Contrast of dose distribution in phantom heads due to aperture and plane wave sources

Current microwave biologic effects research uses both aperture and plane wave sources. In this theoretic investigation, the dose rate (time rate of energy absorption per unit mass) patterns are compared between phantom heads irradiated by both microwave sources. Two brain tissue-equivalent spheres with radii of 3.3 and 7 cm are used to simulate monkey and human heads, respectively. In addition, a five-layered tissue-equivalent sphere is employed to simulate more closely the various tissues in a monkey head. Theoretic formulations of dose rate patterns in multi-layered tissue-equivalent spheres due to separate plane wave and aperture source treatments are derived from the summation of spherical harmonics technique. Calculations are made for the dose rate patterns along two cross-sectional planes and three rectangular axes in the spheres. The results of these calculations indicate variations in dose rate patterns for different sources and phantom head sizes. For aperture irradiation, microwave energy penetration into the phantom heads appears weak compared to the "hot spots" prominent in the plane wave exposure situation. It is concluded that for different radiation sources, direct comparison of biologic results by external field measurements as the only common denominator may not be dosimetrically valid. The results also indicate that for the same measured exposure rate (power density), the microwave energy absorption pattern in a human head may vary according to the type of radiation source.