Analysis and development of quality assurance procedures for radiative hyperthermia systems

This work reports the analysis and development of quality assurance (QA) protocols for systems currently in use in oncological radiative hyperthermia (HT). Radiative hyperthermia is a therapeutic technique used in synergy with radiotherapy and chemotherapy; it involves the use of antennas radiating an electromagnetic (EM) field in the patient’s body. Depending on tumour location and working frequency, HT can be classified in deep (60 – 150 MHz) and superficial (434 MHz; 915 MHz). The absorption of EM energy results in an increase in temperature within the treated tissue: the aim of hyperthermia is to induce a temperature of about 40–44 °C within the tumour and maintain it for about 1 hour. Clinical studies demonstrated that this thermal dose enhances the efficacy of other cancer treatments, thereby reducing the required doses. Quality assurance procedures are crucial for ensuring the proper validation and performance assessment of hyperthermia systems. After the Introduction to the topic, this thesis presents an in-depth analysis of the fundamental aspects of current QA guidelines, highlighting key characteristics, limitations and potential improvements. The study examined aspects related to the use of phantoms, as the role of the dielectric and thermal properties, and the influence of the thermometry systems used in the QA procedures. The analysis also focused on the characterisation of the EM-thermal phenomenon in QA procedures, to investigate the link between electromagnetic and thermal dose parameters and evaluate the feasibility of characterising the device’s performances through electromagnetic measurements. The study was conducted through the development of analytical models representing the interaction between electromagnetic fields and biological tissues, together with numerical simulations and experimental validation measurements. Given the direct relationship between electromagnetic and thermal dose parameters, the work investigated frameworks for measuring the electromagnetic field radiated by hyperthermia devices in tissue equivalent phantoms. In particular, an electric field sensor was developed and optimised for the frequencies (60–915 MHz) and power levels (up to 2 kW) typical of radiative hyperthermia. The corresponding calibration procedure was developed also. In particular, two calibration methods were developed: the temperature-based method, which relies on evaluating the electric field through temperature measurements, and the more accurate analytical-field method, which involves placing the sensor within a well-known EM field. A comparative analysis of the two methods was proposed, aimed at validating the temperature-based method for use in various contexts. The applicability of this calibration approach was therefore assessed in both an industrial and a clinical setting. In the latter, due to the lack of the needed instrumentation (a fibre optic probe) an alternative procedure was proposed and validated. The developed sensor was employed to establish experimental validation procedures for numerical models of hyperthermia applicators. Such models constitute the basis of treatment planning systems, which play a fundamental role in the clinical workflow by predicting the electromagnetic and thermal distributions generated during treatment. In this context, a comparative analysis of two simulation platforms for superficial hyperthermia was performed, while electric field measurements in tissue-equivalent phantoms were used to assess the accuracy of the corresponding numerical models. Overall, this work aims to contribute to the advancement of quality assurance methodologies for radiative hyperthermia by providing a comprehensive analysis of current QA protocols, identifying their key aspects and limitations, and proposing approaches for device characterisation and validation. The developed measurement techniques aimed at assessing the capability of hyperthermia systems to generate the expected electromagnetic field distribution and to maintain it throughout an entire therapy session. The outcomes of this work provide a basis for future developments in quality assurance protocols, contributing to ongoing efforts to improve the reliability and consistency of hyperthermia treatments. The research was funded by Regione Lazio in collaboration with MedLogix S.r.l. (Rome, Italy). Part of the work was carried out during a three-months period of research visit at the Division of Medical Radiation Physics and Department of Radiation Oncology at the Inselspital University Hospital (Bern, CH).