A comparison of three different microwave systems for laparoscopic liver tumor ablation

Our aim was to perform a comparison of three current microwave ablation (MWA) systems widely used for laparoscopic liver ablations in terms of ablation kinetics and geometry of ablation zones.


| INTRODUCTION
2][3] Over the last decade, microwave ablation (MWA) has received more preference over radiofrequency ablation due to its ability to create treatment zones more efficiently with better local tumor control. 4,5As a result, a number of ablation systems has been developed by different companies. 6Although innovation drives efficiency, the availability of multiple systems with different characteristics has created confusion and left liver surgeons puzzled about what system to use in a given patient.Although a number of studies have attempted to compare different systems in dry-lab 7,8 models, there is scant data on the comparison of performance in patients. 9,10his study aims to perform a comparison of three current MWA systems widely used for laparoscopic liver ablations in terms of ablation kinetics and geometry of ablation zones.

| METHODS
This was an institutional review board (IRB) approved retrospective preferred with smaller patients with fewer and smaller tumors located more superficially.On the other hand, Emprint HP was preferred for bigger patients with a larger number and size of tumors in deeper locations, due to its longer antenna length and ability to deliver higher amounts of energy to tissues.
The procedures were done laparoscopically, as previously described. 11In brief, the patients were placed supine on the operating table.Two 12 mm incisions were made in the right upper quadrant for the laparoscope and ultrasound transducer.The MWA antenna was introduced through a separate stab puncture through a metallic sheath.
The ablations were performed under ultrasound guidance, aiming for at least a cm of margin unless limited by surrounding anatomic structures and the amount of total parenchyma treated.The patients were kept overnight and discharged home the next morning.At 2 weeks, either a triphasic liver CT or a liver MRI was performed to confirm complete ablation and rule out missed residual disease.These scans were repeated quarterly for the first 2 years and then biannually.The presence of incomplete ablations and development of recurrent disease in follow-up was assessed by abdominal radiologists with significant experience in interpreting these scans.As a fusion technology was not available, the ablation margin was calculated by subtracting the largest diameter of the lesions on preoperative imaging (within a month of surgery) from the largest diameter of corresponding ablation zones on 2 weeks postablation imaging studies and dividing the value in half to give the radial margin.

| Ablation systems used
1. Emprint TM Ablation System with Thermosphere TM Technology (Medtronic): This 2.45 GHz MWA system uses a 30 cm, 14-G antenna with a 100 W ablation generator (Figure 1).The active microwave antenna is surrounded by saline irrigation channels.An illustrative case is presented in Figure 2.
2. Emprint TM HP Ablation Generator with Thermosphere TM Technology (Medtronic): This 2.45 GHz MWA system uses a 30 cm, 14-G antenna with a 150 W ablation generator (Figure 3).The active microwave antenna is again surrounded by saline irrigation channels.Figure 4 demonstrates a representative case.

NEUWAVE TM Microwave Ablation System (Johnson & Johnson):
This 2.45 GHz MWA system uses a 17-G, 20 cm antenna cooled by CO 2 gas with a 140 W generator (Figure 5). Figure 6 illustrates a representative case.
A "stepwise" ablation technique was used to treat most of the tumors.In this technique that was described in detail before, 12 the ablation was initially started at a low power of 60 W until the first ablation bubble, the "pop" occurred.Then, the power was increased to higher powers (95−150 W based on the equipment used) to create the desired ablation zone.

| DISCUSSION
To our knowledge, this is the first clinical comparison of three popular modern MWA systems in use for laparoscopic liver tumor ablation.
Our results show that all three systems performed well to achieve complete tumor destruction without any residual areas for all lesions treated.Ablation times were also within an acceptable range for all systems.Nevertheless, there were differences in ablation zone creation.
First of all, despite being a less powerful system, the initial "pop" occurred faster and more gradually with the NeuWave versus Emprint and Emprint HP systems.This is related to the fact that the active antenna is exposed directly to liver tissue with the F I G U R E 6 A 1.8 × 1.6 cm colorectal cancer metastasis located in segment II (A) was treated with laparoscopic ablation using the NeuWave device.The ablation was performed initially at 60 W for 45 s, followed by further ablation at 95 W for 3 min.A postablation scan (B) obtained at 2 weeks revealed a 5.0 × 4.8 × 4.2 cm ablation zone.
NeuWave system but surrounded by the saline microchannels with the Emprint systems.As a result, the "pop" occurred later and more forcefully with the Emprint systems.Whether this difference causes any impact on clinical outcomes is unknown.We did not encounter any bleeding or biliary complications with any system in this study.
Second, ablation zones were more spherical with both Medtronic systems and more ellipsoid with the NeuWave system.Although a complete ablation, with no residual tumors, was seen in all patients 2 weeks postablation, the formation of the ablation zones suggests that it would be easier to achieve wide circumferential margins for smaller rather than larger tumors with a single stick when using the NeuWave system.Hence, monitoring the ablation zone with real-time intraoperative ultrasound is essential to ensure this coverage is obtained.
On the other hand, due to the spherical ablation zone creation, it seems easier to create wide circumferential ablation zones with the Emprint systems.This difference raises the question of whether local recurrence rates would differ between the ablation systems used in the study.Nevertheless, due to the more recent usage of the NeuWave system, it is not possible to answer this question at this time.Third, ablations to cover the tumors treated in the study were done faster (a median of 3 min shorter) with the Emprint HP and NeuWave systems versus the Emprint system in the study.This is related to the later creation of the "pop" with the Emprint systems, as we waited to see the "pop" before advancing power to higher settings.Due to the higher power available with the Emprint HP (150 W) versus the Emprint (100 W) system, the ablation was created faster with the former model.Interestingly, despite being a lowerpower system, the overall creation of the ablation was still quite efficient with the NeuWave system due to the antenna design discussed above.With the NeuWave system, the SR probe is a longer antenna designed for surgical ablation.Still ellipsoid, but larger ablation zones can be created with this antenna versus the PR probe used in this study.Nevertheless, ablation zones extend further away from the tip of the PR probe compared to the SR probe; hence, the precision is less.Since the PR probe allows for more precise ablations similar to the probes used in Emprint systems, it was used for the ablations in this study.
A number of studies have compared 915 MHz systems with 2.45 GHz systems 10,13 showing faster and larger ablation zones, with resultant better local tumor control with the latter devices.

