Quasistatic zooming for regional hyperthermia treatment planning

Due to current computer limitations, specific absorption rate (SAR) distrib utions in regional hyperthermia treatment planning (HTP) are limited to cen timetre resolution. However, since patient anatomy is highly structured on a millimetre scale, millimetre-resolution SAR modelling is required. A meth od called quasistatic zooming has been developed to obtain a high-resolutio n SAR distribution within a volume of interest (VOI): using the low-resolut ion E-field distribution and the high-resolution patient anatomy, the high- resolution SAR distribution is computed within a small zoom volume Q (small compared with the wavelength in water (lambda (W))). Repeating this proced ure yields the zoomed-resolution SAR distribution in an arbitrary VOI. To v alidate this method for a VOI that is not small compared with lambda (W), h igh-resolution finite-difference time-domain (FDTD) modelling is needed. Si nce this is impractical for a clinical applicator, a computer model of a sm all applicator has been created. A partial patient anatomy is inserted into the applicator and both high- and low-resolution SAR distributions are com puted for this geometry. For the same geometry, zoomed-resolution SAR distr ibutions are computed with different sizes of Q. To compare the low- and zo omed-resolution SAR distributions with the high-resolution one, the correla tion and averaged absolute difference are computed. These numbers are impro ved considerably using zooming (correlation 58% to 92%; averaged absolute d ifference 43% to 20%). These results appear to be independent of the size o f Q, up to 0.3 lambda (W). Quasistatic zooming is a valuable tool in high-r esolution regional HTP.