The performance of the Dophi™ M150E Microwave Ablation System is satisfactorily predictable for the organs under study.

The temperature trends are consistent with the applied MW settings. For liver and kidney, the temperature rise in time (Fig. 3A and 5A) and the slopes of the temperature curves are very similar, proving that it is a reasonable choice to use the settings defined for the liver also for treating kidney [14], as these organs are characterized by similar thermal properties [15,16,17]. Indeed, the thermal effect and the resulting size and shape of ablation region depends on the heat propagation inside the tissues, which is described by the idiosyncratic thermophysical properties. Lastly, the three independent experiments at the same conditions provide a good reproducibility, as witnessed by an average standard deviation < 10% on all the curves.

In terms of ablation areas, both liver and kidney present high reproducibility and correspondence to the data of the manufacturer when 50 W is used. Increasing the power values to 100 W leads to a lower reproducibility (increased values of the standard deviation), and bigger difference with the data provided by the manufacturer (Figs. 3B–C, 5B–C and Table 1). For the kidney, in particular, at 100 W, the measured a axis is comparable with the manufacturer data, whereas the measured b axis is smaller, with a difference of 0.6 cm and 1.1. cm after 5 min and 10 min of MWA, respectively. The possible cause of this low reproducibility is the small size of the porcine kidney, which poses a challenge in performing complete ablations in the interior portion of the organ without affecting its surface. Conversely, it results to be a useful model to study the MWA effects when the power of 50 W is used.

The sphericity index, which is a measure of the spherical shape of the ablation (sphericity index = 1 corresponds to spherical ablated area), has different trends for liver and kidney. For the liver, it ranges from 0.85 to 1, using a single antenna at 50 W and 100 W. These results agree with outcomes obtained with Emprint ablation system (Covidien/Medtronic, Minneapolis, USA), which holds similar technologies of Dophi™ M150E Microwave Ablation System [2]. In ex vivo experiments conducted on porcine livers using the Emprint Ablation System, the mean sphericity index was found to be 0.87, whereas for the HS Amica-Gen device (HS AMICA PROBE, HS Hospital Service, Aprilia, Italy) and NeuWave Certus Microwave Ablation System (NeuWave Medical, Madison, Wisconsin), it was 0.59 [4] and 0.75, respectively [2, 4, 18]. A multicentre retrospective study, conducted across two hospitals in France and one in the USA, included MWA of liver tumors, comprising 65 metastases and 22 hepatocellular carcinomas, using the Dophi™ M150E MWA system (HDTECH, Lorient, France), and reported a mean sphericity index of the ablation zone as 0.78 ± 0.14 [4].

For the kidney, sphericity index was 1 delivering 50 W with one antenna for both 5 and 10 min, while it ranges between 0.7 and 0.8 for 100 W.

Regarding bovine lung, the maximum temperature rise is similar to what obtained for liver and kidney, but the different temperature rises measured at d = 2 cm (i.e., 45 °C at 50 W, 55 °C at 100 W and 60 °C at 75 W-dual antenna) suggest a peculiar temperature trend (Fig. 3A). This might be ascribed to the specific antenna geometry which is recommended for the lung (active length of 26 mm), and to the tissue characteristics. The standard deviation on the ablation axes of the lung (Fig. 4B) is overall bigger than the one obtained for the liver (Fig. 3B), especially on a axis (which is parallel to the path of the antenna). This deviation might be caused by the presence of several bronchi inside the lungs, which contributes to a heat sink effect and lower ablation reproducibility when high power settings are used [3]. However, in in vivo scenarios, the ablation regions are smaller than the ones obtained in ex vivo models with the same MW settings [3].

The sphericity index obtained in the lung is < 1 for most settings, except for double antenna at 75 W-5 min. For both liver and lung, this setting allows a sphericity index = 1.36, with an ablation area elliptical, larger in the direction perpendicular to the antenna’s axis (b axis) than longer (a axis). MWA performed with more than one antenna allows a tailored ablation area, reproducing the shape and size of the lesion with an adequate safe margin [19, 20]. This aspect is crucial to establish the efficacy of MWA, as it has been observed that tumor size increases the risk of local recurrence [21, 22].

The target to reach for every ablation treatment is A0 (Ablation Zero), which means no pathological tissue residual. To achieve  a complete tumor eradication the operator must consider the ablative margin, which is the region ablated beyond the tumor’s perimeter; this margin, ideally, should measure 0.5–1.0 cm in its smallest width, depending on tumor histological type [23].

Considering the same tissue features, the technical parameters that can affect the results are predictability, reproducibility and sphericity index.

Using Dophi™ M150E Microwave Ablation System, the comparison of the experimental ablation axes and the values provided by the manufacturer provides good results especially at lower power and with one antenna (average difference ranges between 2 and 7 mm, Table 1). Using the double antenna setting decreases the performance in terms of predictability of the ablation axes obtained in the liver and in the lung (Fig. 3B, 4B), and provides sphericity index > 1 (except for the lung treated at 75 W-10 min).

However, our results are valid for ex vivo healthy organs (for direct comparison with manufacturer’s indications): data obtained in in vivo scenarios might reveal increased variation compared to company’s values, because of heterogeneity in tumor type.

Variations in heating during longer treatments may be related to thermal conduction and convection of heterogeneous tissues, and suggest preferring short, high-power ablations over long, low-power ablations for the sake of reproducibility [4].

There are some limiting aspects of the study that require a deeper analysis: ablations were carried out in unperfused healthy porcine organs, thus the ablation systems used in this work can perform differently when applied in clinical scenarios. The company provides several settings and needles, but we focused this initial analysis on the most used settings (following the recommendation from radiology experts) and one single needle type for each organ. In future, it will be useful to study the performance of the system at several combinations in power and ablation time, and with different needles.

To conclude, the results of our ex vivo experience with Dophi™ M150E can affect the clinical practice and impact on ablative treatments outcome. In the future, more power-time combinations would be assessed on target organs, and dedicated imaging approaches will be used to evaluate the ablated volumes.