Numerical evaluation of microwave thermal ablation to treat small adrenocortical masses

Abstract

In this study, microwave thermal ablation is numerically evaluated in the context of a treatment for Conn's Syndrome. This condition is caused by a benign shallow tumor in the cortex of adrenal gland. The modeling and design of microwave applicator to deliver thermal ablation to the adrenal gland requires accurate tissue characterization. Measuring the dielectric properties of the constituent tissues in the adrenal gland, that is cortex and medulla, enables more accurate numerical modeling for electromagnetic and thermal simulations. This study presents an anatomically and dielectrically realistic numerical model of the adrenal gland, and investigates the feasibility of applying controlled heating to small targets in the adrenal cortex. In addition, the use of dielectric contrast between the fat and the cortex of the adrenal gland to focus the thermal energy in the gland has also been studied. Being conscious of limitations of numerical simulation of complex multi‐physics problems like the microwave ablative treatment, calculated results provide a preliminary description of the electromagnetic and thermal phenomena involved.

Microwave Ablation Antenna for Functional Adenomas in the Adrenal Gland

“…Two different antennas insertion depth (10 and 30 mm from the feed point) were also tested to verify the improved robustness of the triaxial antenna with respect to the coaxial one. Finally, the triaxial antenna was placed in the adrenal numerical model presented in [15], parallel to the interface between adrenal cortex and fat. The dielectric properties of liver and fat, as well the values related to heat capacity, thermal conductivity and density were manually loaded in the material settings of the CST MWS software from the ITIS database (ITIS Foundation, Zurich, CH thermal properties of the adrenal gland were obtained from the same source, while the dielectric properties were obtained from [18].…”

“…The triaxial antenna was adopted to conduct coupled EM-thermal simulation to investigate the effect of the target area heterogeneity on the resulting ablation zone. A single setting of 60 W -60 s was tested placing the antenna in liver and in an adrenal numerical model [15]. The adrenal model is proposed as generic representative of an heterogeneous scenario.…”

“…Second, directive antennas must be precisely oriented accordingly with the preferred direction: this requirement represents a complication of the clinical procedure and discourages clinicians from adopting this solution in the treatment of functional adenomas of the adrenal gland. Other studies have suggested the exploitation of the natural presence of the fat layer surrounding the adrenal gland to obtain directive pattern of ablation [15] - [17]. These studies illustrated how the interface between two tissue with high contrast in dielectric properties (e.g.…”

“…The drawbacks of this proposed solution are mainly two. First, the increase of the transversal dimension of the antenna and thus the increase of its invasiveness: in a target as small as the adrenal gland (about 15 cm 3 [15]) an increase of even less than 1 mm in diameter can be noticeable. Second, directive antennas must be precisely oriented accordingly with the preferred direction: this requirement represents a complication of the clinical procedure and discourages clinicians from adopting this solution in the treatment of functional adenomas of the adrenal gland.…”

“…In Section 2, we present the numerical and experimental methodology, providing the details about the antenna designing and prototyping. The adrenal gland model presented in [15] is exploited to perform numerical studies. The prototyped antenna is experimentally tested in ex vivo liver and muscle.…”