provide guidelines for treatment planning and execution. Knowledge of the dynamic electrical conductivity of the tissue and electric field that correlates to lesion volume is crucial to ensure predictable complete NTIRE treatment while minimizing damage to surrounding healthy tissue.

Magnetic resonance imaging characteristics of nonthermal irreversible electroporation in vegetable tissue

We introduce and characterize the use of MRI for studying nonthermal irreversible electroporation (NTIRE) in a vegetative tissue model. NTIRE is a new minimally invasive surgical technique for tissue ablation in which microsecond, high electric-field pulses form nanoscale defects in the cell membrane that lead to cell death. Clinical NTIRE sequences were applied to a potato tuber tissue model. The potato is used for NTIRE studies because cell damage is readily visible with optical means through a natural oxidation process of released intracellular enzymes (polyphenol oxidase) and the formation of brown-black melanins. MRI sequences of the treated area were taken at various times before and after NTIRE and compared with photographic images. A comparison was made between T1W, T2W, FLAIR and STIR MRIs of NTIRE and photographic images. Some MRI sequences show changes in areas treated by irreversible electroporation. T1W and FLAIR produce brighter images of the treated areas. In contrast, the signal was lost from the treated area when a suppression technique, STIR, was used. There was similarity between optical photographic images of the treated tissue and MRIs of the same areas. This is the first study to characterize MRI of NTIRE in vegetative tissue. We find that NTIRE produces changes in vegetative tissue that can be imaged by certain MRI sequences. This could make MRI an effective tool to study the fundamentals of NTIRE in nonanimal tissue.

Advanced hepatic ablation technique for creating complete cell death: irreversible electroporation

PURPOSE

To evaluate the effectiveness of irreversible electroporation (IRE) in hepatic tissue ablation and the radiologic-pathologic correlation of IRE-induced cell death.

MATERIALS AND METHODS

On approval of the animal research committee, 16 Yorkshire pigs underwent ultrasonography (US)-guided IRE of normal liver. A total of 55 ablation zones were created, which were imaged with US, magnetic resonance (MR) imaging, and computed tomography (CT) and evaluated with immunohistochemical analysis, including hematoxylin-eosin (H-E), Von Kossa, and von Willibrand factor (vWF) staining and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay.

RESULTS

At gross section examination, the mean diameter of the ablation zones was 33.5 mm + or - 3.0 (standard deviation) and was achieved in 6.9 minutes (mean total procedure time per ablation), with a mean difference of 2.5 mm + or - 3.6 between US and gross section measurements (r = 0.804). No complications were seen in any of the 16 animals. IRE ablation zones were well characterized with US, CT, and MR imaging, and real-time monitoring was feasible with US. H-E, Von Kossa, and vWF staining showed complete cell death, with a sharply demarcated treatment area. Bile ducts and vessels were completely preserved. Areas of complete cell death were stained positive for apoptotic markers (TUNEL, BCL-2 oncoprotein), suggesting involvement of the apoptotic process in the pathophysiology of cell death caused by IRE.

CONCLUSION

In an animal model, IRE proved to be a fast, safe, and potent ablative method, causing complete tissue death by means of apoptosis. Cell death is seen with full preservation of periablative zone structures, including blood vessels, bile ducts, and neighboring nonablated tissues.

Preclinical analysis of irreversible electroporation on rat liver tissues using a microfabricated electroporator

A microfabricated electroporator (MFE) for the irreversible electroporation (IRE) of tissues has been developed by miniaturizing a clinical electroporator with a two-needle array while keeping the same electric field strength distribution. Since IRE was brought to special attention as one of the local tissue ablation techniques to treat tumors, many preclinical studies have been conducted to investigate the efficacy of IRE on animal tissues. However, some technical difficulties have been frequently encountered due to the macroscale dimension of clinical electroporators, particularly in experiments on small animal models such as the mouse or rat. Here, the MFE was proposed to solve the associated problems, resulting in time- and cost-effective experimental procedures. With the developed MFE, the effect of IRE on rat liver tissues was analyzed with time by immunohistological stainings and electrical measurement, and the experimental results were compared with those operated with the corresponding real-scale clinical electroporator.

