Hyperthermia properties of fiducial markers used in Image-Guided Radiation Therapy at Low Magnetic Fields

Current treatment strategies for several oncological pathologies are based on the combination of different techniques. Surgery, radiotherapy and chemotherapy are nowadays used synergistically because of their greater effectiveness than individual approaches. In radiotherapy, the most advanced delivery techniques allow high conformal radiation dose distributions to the tumor target with good sparing of the surrounding healthy tissues. Prerequisite for the success of these techniques is the correct visualization of the targets immediately before or during irradiation. This aspect is even more important in the case of treatment of moving organs. The most used imaging techniques in the so-called Image-Guided Radiation Therapy (IGRT) are those based on X-rays. In such case, radiopaque metallic fiducial markers implanted in the lesions, or in their close proximity, are easily visible by X-ray imaging, allowing the optimal patient set up and immobilization. Special methods of tracking of the lesion during the treatment, as in the case of Cyberknife, are also possible thanks to gold fiducial markers. Meanwhile, Fluid Magnetic Hyperthermia (MFH) is gaining importance due to the recent successful results in various clinical trials. In MFH technique nanomaterials, normally magnetic, injected directly on the tumor, under the action of an externally applied alternating current (AC) field, HAC, can generate a certain amount of heat proportional to the frequency f of that field.2 The aim of this work is to investigate the hyperthermia properties of the fiducial markers employed in IGRT in order to find out possible therapeutic effects, currently unexplored, and to study the physics mechanisms of the observed macroscopic effects. In the framework of this research project, hydrogel matrix samples of different concentration, shape and size are prepared and used as tissue mimicking materials. Typical gold fiducial markers, (cylinders 3 mm long and 1.2 mm in diameter) are dispersed in the gel samples, using various geometric configurations simulating the actual position of the markers in the clinical practice. Hyperthermic properties of gold fiducial markers are studied varying discontinuously the field HAC from 7 to 20 kA/m and the frequency from 100 to 1000 kHz. During the stimulation, an optical fibre based sensor enables the measurement of the temperature in various points of the hydrogel samples. Beside these temperature measurements, a high resolution thermal camera (FLIR A065, 640x520 pixel, 0.05 K thermal resolution) is used to map temperature distribution over the samples with a spatial resolution of 5 pixel/mm. The low emissivity of the gold material can be overcome by measuring the temperature distribution over the neighboring gel areas, being composed mostly by water, assuming an emissivity of 0.95. Preliminary results attesting an increase of the temperature in the gel matrices as effect of the heating of the gold fiducial markers under a suitable magnetic stimulation are presented. Such evidences pave the way to extend the study to other types of fiducial markers and tissue samples, in order to interpret and model the observed phenomena.