Implementation of Time Reversal Focusing on Hyperthermia Treatment of Brain Tumours

According to NBTS (National Brain Tumor Society), glioblastoma is the most occurring primary malignant brain tumor [1]. Current research shows the benefit of combining hyperthermia and radiotherapy for these brain tumors. The hyperthermia treatment planning (HTP) is used for treatment guidance. HTP is usually divided into three individual steps: segmentation, electromagnetic (EM) simulations, and phase-amplitude optimization. To find the best phase and amplitude of the antenna element input signals using either specific absorption rate (SAR)-based or temperature-based optimizations. The main advantage of SAR-base optimization is the speed and computational complexity, but these optimizations need to consider the cooling mechanisms of the human body and the water bolus. Currently, several methods for SAR-based optimization like particle swarm optimization (PSO), Nelder-Mead simplex algorithm (NMS), genetic algorithm (GA), or Time reversal focusing (TRF) have been introduced. Time reversal focusing consists of forward and reverse steps. First, the virtual EM source is placed in the tumor. According to [2], it is not a suitable source directly to the center; better results are achieved if the source is near the surface of the tumor. Next, a wavefront is propagated from a virtually located source, recorded by peripheral antenna elements. These signals are time reversed for detection amplitude and phase. In the second step, these values are set, and the SAR is calculated. Treatment was planned for an applicator with twelve antenna elements (six elements in one ring) operating at 434 MHz. The results of this study show the benefits of using TRF in the hyperthermic treatment of glioblastoma. Better results were achieved for small tumors with regular shapes. The total volume was not targeted for larger tumors but only the part where the virtual source was located. The advantage of this method is that there is no significant absorption of the SAR outside the tumor tissue, and therefore no hotspots should occur.