Quasistatic zooming for regional hyperthermia treatmentplanning

Abstract

Due to current computer limitations, specific absorption rate (SAR)distributions in regional hyperthermia treatment planning (HTP) are limited tocentimetre resolution. However, since patient anatomy is highly structuredon a millimetre scale, millimetre-resolution SAR modelling is required. Amethod called quasistatic zooming has been developed to obtain ahigh-resolution SAR distribution within a volume of interest (VOI): using thelow-resolution E⃗-field distribution and the high-resolution patientanatomy, the high-resolution SAR distribution is computed within a small zoomvolume Q (small compared with the wavelength in water(λw)). Repeating this procedure yields thezoomed-resolution SAR distribution in an arbitrary VOI. To validate thismethod for a VOI that is not small compared with λw,high-resolution finite-difference time-domain (FDTD) modelling is needed.Since this is impractical for a clinical applicator, a computer model of asmall applicator has been created. A partial patient anatomy is inserted intothe applicator and both high- and low-resolution SAR distributions arecomputed for this geometry. For the same geometry, zoomed-resolution SARdistributions are computed with different sizes of Q. To compare the low-and zoomed-resolution SAR distributions with the high-resolution one, thecorrelation and averaged absolute difference are computed. These numbers areimproved considerably using zooming (correlation 58% to 92%; averagedabsolute difference 43% to 20%). These results appear to beindependent of the size of Q, up to 0.3λw. Quasistaticzooming is a valuable tool in high-resolution regional HTP.