Microwave antennas for thermal ablation of benign adrenal adenomas

Abstract

Microwave thermal ablation is under consideration as a minimally invasive modality to treat 10-20 mm benign adrenal adenomas, while preserving normally functioning adjacent adrenal tissue, and returning the gland to a normally functioning status that is under normal regulation. In contrast to applications for tumor ablation, where devices have been developed with the objective of maximizing the size of the ablation zone for treating large tumors, a challenge for adrenal ablation is to minimize thermal damage to non-targeted adrenal tissue and thereby preserve adrenal function. Here, we investigate methods for creating small spherical ablation zones of volumes in the range 0.5-4 cm 3 for the treatment of benign adrenal adenomas using water-loaded microwave monopole antennas operating at 2.45 GHz and 5.8 GHz. Coupled electromagnetic and bioheat transfer simulations and experiments in ex vivo tissue were employed to investigate the effect of frequency, applied power, ablation duration, and coolant temperature on the length and width of the ablation zone. Experimental results showed that small spherical ablation zones with diameters in the range of 7.4-17.6 mm can be obtained by adjusting the applied power and ablation duration. Multi-way ANOVA analysis of the experimentally-measured ablation zone dimensions demonstrated that frequency of operation and ablation duration are the primary parameters for controlling the ablation zone length and width, respectively. Additionally, it was demonstrated that the coolant temperature provides another effective parameter for controlling the ablation zone length without affecting the ablation zone width. Our study demonstrates the feasibility of creating small spherical microwave ablation zones suitable for targeting benign adrenal adenomas.

Millimeter-wave pulsed heating in vitro: cell mortality and heat shock response

“…Microwave (MW) frequencies have been exploited for invasive and non-invasive thermal treatments, mainly in the Industrial Scientific Medical (ISM) bands around 434 MHz, 915 MHz, 2.45 GHz, and 5.8 GHz 15–17 . The feasibility of using MW up to 18 GHz (e.g.…”

Experimental assessment of microwave ablation computational modeling with MR thermometry

“…Similar to state‐of‐art MWA computational models, 5,9 changes in tissue physical properties as a function of temperature change were incorporated within our model. Similar to prior modeling studies of water‐cooled microwave ablation applicators, 10,37 we employed a convective heat transfer boundary condition assuming a constant cooling temperature for the circulating water flowing inside the applicator (during ablation experiments, a 1 °C change in circulating water was measured, see Fig. .…”

Thermal ablation in adrenal disorders: a discussion of the technology, the clinical evidence and the future

“…Microwave systems typically provide better tissue penetration compared to RFA as they do not require paths of low electrical impedance [ 23 ]. They can also produce larger, more precise, spherical ablation zones within a shorter application time, using constant rather than pulsed energy delivery [ 24 ▪▪ , 25 ▪▪ , 26 ▪ ]. Multiple synergistic antennae can also be synchronously used to deliver focused energy to single large tumours or even to simultaneously target multiple tumour loci [ 27 ].…”

“…Generation and transmission of microwave energy have traditionally been difficult to achieve and must be carried in coaxial cables, which of wider gauge than that of the simple wires used in RFA applicators [ 28 ]. Cable heating presents a greater challenge for MWA, when compared to RFA and therefore these systems must be cooled, using water or gas, both of which add to the complexity and diameter of the applicator antennae [ 22 , 25 ▪▪ , 28 , 29 ▪▪ , 30 , 31 ]. It is due to these complexities that the advancement and broader use of MWA in the clinic have lagged behind RFA [ 32 ].…”