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1993 Astrid de Leeuw

The Coaxial TEM regional hyperthermia system

Astrid de Leeuw
Department of Radiotherapy, University Hospital Utrecht, The Netherlands
Thesis Advisors: J.J. Battermann and J.J.W. Lagendijk
Ph. D. awarded November 1993 by University Utrecht, The Netherlands

Summary

Hyperthermia, i.e. the elevation of tissue temperature, can be used for the treatment of malignant tumours. The combination of hyperthermia and radiotherapy is strongly indicated for certain deep seated tumours. This calls for systems which are capable of deep heating. Using electromagnetic Radio-Frequency techniques, radiative antennas are theoretically most optimal for this purpose. This automatically implies regional heating because focusing is limited at these frequencies.
In this thesis the design and clinical introduction of the Coaxial TEM regional hyperthermia system are described. This system is designed especially for application on pelvic tumours.

The introduction (chapter 1) briefly presents the biological rationale and the clinical application with an accent on deep hyperthermia as adjuvant to radiotherapy. A brief review is given of general hyperthermia technology, with emphasis on deep-body hyperthermia techniques. From the historical development of radiative types of regional deep-body hyperthermia systems we noticed that early experience was characterized by several clinical problems, most of which were system specific, rather than related to the fundamentel principle of circumferential radiative field. This sets the background of our project.

In Chapter 2 several aspects of the Coaxial TEM system have been described. This rather technical chapter includes characteristics of the applicator such as its basic principle, facets as impedance and tuning, but also a description of the patient support system. A special open waterbolus has been designed, which provides not only an optimal electromagnetic coupling and a good control of skin and body temperature but also easy patient positioning. Operational safety and feasibility of our system have been demonstrated by pre-clinical experiments on pigs. An important aspect of each hyperthermia treatment is thermometry, which in our case has been based on the use of thermocouples. Chapter 2 reports on both the thermometry system and on studies on RF disturbance. Besides obtaining information on temperature itself the fast temperature measurements possible with our system are also essential for in-vivo determination of the absorption of the electromagnetic energy in tissue.

Chapter 3 deals with techniques to determine specific absorption rate (SAR) in phantoms as well as in-vivo. Essentially two types of SAR determination can be distinguished; direct measurement of the electromagnetic field or measuring the temperature change due to the absorbed power density. The effect of tissue heterogenities on the SAR distribution was studied in phantoms which clearly showed the effect and importance of boundary conditions. For the direct E-field measurements an E-field antenna has been applied. A comparison was made between scans with a simple E-field antenna and a LED matrix; a transverse plane with multiple LED dipole antennas. This study revealed the restricted capability of the LED matrix for quantitative determination of SAR patterns. This technique is nevertheless very valuable for fast qualitative measurements.
Concerning the indirect method of SAR determination using the temperature change we concluded that this method, in combination with our multi-sensor thermocouple probes, is feasible for in-vivo SAR quantification. In-vivo measurements in patients illustrating this were given. A combination of SAR and temperature determination was used to investigate local tissue cooling.

Chapter 4 describes different investigations of applicator performance and characteristics. From experiments in a 50 cm diameter prototype of the applicator, it was concluded that the applicator produced the standard radiative SAR distribution in the aperture midplane. Unexpected was the remarkable effect of PVC layers on the axial field length. To control this effect so-called inner-mirrors were introduced in the clinical applicator.
The steering capability was investigated in the clinical system in cylindrical and elliptical homogeneous phantoms (section 4.2). Both theoretically and experimentally, the effect of phantom positioning on the SAR distribution was studied. Theoretical predictions indicated that the maximum of the SAR distribution was stationary around the central axis of the applicator system, irrespective of phantom position. The maximum SAR can therefore be located at any desired location within the phantom. Although these two- dimensional models give a qualitative insight in the phenomena studied, they lack quantitative agreement. The predicted steering capacity has been confirmed experimentally and is now applied clinically by positioning the tumour mass according to these insights.
In section 4.3 results of SAR measurements along the central axis of phantoms were presented. The influence of the waterbolus level was investigated. It was found that the SAR profile was characterized by a clear absolute maximum at the aperture midplane and a small secondary maximum at the top of the phantom. These two maxima interfere for certain waterbolus levels, causing a single broad, shifted absorption maximum.

Chapter 5 reports on the clinical feasibility study on the first 22 patients, treated between August 1989 and July 1992. Feasibility, acute toxicity and temperatures were evaluated, tumour response was no goal of that study. From the study we concluded that the treatment is feasible, but that a limiting factor was the maximum available power of the generator. Concerning local toxicity the low incidence of local pain is noticeable, while systemic stress was manifest but less treatment limiting than expected. The open waterbolus improved patient comfort and limited local pain. Whereas the relatively large field length in the open bolus contributed to high systemic temperatures, the strong cooling capacity assisted in the control of those temperatures. Although the temperatures achieved were not yet optimal, they were comparable with studies using other types of radiative systems.

In Chapter 6 a general discussion is given concerning optimization, not only for our system, but for radiative systems in general. Although steering of the SAR distribution should and can be optimized, all radiative techniques will be restricted to 'large volume heating'. This is not necessarily an impediment. The application of three dimensional computer models is an important tool in enhancing insight in and performance of the clinical systems. Progress in treatment planning is expected to improve the treatments.
In conclusion: our Coaxial TEM applicator turned out to be a clinically useful system. The quality of treatments is continuously being improved due to more clinical experience and increased insight in performance.

Email: A.A.C.deLeeuw[at]umcutrecht.nl