Design and characterisation of miniaturised cavity-backed patch antenna for microwave hyperthermia

Purpose: The aim of this study was to describe the design and characterisation of a miniaturised 434MHz patch antenna enclosed in a metal cavity for microwave hyperthermia treatment of cancer. Materials and methods: Electromagnetic (EM) field distribution in the near field of a microstrip patch irradiating body tissue was studied using finite element method (FEM) simulations. Antenna miniaturisation was achieved through dielectric loading with very high permittivity, metal enclosure, patch folding and shorting post. Frequency dependent electrical properties of materials were incorporated wherever appropriate using dispersion model and measurements. Antenna return loss and specific absorption rate (SAR) at 434MHz were measured on muscle phantoms for characterisation. Results: The design was progressively optimised to yield a compact 434MHz patch (22mmx8.8mmx10mm) inside a metal cavity (40mmx12mm) with integrated coupling water bolus (35mm). The fabricated antenna with integrated water bolus was self resonant at 434MHz without load, and has better than -10dB return loss (S-11) with 13-20MHz bandwidth on two different phantoms. SAR at 434MHz measured using an infrared (IR) thermal camera on split phantoms indicated penetration depth for -3dB SAR as 8.25mm compared to 8.87mm for simulation. The simulated and measured SAR coverage along phantom depth was 3.09cm(2) and 3.21cm(2) respectively at -3dB, and 6.42cm(2) and 9.07cm(2) respectively at -6dB. SAR full width at half maximum (FWHM) at 5mm and 20mm depths were 54.68mm and 51.18mm respectively in simulation, and 49.47mm and 43.75mm respectively in experiments. Performance comparison of the cavity-backed patch indicates more than 89% co-polarisation and higher directivity which resulted in deeper penetration compared to the patch applicators of similar or larger size proposed for hyperthermia treatment of cancer. Conclusion: The fabricated cavity-backed applicator is self-resonant at 434MHz with a negligible shift in resonance when coupled to different phantoms, f/f(0) less than 1.16%. IR thermography-based SAR measurements indicated that the -3dB SAR of the cavity-backed aperture antenna covered the radiating aperture surface at 5mm and 20mm depths. It can be concluded that the compact cavity-backed patch antenna has stable resonance, higher directivity and low cross polarisation, and is suitable for design of microwave hyperthermia array applicators with adjustable heating pattern for superficial and/or deep tissue heating.