Physical basis of RF hyperthermia for cancer therapy (2). Measurement of distribution of absorbed power from radiofrequency exposure in agar phantom.

Abstract

Distribution of the absorbed RF power in a phantom was measured by scanning the thermocouple immediately after RF-irradiation of 13.56 MHz; a phantom used was composed of 2.0 W/W % agar, 0.5 W/W % NaCI and 97.5 W/W % distilled water and equivalent to high water content tissue both in the electric conductivity and the dielectric constant. Absorbed RF power at the midpoint on the central axis of the phantom decreased exponentially in accordance with increasing in its thickness. And the decrease at the midpoint was more remarkable at the RF electrodes got smaller. Relationship between relative absorbed RF power at the midpoint to that at 1 cm in the depth and relative thickness of the phantom to the diameter of electrodes was shown as a sort of exponential function regardless the diameter of electrodes. The effective heating diameter at the midpoint was equal to the diameter of electrode if the thickness of phantom was smaller than the diameter of electrode, while the effective heating diameter became larger than the diameter of electrode as the thickness of phantom became larger than the diameter of electrode.