915 MHz interstitial hyperthermia: Dosimetry from heating pattern reconstruction based on radiometric temperature measurements

Summary form only given. Using hyperthermia in conjunction with other therapeutic against human cancer continues to be a high level of interest. Most known methods are based on the use of electromagnetic energy to elevate the malignant tissue up to 42°C, generally using external non invasive techniques. In the case of deep or semi-deep sited tumors it appears very interesting to deliver the heating power directly inside the tumor. For this purpose we have design several thin antennas different in their active length inserted in the catheter already implanted for the Ir 192 brachytherapy. The main problem in this technique is the temperature control and moreover the dosimetry since implanting thermoprobes only provides temperature data limited to a small volume around the measuring point. We solved this problem using radiometric temperature measurement. One or several antennas, already used for heating, are used as receivers : they pick up the noise power spontaneously emitted by the tissues around them. This power is directly related to the temperature and is measured by means of two radiometers centered on 3 and 9 GHz. These measurements give an "in volume" temperature information. Every elementary subvolume in the tissue volume coupled to the antenna contributes to the radiometric temperature value. In order to take advantage of the measurements we had to completely characterize our antennas by the study of the volume coupled with the antenna at the heating frequency as at the radiometric frequencies. Results of this study are then used in conjunction with the bioheat equation, to reconstruct heating patterns in cross section planes normal to the antennas. By means of radiative transfer a 3 GHz and 9 GHz radiometric temperature are computed. These values are compared to the measured one and allows the optimisation of the blood exchange coefficient appearing in the bioheat equation. All the steps of computation are used in a complete dosimetry software. Res- - ults are displayed as temperature evolution versus time (radiometric or at a done point of the heated volume) or as heating pattern in a cross section plane. Efficiency of the reconstruction is proved through a comparison between computed temperature evolution and measured one . Experiments have been done, first on polyacrylamid gel secondly on anesthetized animals, finally in PHASE I and II clinical trials. Calculations are made on a desktop computer in a CPU time as short as 2 min so it is possible to display in real time during hyperthermia session, or to reconstruct heating pattern from memorized data making a-posteriori dosimetry.