This paper assess initial collateral effects of the use of hyperthermia in breast cancer treatment by means of an electromagnetic simulation. The breast model was radiated by microwaves with a waveguide at 2.45 GHz and the power density distribution inside the breast was calculated. A comparison between the power density in the tumor and other breast tissues (skin, fat, and muscle) is presented. Results show that the location of the microwave applicator is a factor that determines the unwanted overheating of tissues close the tumor.