Physical–technical basis of invasive microwave hyperthermia—Simulation, construction, and characterization of miniaturized interstitial microwave antennas for the treatment of malignant tumors (in German)

Abstract

The electromagnetic field of dipole antennas for interstitial microwave hyperthermia was investigated using the field evaluation program MAFIA (MAxwell's equations using the Finite Integration Algorithm). The numerical method was applied to clinically used applicators running at ISM frequencies: 434 MHz and 915 MHz. The method was also used for improvement and optimization of more sophisticated applicators, e.g., those in a triaxial technique having a λ/4 choke section to reduce RF currents on the outer coaxial conductor. Simulations of the frequency dependent impedance match, the electrical field and the specific absorption rate (SAR) distribution of different applicators immersed in a muscle phantom were compared with measurements. Moreover, arrays of two and four applicators, various surrounding tissue properties (bone, fat, tissue), catheter types, and semi‐rigid coaxial cable materials were investigated. Compared with coaxial antennas only triaxial antenna designs produce a homogeneous, symmetric energy deposition pattern below the choke section at all insertion depths. Only triaxial dipole antennas can be designed to run at a specific frequency while the reflection factor ( parameter) does not change with insertion depth. The field simulation allowed studying realistic effects at all discontinuities of the rotational symmetric applicator‐catheter‐tissue system and gave a better understanding of heating phenomena occurring during interstitial hyperthermia treatment.