To overcome the limits of conventional microwave ablation, a new frequency spectrum above 6 GHz has been explored for low-power and low collateral damage ablation procedure. A planar coaxial probe-based applicator, suitable for easy insertion into the human body, was developed for our study to cover a wideband frequency up to 30 GHz. Thermal ablations with small input power (1-3 W) at various microwave frequencies were performed on nude mice xenografted with human breast cancer. Comparative study of ablation efficiencies revealed that 18-GHz microwave results in the largest difference in the temperature rise between cancer and normal tissues as well as the highest ablation efficiency, reaching 20 times that of 2 GHz. Thermal profile study on the composite region of cancer and fat also showed significantly reduced collateral damage using 18 GHz. Application of low-power (1 W) 18-GHz microwave on the nude mice xenografted with human breast cancer cells resulted in recurrence-free treatment. The proposed microwave ablation method can be a very effective process to treat small-sized tumor with minimized invasiveness and collateral damages.Breast cancer is a major threat to women's health psychologically as well as physically. 1 Despite the high potential of fatality, most women show reluctance for large-scale breast removal due to cosmetic concerns, which brings about the need for local surgical control. For this reason, minimally invasive techniques to achieve better local surgical control have collected extensive attention as a possible first-line treatment in lieu of conservative breast surgery procedures such as lumpectomy. 2,3 Among several alternative methods, microwave ablation entails many benefits in comparison with the others such as laser ablation, cryoablation and ethanol ablation. [4][5][6][7][8] Microwave ablation method is much safer and easily manageable 9 and can, in principle, offer material-specific responsiveness, where tissues with high water content such as cancer are preferentially heated and damaged. 10,11 This characteristic of microwaves makes microwave ablation well suited for local treatment of early stage of breast cancer. So far, the frequencies used in the existing microwave ablation systems have been limited to the low-frequency spectrum such as 915 MHz or 2.4 GHz. Because of the inherent low radiation efficiency, very high microwave input power of several tens of Watts is required at these frequencies, thus greatly increasing the cost and size of the equipment. 12 Moreover, because of the self-heating of the applicator by the excessive microwave power, additional means for cooling the applicator are required to avoid the damage of healthy tissues near the passage of the applicator. 13 Besides, the conventional low-frequency spectrum ablation has poor material selectivity when compared to the alternative high-frequency spectrum proposed in our work.To overcome the limitations of the existing microwave ablation methods, we have investigated the properties of microwaves up to 30 GHz wit...

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