Theoretical characterization of dual concentric conductor microwave applicators for hyperthermia at 433 MHz.

Radiation patterns from multi-aperture arrays of Dual Concentric Conductor (DCC) applicators for superficial microwave hyperthermia were calculated numerically using the Finite Difference Time Domain (FDTD) method at an operating frequency of 433 MHz. Different aperture sizes were studied from 3.5-7 cm square in six-element array configurations which may be considered as building block elements of much larger arrays. Once optimum parameters were established for gap width, inter-element spacing, and bolus thickness for each aperture size using repetitive parametric FDTD studies, a direct comparison of power deposition (SAR) patterns was performed for similar DCC arrays driven at 915 MHz and 433 MHz. Results show that clinically desirable uniformity and penetration of SAR can be obtained for square apertures in the range of 4-6 cm per side at 433 MHz, using 9.5-12.5 mm thick water coupling boluses, as compared to the optimum 2-4.5 cm square apertures and 5-10 mm water bolus dimensions determined previously for 915 MHz arrays. With an accompanying reduction in the lateral adjustability of power control, the 433 MHz applicators can provide a significantly larger heating area than arrays with the same number of elements driven at a frequency of 915 MHz. Penetration depth is similar for optimally sized apertures at both frequencies of operation, providing a maximum heating depth of 1-2 cm for the DCC array applicators.