Antenna Development for Radio Frequency Hyperthermia Applications

This thesis deals with the design steps, development and validation of an applicator for radio frequency hyperthermia cancer therapy. An applicator design to enhance targeted energy coupling is a key enabler for preferential temperature increments in tumour regions. A single-element, near-field approach requires a miniaturised solution that addresses ergonomic needs and is tolerant to patient anatomy. The antenna near-field modality and the high-dielectric patient loading introduce significant analytical and computational resource challenges. The antenna input impedance has to be sufficiently insensitive to in-band resonant detuning and the fields in the tissue can be targeted to selected areas in the patient.

An introduction to the medical and biological background of hyperthermia is presented. The design requirements of antennas for medical and in particular for hyperthermia applications are highlighted. Starting from a conventional circular patch, the antenna evolved into a compact circular patch with a concentric annular ring and slotted groundplane, operating at the 434MHz Industrial Scientific and Medical frequency band. Feed point location is optimized for an energy deposition pattern aligned with the antenna centre.

The applicator is assessed with other published approaches and clinically used loop, dipole and square patch antennas. The antennas are evaluated for the unloaded condition and when loaded with a tri-layer body tissue numerical model. This model comprises skin, fat and transverse fiber of muscle of variable thicknesses to account for different body locations and patient anatomy. A waterbolus containing de-ionized water is added at the skin interface for superficial tissue cooling and antenna matching. The proposed applicator achieves a penetration depth that supersedes other approaches while remaining compact and an ergonomic fit to tumour areas on the body.

To consider the inner and peripheral complex shapes of human bodies, the full human body numerical model developed by Remcom is used. This model was segmented from 1mm step computed tomography (CT) and magnetic resonance imaging (MRI) cross-sections through and adult male and it comprises twenty-three tissue types with thermal and frequency-dependent dielectric properties. The applicator performance is evaluated at three anatomical body areas of the model to assess its suitability for treatment of tumours at different locations. These three anatomical regions present different aperture coupling and tissue composition. Different conformal waterbolus and air gap thickness values are evaluated.
The models used in this work are validated with measurements performed in a phantom containing a lossy liquid with dielectric properties representative of homogeneous human body tissue. The dosimetric assessment system (DASY) is used to evaluate the specific absorption rate (SAR) generated for the antenna into the liquid. The measurement setup with the antenna, phantom and liquid are simulated. Simulated and measured results in terms of specific absorption rate and return loss are evaluated.