Pilot study of irreversible electroporation for intracranial surgery

Irreversible electroporation (IRE) is a new minimally invasive technique to treat cancer using intense but short electric pulses. This technique is unique because of its non-thermal mechanism of tissue ablation. Furthermore it can be predicted with numerical models and can be confirmed with ultrasound and MRI. We present some preliminary results on the safety of using irreversible electroporation for canine brain surgery. We also present the electric field (460 V/cm - 560 V/cm) necessary for focal ablation of canine brain tissue and provide some guidelines for treatment planning and execution. This preliminary study is the first step towards using irreversible electroporation as a brain cancer treatment.

Treatment of breast cancer through the application of irreversible electroporation using a novel minimally invasive single needle electrode

Irreversible electroporation (IRE) is a therapeutic technology for the ablation of soft tissues using electrodes to deliver intense but short electric pulses across a cell membrane, creating nanopores that lead to cell death. This phenomenon only affects the cell membrane, leaving the extracellular matrix and sensitive structures intact, making it a promising technique for the treatment many types of tumors. In this paper, we present the first in vivo study to achieve tumor regression using a translatable, clinically relevant single needle electrode for treatment administration. Numerical models of the electric field distribution for the protocol used suggest that a 1000 V/cm field threshold is sufficient to treat a tumor, and that the electric field distribution will slightly decrease if the same protocol were used on a tumor deep seated within a human breast. Tumor regression was observed in 5 out of 7 MDA-MB231 human mammary tumors orthotopically implanted in female Nu/Nu mice, with continued growth in controls.

A statistical model for multidimensional irreversible electroporation cell death in tissue

Background

Irreversible electroporation (IRE) is a minimally invasive tissue ablation technique which utilizes electric pulses delivered by electrodes to a targeted area of tissue to produce high amplitude electric fields, thus inducing irreversible damage to the cell membrane lipid bilayer. An important application of this technique is for cancer tissue ablation. Mathematical modelling is considered important in IRE treatment planning. In the past, IRE mathematical modelling used a deterministic single value for the amplitude of the electric field required for causing cell death. However, tissue, particularly cancerous tissue, is comprised of a population of different cells of different sizes and orientations, which in conventional IRE are exposed to complex electric fields; therefore, using a deterministic single value is overly simplistic.

Methods

We introduce and describe a new methodology for evaluating IRE induced cell death in tissue. Our approach employs a statistical Peleg-Fermi model to correlate probability of cell death in heterogeneous tissue to the parameters of electroporation pulses such as the number of pulses, electric field amplitude and pulse length. For treatment planning, the Peleg-Fermi model is combined with a numerical solution of the multidimensional electric field equation cast in a dimensionless form. This is the first time in which this concept is used for evaluating IRE cell death in multidimensional situations.

Results

We illustrate the methodology using data reported in literature for prostate cancer cell death by IRE. We show how to fit this data to a Fermi function in order to calculate the critical statistic parameters. To illustrate the use of the methodology, we simulated 2-D irreversible electroporation protocols and produced 2-D maps of the statistical distribution of cell death in the treated region. These plots were compared to plots produced using a deterministic model of cell death by IRE and the differences were noted.

Conclusions

In this work we introduce a new methodology for evaluation of tissue ablation by IRE using statistical models of cell death. We believe that the use of a statistical model rather than a deterministic model for IRE cell death will improve the accuracy of treatment planning for cancer treatment with IRE.