Regarding the comparison of ablation zones created with
NeuWave versus Emprint systems, Lee et al. 14  Although the authors did not present an explanation for the difference, we speculate that this was related to the differences in antenna design, with the energy being directly applied to the tissue with the NeuWave system versus being applied through the saline irrigation channel barrier in the Emprint system.
There are certain limitations of the study.First of all, it was a retrospective study.Second, there was a bias in selecting patients with larger and higher numbers of tumors for the Emprint systems versus the NeuWave system.In addition, due to the shorter antenna length with the NeuWave system, patients with more favorable anatomy and tumors could have been selected for this system versus the Emprint systems.Nevertheless, we still believe that we were able to elucidate some key differences in the creation of ablation zones per device with the study design.

| CONCLUSIONS
To our knowledge, this is the first study focusing on comparisons of laparoscopic liver ablations created with three current MWA systems.
Despite the retrospective nature, we quantified technical and geometric differences in ablation zone creation between the systems.
Although a saline-cooling system that covers the active microwave antenna with the Emprint system allows for larger diameter spherical ablation zones to be created, it leads to decreased efficiency compared to the CO 2 -cooled NeuWave system, which exposes the active antenna directly to tissue, as hinted by Ruiter et al. 9 Increased power delivered by Emprint HP improves the efficiency of the salinecooled design, as demonstrated by faster ablation times in our analysis.We believe that ablation surgeons should be aware of the b Range.
c Blood vessel proximity: lesions in direct contact with a vessel were considered near a large blood vessel, and otherwise away.
study.Between 2014 and 2023, laparoscopic MWA for malignant liver tumors performed by the senior author (E.B.) were entered prospectively into an IRB-approved database.Emprint TM Ablation System with Thermosphere TM Technology (Medtronic) was used between 2014 and 2020.The Emprint system was replaced by Emprint TM HP Ablation Generator with Thermosphere TM Technology (Medtronic) in 2020.The NEUWAVE TM Microwave Ablation System (Johnson & Johnson) was also added to the armamentarium in 2021.The selection of NeuWave versus Emprint HP after 2021 depended on patient and tumor characteristics.Neuwave was generally At the time of this writing, 23 cases with the NeuWave system had been done by the senior surgeon (E.B.).For comparison, two groups F I G U R E 1 Photo showing the Emprint generator and the laparoscopic probe used in the study. of 23 consecutive patients, each ablated using Emprint and Emprint HP systems, were included in the analysis.Clinical and operative parameters were compared using Mann−Whitney U and χ 2 tests using JMP ® 16 software (SAS Institute).Statistical significance was reached at p < 0.05.Continuous data are presented as median and interquartile range (IQR).

F
I G U R E 2 A 1.7 × 1.7 cm neuroendocrine metastasis located in segment VIII (A) was treated with laparoscopic ablation using the Emprint device.The lesion was initially treated at 60 W for 1 min and then at 100 W for 5 min.Postablation scan (B) done at 2 weeks revealed a 4.2 × 4.1 × 4.2 cm ablation zone.F I G U R E 3 Photo showing the Emprint HP generator and the laparoscopic probe used in the study.

F
I G U R E 4 A 1.7 × 1.2 cm colorectal cancer metastasis located in segment VI (A)was treated with laparoscopic ablation using the Emprint HP device.Ablation was performed initially at 60 W for 2 min and then at 150 W for 3 min.Postablation scan (B) at 2 weeks demonstrated a 4.6 × 4.0 × 4.6 cm ablation zone.F I G U R E 5 Photo showing the NeuWave generator and the PR probe used in the study.and NeuWave groups were 49, 60, and 52, with colorectal cancer and neuroendocrine tumors comprising the majority in each group (Table reported that spherical indices for ablations created in bovine liver at the benchtop were closer to one with Emprint versus NeuWave T A B L E 1 Demographic and clinical details of study patients.The Emprint MWA system was utilized as a standalone intervention in 13 patients, none of them encountered complications.The Emprint HP MWA system was utilized as a standalone intervention in 14 patients, among whom 1 patient developed urinary retention.The Neuwave MWA system was utilized as a standalone intervention in 12 patients, among whom 1 patient developed urinary tract infection. Abbreviation: SEM, standard error of the mean.aMedian (IQR).bRange.cPatients who underwent concomitant surgical procedures excluded.d Comparison of clinical and ablation parameters on a per lesion basis.
technical differences between different MWA systems.Selection of the appropriate ablation technology for a given tumor by T A B L E 2 a Median (IQR).