Evaluation of calculated temperature distributions for a 27 MHz ridged waveguide used in localized deep hyperthermia

The computational model developed by van den Berg has been used to perform two-dimensional simulations of tissue-equivalent phantoms heated by a 27 MHz ridged waveguide. The program is able to calculate temperature distributions for realistic inhomogenous tissue configurations (as derived from CT scans) in about 20 min on a PDP 11-44. Results for tissue-equivalent phantoms are evaluated with respect to their accuracy in predicting actual temperature distributions by comparing them with measured temperature distributions. In general the computer simulation tends to overestimate the depth of penetration; this can be ascribed to the two dimensional approach followed which does not take into account the divergence of the electric field in the third dimension. This effect can to a certain degree be compensated by assigning a higher value for the electrical conductivity of the medium. For homogenous phantoms this results in a satisfactory agreement between measured and calculated temperature distributions. Furthermore, a correct approximation for the electric field distribution over the aperture appears to be of importance, since it significantly influences the calculation results, especially for waveguide loads whose size exceeds the width of the ridge area. For inhomogeneous media the simulation results, using realistic (literature) values for the electrical conductivity, can be used with these limitations in mind, as a best-case estimate of the absorbed power distribution at depth. For more accurate simulations a three-dimensional computer model is ultimately needed.