Percutaneous Irreversible Electroporation (IRE) of Hepatic Malignancy: A Bi-institutional Analysis of Safety and Outcomes
Irreversible electroporation (IRE) is a non-thermal ablative option in patients unsuitable for standard thermal ablation, due to its potential to preserve collagenous structures (vessels and ducts) and a reduced susceptibility to heat sink effects. In this series from two large tertiary referral hepatobiliary centres, we aim to assess the safety/outcomes of hepatic IRE.
Materials and Methods
Bi-institutional retrospective, longitudinal follow-up series of IRE for primary hepatic malignancy; [hepatocellular carcinoma (n = 20), cholangiocarcinoma (n = 3)] and secondary metastatic disease; colorectal (n = 28), neuroendocrine (n = 1), pancreatic (n = 1), breast (n = 1), gastrointestinal stromal tumour (GIST, n = 1) and malignant thymoma (n = 1). Outcome measures included procedural safety/effectiveness, time to progression and time to death.
Between 2013 and 2017, 52 patients underwent percutaneous IRE of 59 liver tumours in 53 sessions. All tumours were deemed unsuitable for thermal ablation. Cases were performed using ultrasound (US) or computed tomography (CT) guidance. A complete ablation was achieved in n = 44, (75%) of cases with an overall complication rate of 17% (n = 9). Of the complete ablation group, median time to progression was 8 months. At 12 months, 44% were progression-free (95% CI 30–66%). The data suggest that larger lesion size (> 2 cm) is associated with shorter time to progression and there is highly significant difference with faster time to progression in mCRC compared with HCC. Median survival time was 38 months.
This bi-institutional review is the largest UK series of IRE and suggests this ablative technology can be a useful tool, but appears to mainly induce local tumour control rather than cure with HCC having better outcomes than mCRC.
KeywordsIrreversible electroporation Hepatic malignancy Tumour ablation
Materials and Methods
A retrospective analysis was performed to identify all patients who had undergone percutaneous hepatic IRE at two large hepatobiliary tertiary referral centres in the UK.
Patients and Tumour Characteristics
All cases were performed using the NanoKnife system in accordance with the manufacturer’s guidance. General anaesthetic with neuromuscular blockade (most commonly using rocuronium bromide) is mandatory to minimise unwanted muscular contraction.
IRE electrodes were percutaneously placed using image guidance as per operator preference. A variety of imaging modalities were employed, the most common being computed tomography guidance (n = 35). Other modalities included ultrasound alone (n = 13), contrast-enhanced ultrasound (CEUS) in n = 8, CT/US fusion (n = 2) and combined CT/US in (n = 1). Electrodes were placed in a parallel direction with a distance of 1.0–2.0 cm apart. A mean number of electrodes used were 3 (range 2–7) which were sited in order to build an ablation zone encompassing the target lesion and rim of surrounding tissue. Electrode repositioning was carried out as required to enable ablation of the whole lesion. Including initial test pulses, 90 pulses of 1500 v/cm were applied between each electrode pair. Parameters were adjusted if necessary in order to achieve a range of 20–50 Amperes, a level associated with irreversible electroporation and cell death.
The pulses are delivered with ECG gating in the refractory phase after myocardial depolarisation to minimise the risk of cardiac arrhythmias.
Post-intervention patients were transferred to the post-anaesthesia care unit (PACU) for recovery and then transferred back to the ward with a minimum of 6 h bed rest. Mean hospital stay was 3 days (range 1–12).
Outcome Measures and Statistical Analysis
There were two outcome measures: time to progression and time to death. As not all subjects progressed or died, these were considered as survival outcomes. The outcome was recorded as the time to the progression or death. Those where these outcomes had not occurred were censored to the last time where either progression or death was recorded not to have occurred.
For patients with more than one lesion, progression was deemed to have occurred, if one or more of the lesions progressed. The time to the two outcomes was summarised and displayed graphically using Kaplan–Meier methods. Median survival was calculated, as was survival at specific points in time. Corresponding confidence intervals were calculated for the estimates at specific time points.
Patients were divided into three groups based on their lesion size. For patients with more than one lesion, the largest lesion size was considered. The logrank test was used to compare time to progression and overall survival between groups.
Patients were also divided into groups based on their pathology. The majority of patients had either a CRLM or HCC pathology, and specific comparisons of the outcomes of these two groups only were made using the logrank test. Patients with other pathologies were omitted from these analyses.
Adverse Events and Complications
All complications during and after 53 IRE procedures. Categorised by CIRSE classification system
Gallbladder perforation with resultant bile leak and peritonitis
Systemic inflammatory response syndrome
Outcomes, Tumour Response and Survival
Technical success was defined as a complete response on first follow-up imaging using either contrast-enhanced CT or MRI at 4–8 weeks post-ablation. A complete ablation was achieved in n = 44, (75%) of ablations in 37 patients. All radiology images were reviewed by radiologists with experience in post-ablation and hepatobiliary imaging. For HCC, a complete response was reported according to mRECSIST. An incomplete ablation was observed in n = 13 cases (22%), one patient was lost to follow-up, and 1 patient died as outlined above. Where patients were found to have an incomplete ablation, a second attempt at IRE was not performed, and instead, patients were managed by non-interventional treatments.
