High‐resolution regional hyperthermia treatment planning

Abstract

The major difficulties in electromagnetic regional hyperthermia are the occurrence of systematic stress and local high temperatures (hot spots). This thesis describes the development of a planning system to get more insight in hyperthermia treatments. The system is based on a finite‐difference time‐domain computer model to calculate the specific absorption rate (SAR) distribution within a patient. Segmenting a CT data set into fat, muscle, and bone and using literature‐based values of the dielectric properties results in the required dielectric patient model. Because the SAR computation on CT‐resolution (1×1×5 would require 310 days, this high‐resolution is down‐scaled to a lower resolution of 1 (computation time 2.8 hours). The use of the HTP system reveals the close relation between SAR distribution and patient anatomy, stressing the need for high‐resolution computations. For this purpose a method called quasistatic zooming has been developed: from the low‐resolution E‐field distribution the high‐resolution potential distribution at the surface of a small subvolume is calculated. With this potential distribution and the high‐resolution patient geometry, the high‐resolution SAR distribution is computed using a quasistatic SAR model. Repeating this procedure yields the high‐resolution SAR distribution for an arbitrary volume of interest. With this method it takes 4.3 days to compute CT‐resolution SAR distributions. SAR maxima that are predicted by both quasistatic zooming and high‐resolution SAR modeling are not predicted by low‐resolution computations and vice versa. Hence, zooming is required for reliable treatment planning. Whether SAR maxima lead to hot spots depends on local blood flow (discrete vasculature, local perfusion) and will be investigated in a succeeding project.