The results of our study demonstrate that multiple smaller antennas can be used to distribute microwave power more uniformly over an area of tissue. Instead of delivering all the power at the center of the ablation zone, the power is distributed throughout the target volume. This leads to lower (but still cytotoxic) temperatures at the center of the ablation zone, higher temperatures toward the periphery of the ablation zone, and an increase in the size of the ablation zones. Heating tissue uniformly is a more efficient use of energy, given that cells heated to 60 °C are already necrotic, and that tissue temperatures fall off rapidly further away from antennas. Despite the linear and triangular configurations of the arrays, multiple-antenna zones of ablation were as circular in cross-section as single-antenna ablations. Importantly, the increase in the size of the ablation zone was accomplished without a corresponding increase in invasiveness (ie, total puncture area of the applicators). In fact, the total puncture area of the 17- and 18-gauge configurations (3.54 and 3.39 mm2, respectively) was less than that of the 13-gauge applicator (4.15 mm2). In other words, the efficiency of power delivery was increased with multiple antenna arrays, reflected by a higher ablation-to-invasiveness ratios observed in the 17- and 18-gauge groups.
Brace et al.7
and Wright et al.1
have previously discussed some of the advantages of multiple-antenna microwave ablation when compared with single-antenna ablation. In particular, multiple-antenna systems can rapidly ablate disproportionately larger volumes of tissue in less time and with less complexity than overlapping single ablations due to the concept of thermal synergy. The disadvantage of using the techniques described in those studies is that each antenna was the same size, and operated at the same power level as a single antenna. Therefore, the invasiveness of the multiple-antenna groups was three times that of each single-antenna ablation. While using multiple antennas was more efficient than overlapping sequential single ablations, techniques are needed that increase the size of the ablation zone while maintaining (or decreasing) the invasiveness of the procedure. For example, when compared with the results of Hines-Peralta et al.,3
Brace et al.7
achieved similar volumes of ablation by delivering similar input power through three smaller antennas (versus a single larger one) while reducing the cross-sectional area of the tissue punctured (5.3 mm2
versus 25.5 mm2
Uniformly distributing power is not the only technique by which the size of the ablation zone can be increased without a corresponding increase in invasiveness. Switching between multiple antennas delivers more power to the target area and creates larger ablation zones when compared with a single larger antenna.15
Constructively phasing arrays of antennas can also be used to dramatically increase heating in certain areas of tissue (by N2
, where N
is the number of antennas).16–18
It is likely that further increases in the efficiency of energy delivery would be achieved by combining one or more of these techniques.
This study was limited by the use of an ex vivo model. Perfusion has well documented detrimental effects on thermal ablation.12,19
However, given that the periphery of the ablation zone is the most vulnerable to perfusion-mediated cooling, it is likely that the benefit achieved by uniformly distributing power will translate to an in vivo
environment. Percutaneous placement of several antennas in the optimal spatial configuration may be difficult to achieve in certain cases due to limited access. However, it is not anticipated that this will be a problem given the extensive experience using other multiple-applicator systems in widespread clinical use today (eg, cryoablation, RF ablation and laser ablation).20–22