Microwave heating patterns in simple tissue models

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Abstract
The effects of microwave radiation on 2 spherical models of differing size are compared and discussed in relation to biological experiments and to possible therapeutic applications. [Application of hyperthermia to cancer therapy is discussed.] For the spherical model with dimensions representative of a mouse, plane wave irradiation at the standard microwave diathermy frequency (2.45 GHz) produces a heating potential distribution with an internal maximum. The common procedure of monitoring body temperature at a single site (rectum, intraperitoneally, etc.) in small rodents subjected to whole body microwave irradiation at or close to this frequency is therefore questionable. For a sphere which could be representative of a larger animal or part of a human torso it has been shown that plane wave irradiation at 2.45 GHz produces a heating potential which has its maximum at the front surface and which decays approximately exponentially towards the center of the sphere with a penetration depth (the distance required for the heating potential to be reduced by approximately 87%) of 17 mm. If a therapeutic application requires human tissue to be heated at depth, a heating potential distribution such as that produced by plane wave irradiation at 2.45 GHz is disadvantageous since skin and subcutaneous tissues will receive more thermal energy than the region of interest. The use of microwaves of lower frequency and with a greater penetration depth is necessary. The model described is an extremely simple one in that the target is assumed to consist of an homogeneous, spherical core surrounded by homogeneous spherical shells. Actual temperature increases in such a model depend not only on the heating potential at a given point, but also on the specific heat of the tissue at that point and upon the distribution and degree of cooling throughout the model.

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