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ABSTRACT The use of tumor-specific resonances is investigated for more efficient microwave hyperthermia of breast cancer. This concept is based on the idea that an increased power coupling (or absorption) would take place at the resonance. For hyperthermia treatment, higher power absorption means faster temperature rise to a desired therapeutic level. The natural resonances would vary depending on the tumor size, shape, and breast tissue configuration. Therefore, by adaptively tuning the source frequency of an applicator corresponding to the tumor-specific natural resonance, it could improve the overall efficiency of hyperthermia treatment. In this article, we demonstrate this concept by applying the singularity-expansion method to estimate the natural resonances (poles) and using the frequency of the dominant pole as the tumor-specific frequency. The performance is evaluated via the power absorption density in the breast. Results from numerical simulations and a simple experiment shows that the optimal power absorption in the tumor takes place at the tumor-specific resonance frequency in comparison to other frequencies. Using a multielement array setup for focused microwave, we also demonstrate that the relative phase shift between the residues (of the poles) obtained at different array elements can be used for phasing the array to create a focus at the tumor. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2659-2665, 2013 [ABSTRACT FROM AUTHOR]