Focusing microwave hyperthermia in realistic environment for breast cancer treatment

Abstract

Microwave hyperthermia, a process that involves heating tumour cells, has been proposed as an alternative method of treating breast cancer. By sending microwave energy of a suitable frequency range to the breast, localised heating is induced by the applied electromagnetic radiation. It is important that the tumour is heated to a temperature greater than 42℃, (the required temperature for hyperthermia treatment) while keeping healthy tissue at normal body temperature. Several useful approaches have been proposed to direct microwave hyperthermia to cancerous tissue, including novel focusing methods and innovative microwave hardware designs. However, a number of challenges restricting the use of microwave systems in clinical medicine remain. Arguably, the most persisting engineering problem to overcome is achieving locally focused microwave power in the heterogeneous environment of the breast. In order to understand the challenges of microwave hyperthermia, considerably large computational efforts are required to solve 3D Electromagnetic (EM) problems in a complex model of breast tissue. The success of using microwave hyperthermia to treat breast cancer is highly dependent on the accuracy of the excitation signals utilised for each antenna element. Several research papers have reported various approaches on this topic. Although recent progress has been made in the investigation of microwave hyperthermia, the studies were mainly reliant upon oversimplified Radio Frequency (RF) models which assume point source applicators, and human breast phantoms which are mostly 2D models lacking tissue thermal-electric properties. As a result, it is infeasible to implement the proposed systems for clinical applications. This thesis aims to address the limitations of these earlier studies by modelling and constructing realistic human breast models and 3D antenna arrays. This will require the determination of correct phases and amplitudes for the excitation signals in a realistic environment by using a global optimisation method. The approaches proposed use actual antenna arrays to transmit microwave power while the excitation signals are optimised based on power distributions and thermal profiles induced in the patient-specific breast models. The first study based on 2D antenna arrays and patient-specific breast models is demonstrated in Chapter 3. In this study, microwave hyperthermia is performed in a realistic environment rather than over-simplified scenarios. In the recommended technique, electromagnetic focusing on patient-specific breast models concentrates the power at the tumour position while successfully keeping the power levels at other positions (healthy tissue) at minimum values. Several patient-specific breast models ranging from fatty tissue to highly dense tissue are used to investigate the effect of breast anatomy on the proposed focusing technique. Chapter 4 introduces an improved hyperthermia approach for breast cancer treatment, overcoming the limitations of previously analysed techniques. In this approach, microwave hyperthermia is conducted in a 3D environment which allows targeting microwave power to tumours located at various locations by using a fixed antenna array. The capability of using 3D focusing microwave hyperthermia for treating the breast is extensively studied and confirmed. The technique suggested in this chapter uses a 3D antenna array with the excitations (amplitude and phase) optimised using the global Particle Swarm Optimisation method. To implement the technique, a link between Matlab and the full-wave electromagnetic simulator (CST Microwave Studio) is conducted. This unique technique enables the possibility of finding optimum excitations for antenna elements in a complicated 3D medium. The results show the possibility of focusing hyperthermia treatment on the exact volume of tumours situated at different on-axis and off-axis locations within a very dense breast, including the challenging case of a tumour embedded in a gland, while preventing any hot spots in healthy tissue. In order to validate the proposed focusing technique presented in Chapter 4, the 3D focusing technique for non-invasive microwave hyperthermia treatment of breast cancer is experimentally confirmed and reported in Chapter 5. The experimental system employs a 3D array of corrugated tapered slot antennas operating at a frequency band of 4 GHz to focus microwave power to a target located inside a breast model embedded in a coupling/cooling medium. Prior to implementing the system, Chapter 5 introduces a methodology to fabricate a thermo-dielectric breast model. The exclusive feature of the model is that it reflects both thermal and dielectric properties of each tissue. To fabricate the phantom materials, mixtures of low-cost and stable materials are utilised to meet the tissues’ dielectric (permittivity and conductivity) and thermal (specific heat capacity and thermal conductivity) properties across the frequency band 3-5 GHz, which is suitable for breast hyperthermia. A 3D printer is used to create accurate moulds ensuring the fabricated breast phantom is anatomically correct. Further validation is achieved by confirming the capability of locally heating a 1 cm3 tumour located in a very dense thermo-dielectric breast phantom. The similarity between the measured and simulated results observed at different heating times confirms that the effectiveness of the proposed 3D microwave focusing technique is successful.