Irreversible electroporation therapy in the liver: longitudinal efficacy studies in a rat model of hepatocellular carcinoma

Irreversible electroporation (IRE) is an innovative local-regional therapy that involves delivery of intense electrical pulses to tissue to induce nanoscale cell membrane defects for tissue ablation. The purpose of this study was to investigate the feasibility of using IRE as a liver-directed ablation technique for the treatment of hepatocellular carcinoma (HCC). In the N1-S1 rodent model, hepatomas were grown in 30 Sprague-Dawley rats that were divided into treatment and control groups. For treatment groups, IRE electrodes were inserted and eight 100-mus 2,500-V pulses were applied to ablate the targeted tumor tissues. For both groups, magnetic resonance imaging scans were performed at baseline and 15-day follow-up intervals to determine tumor sizes (one-dimensional maximum diameter, D(max); estimated two-dimensional cross-sectional area, C(max)) as a tactic to assess longitudinal outcomes. Additional groups of treated animals were sacrificed at 1-, 3-, and 7-day intervals posttherapy for pathology assessment of treatment response. Magnetic resonance images showed significant tumor size reductions within 15 days posttherapy (32 +/- 31% D(max) and 52 +/- 39% C(max) decreases compared with 110 +/- 35% D(max) and 286 +/- 125% C(max) increases for untreated tumors). Pathology correlation studies documented progression from poorly differentiated viable HCC tissues before treatment to extensive tumor necrosis and full regression in 9 of 10 treated rats 7 to 15 days after treatment. Our findings suggest that IRE can be an effective strategy for targeted ablation of liver tumors, prompting its further evaluation for HCC therapy.

Liver tumor ablation: percutaneous and open approaches

The global incidence of liver cancer is greater than a million cases a year. Surgical resection where applicable is still the standard of care for these patients. Various liver-directed regional therapies have been developed in an effort to treat the vast majority of unresectable liver tumors. This article reviews the principles behind various ablation therapies currently available for malignant liver tumors and their outcomes.

A study using irreversible electroporation to treat large, irregular tumors in a canine patient

Irreversible electroporation (IRE) has shown promise for the therapeutic treatment of focal disease, including tumors. The effects of treatment are dependent on the electric field distribution, which may be predicted with numerical modeling. In order to improve the effectiveness and scope of IRE therapies, techniques must be developed for designing protocols capable of treating large and irregular tumors. We present the findings of a study designing an IRE treatment plan for a canine patient using medical imaging analysis and reconstruction, numerical modeling, and real-time electrode placement guidance. The executed plan was able to alleviate the patient's clinical symptoms without damaging any of the nearby sensitive tissues in a complex heterogeneous environment.

Electrical field and temperature model of nonthermal irreversible electroporation in heterogeneous tissues

Nonthermal irreversible electroporation (NTIRE) is a new minimally invasive surgical technique that is part of the emerging field of molecular surgery, which holds the potential to treat diseases with unprecedented accuracy. NTIRE utilizes electrical pulses delivered to a targeted area, producing irreversible damage to the cell membrane. Because NTIRE does not cause thermal damage, the integrity of all other molecules, collagen, and elastin in the targeted area is preserved. Previous theoretical studies have only examined NTIRE in homogeneous tissues; however, biological structures are complex collections of diverse tissues. In order to develop electroporation as a precise treatment in clinical applications, realistic models are necessary. Therefore, the purpose of this study was to refine electroporation as a treatment by examining the effect of NTIRE in heterogeneous tissues of the prostate and breast. This study uses a two-dimensional finite element solution of the Laplace and bioheat equations to examine the effects of heterogeneities on electric field and temperature distribution. Three different heterogeneous structures were taken into account: nerves, blood vessels, and ducts. The results of this study demonstrate that heterogeneities significantly impact both the temperature and electrical field distribution in surrounding tissues, indicating that heterogeneities should not be neglected. The results were promising. While the surrounding tissue experienced a high electrical field, the axon of the nerve, the interior of the blood vessel, and the ducts experienced no electrical field. This indicates that blood vessels, nerves, and lactiferous ducts adjacent to a tumor treated with electroporation will survive, while the cancerous lesion is ablated. This study clearly demonstrates the importance of considering heterogeneity in NTIRE applications.