In patients with resectable hepatic malignancy, surgical resection is regarded as the gold standard of care. However, data from two of the most common hepatic cancers: metastatic colorectal cancer (mCRC) and hepatocellular carcinoma (HCC), suggest that less than 30% of patients are typically suitable for surgery. Thermal ablation with RFA and MWA has a proven role in the management of unresectable hepatic malignancy [12, 13]. In small lesions, ablation can rival surgical resection in terms of local tumour control . Ablation is also associated with a low morbidity and mortality. In a meta-analysis of over 15,000 patients, the morbidity (major complication rate defined as any symptom that developed after ablation and persisted for more than 1 week, or those that delayed hospital discharge, threatened the patient’s life or led to substantial morbidity and disability) was 4.1% for RFA and 4.6% for MWA [14, 15]. Mortality was calculated at 0.15% for RFA and 0.23% for MWA. Not all hepatic lesions are suitable for thermal ablation due to the danger of damaging adjacent thermosensitive structures such as the gallbladder and central bile ducts. Furthermore, ablation adjacent to large vessels can be ineffective due to heat sink effects or result in vessel thrombosis . IRE is a comparatively novel non-thermal ablation technology which can induce tumour necrosis while sparing adjacent vulnerable structures. Emerging small data series support IRE’s safety and effectiveness in the liver [7, 8, 9, 10].
A recent systematic review has suggested a complication rate for hepatic IRE ranging from 11 to 36% . The data from our series are at the lower end of this rate at 17%. However, these figures are considerably higher than thermal ablation complication rates. This increase may be related to the fact IRE requires a minimum of two electrodes and ablations are more frequently adjacent to vital hepatic structures. Of note, the cardiac arrhythmias appear more frequent with IRE, while the vicinity of electrodes close to the heart has been suggested as a possible cause, but the precise mechanism is not fully understood. It has also been shown that subclinical myocardial injury can occur with IRE as evidenced by elevations in high-sensitive troponin I . Cardiac arrhythmias rarely impair completion of an IRE procedure, as was the case in our three instances of atrial fibrillation which were all either self-limiting or medically managed . Ablation of large tumour volumes (40% of liver) has been shown in an animal model to induce alterations in serum potassium levels due to ionic shifts with nanopore formation resulting in the potential for electrocardiogram arrhythmias, but this mechanism is not felt to be applicable in this study as lesional volume was considerably smaller .
A systematic review of mainly single-centre retrospective reviews has shown hepatic IRE to have a primary efficacy of 67–100% . Published data from multicentre prospective trials in hepatic IRE are still awaited, but preliminary presented results from (clinicaltrials.gov ID:NCT01078415) for biopsy proven early-stage HCC have indicated a 1-month complete response rate of 77% . Data from our study have indicated a 75% complete response rate at the first follow-up imaging, which is within the range of the aforementioned published studies. One challenging factor which is rarely mentioned in the literature is the difficulty in interpretation of post-IRE imaging which may impact on the primary efficacy outcomes. Imaging appearances post-IRE remain at an investigative stage; therefore, reported efficacy outcomes should be interpreted with caution [22, 23]. Small histopathologic studies have suggested that imaging responses to IRE may be an inaccurate reflection of the ablation zone [24, 25].
This study and the literature as a whole on hepatic IRE currently suggest that IRE is not as effective as its thermal ablation counterparts, which are able to rival surgical resection . Caution should be observed in case selection with IRE as we have shown statistically significant differences in outcomes with both size and pathology with small (< 2 cm) HCCs having the best outcomes in terms of primary efficacy and longer-term tumour control. Other studies have indicated tumour volume of > 5 cm3 and underlying disease type (HCC, cholangiocellular carcinoma or metastatic disease) as independent risk factors for early local recurrence .
The reasons for differences between IRE and thermal ablation in terms of primary efficacy and local recurrence are uncertain. The technical challenges of placing multiple electrodes in parallel orientation into challenging hepatic locations may be a factor, and a learning curve of at least five cases has been suggested . Traditional teaching with thermal ablation suggests a 1-cm circumferential ablation zone around a lesion to achieve recurrence-free survival results rivalling surgery . With IRE, electrodes are typically placed at the periphery of a lesion with the electroporation effect extending up to 5 mm beyond the electrode position, and this means the margins achieved using current IRE ablation protocols are not equivalent to thermal ablation. Further investigation into optimum ablation protocols beyond mathematical models is needed .
The retrospective nature, relatively small sample size and heterogeneous group of patients with varying pathologies represent the largest limitation to this review. Further heterogeneity exists with the imaging follow-up protocol which employs both CT and MRI at varied time points post-procedure. All patients were, however, planned to have imaging follow-up at 4–8 weeks post-treatment. As the current study was not designed as a trial, ultimately, the information presented reflects ‘real-world data’ from two large hepatobiliary centres. The nature of IRE is such that only a limited patient cohort is suitable for this type of ablation, and it therefore unlikely a gold standard randomised control trial would be possible for this ablation technology. The data obtained for the two largest groups: HCC and CRLM add to the existing literature and should therefore be of use in the process of clinical decision-making. No definite conclusions can be drawn regarding other metastatic malignancies.
This bi-institutional study represents the largest follow-up series to date in the UK regarding hepatic IRE. Our data suggest that lesion size < 2 cm and HCC represent the ‘optimum’ case selection for IRE. However, even within this group results do not equal the response rates published for thermal ablation. IRE may therefore be an attractive ablation option in patients with no other treatment option, but interventional radiologists should remain aware of the uncertainties regarding this technology.
Compliance with Ethical Standards
Conflict of interest
All authors declare that they have no conflict of interest.
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.
Informed consent was obtained from all individual participants included in the study.
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