A preliminary study to delineate irreversible electroporation from thermal damage using the arrhenius equation

Intense but short electrical fields can increase the permeability of the cell membrane in a process referred to as electroporation. Reversible electroporation has become an important tool in biotechnology and medicine. The various applications of reversible electroporation require cells to survive the procedure, and therefore the occurrence of irreversible electroporation (IRE), following which cells die, is obviously undesirable. However, for the past few years, IRE has begun to emerge as an important minimally invasive nonthermal ablation technique in its own right as a method to treat tumors and arrhythmogenic regions in the heart. IRE had been studied primarily to define the upper limit of electrical parameters that induce reversible electroporation. Thus, the delineation of IRE from thermal damage due to Joule heating has not been thoroughly investigated. The goal of this study was to express the upper bound of IRE (onset of thermal damage) theoretically as a function of physical properties and electrical pulse parameters. Electrical pulses were applied to THP-1 human monocyte cells, and the percentage of irreversibly electroporated (dead) cells in the sample was quantified. We also determined the upper bound of IRE (onset of thermal damage) through a theoretical calculation that takes into account the physical properties of the sample and the electric pulse characteristics. Our experimental results were achieved below the theoretical curve for the onset of thermal damage. These results confirm that the region to induce IRE without thermal damage is substantial. We believe that our new theoretical analysis will allow researchers to optimize IRE parameters without inducing deleterious thermal effects.

The feasibility of irreversible electroporation for the treatment of breast cancer and other heterogeneous systems

Developments in breast cancer therapies show potential for replacing simple and radical mastectomies with less invasive techniques. Localized thermal techniques encounter difficulties, preventing their widespread acceptance as replacements for surgical resection. Irreversible electroporation (IRE) is a non-thermal, minimally invasive focal ablation technique capable of killing tissue using electric pulses to create irrecoverable nano-scale pores in the cell membrane. Its unique mechanism of cell death exhibits benefits over thermal techniques including rapid lesion creation and resolution, preservation of the extracellular matrix and major vasculature, and reduced scarring. This study investigates applying IRE to treat primary breast tumors located within a fatty extracellular matrix despite IREs dependence on the heterogeneous properties of tissue. In vitro experiments were performed on MDA-MB-231 human mammary carcinoma cells to determine a baseline electric field threshold (1000 V/cm) to cause IRE for a given set of pulse parameters. The threshold was incorporated into a three-dimensional numerical model of a heterogeneous system to simulate IRE treatments. Treatment-relevant protocols were found to be capable of treating targeted tissue over a large range of heterogeneous properties without inducing significant thermal damage, making IRE a potential modality for successfully treating breast cancer. Information from this study may be used for the investigation of other heterogeneous tissue applications for IRE.

Towards Solid Tumor Treatment by Nanosecond Pulsed Electric Fields

Local and drug-free solid tumor ablation by large nanosecond pulsed electric fields leads to supra-electroporation of all cellular membranes and has been observed to trigger nonthermal cell death by apoptosis. To establish pore-based effects as the underlying mechanism inducing apoptosis, we use a multicellular system model (spatial scale 100 μm) that has irregularly shaped liver cells and a multiscale liver tissue model (spatial scale 200 mm). Pore histograms for the multicellular model demonstrate the presence of only nanometer-sized pores due to nanosecond electric field pulses. The number of pores in the plasma membrane is such that the average tissue conductance during nanosecond electric field pulses is even higher than for longer irreversible electroporation pulses. It is shown, however, that these nanometer-sized pores, although numerous, only significantly change the permeability of the cellular membranes to small ions, but not to larger molecules. Tumor ablation by nanosecond pulsed electric fields causes small to moderate temperature increases. Thus, the underlying mechanism(s) that trigger cell death by apoptosis must be non-thermal electrical interactions, presumably leading to different ionic and molecular transport than for much longer irreversible electroporation pulses.