A 3-D SAR model for current source interstitial hyperthermia

Abstract

A three-dimensional (3-D) model is presented for the calculation of the specific absorption rate (SAR) in human tissue during current source interstitial hyperthermia. The model is capable of millimeter resolution and can cope with irregular implants in heterogeneous tissue. The SAR distribution is calculated from the electrical potential. The potential distribution is determined by the dielectric properties of the tissue and by the electrode configuration. The dielectric properties and the current injection of the electrodes are represented on a 3-D uniform grid. The calculated potential at an electrode current injection point is not the actual electrode potential at that point. To estimate this potential a grid independent representation of an electrode together with an analytical solution in the neighborhood of the electrode are used. The calculated potential on the electrode surface is used to estimate the electrode impedance. The tissue implementation is validated by comparing calculated distributions with analytical solutions. The electrode implementation is verified by comparing different discretizations of an electrode configuration and by comparing numerically calculated electrode impedances with analytically calculated impedances.

Planning, optimisation and evaluation of hyperthermia treatments

“…Currently, simulations are performed on fixed resolution meshes. When necessary, relatively thin or small structures compared with the voxel size can be modelled accurately by implementing additional correction strategies similar to the correction for the small diameter of current source electrodes modelled in quasar [35,[69][70][71]. Implementation of odd-shaped or irregular structures can require more sophisticated techniques.…”

“…The quasi-static implementation of Maxwell's equations in quasar has been validated by comparing simulations and analytical solutions for absorbed power in tissue [35]. The implementation of the current source electrodes has been validated by comparing different discretizations of an electrode configuration, as well as by comparing numerically estimated electrode impedances with analytically calculated impedances [35].…”

“…In case of interstitial heating with local current field electrodes, the diameter of the electrodes is typically quite small compared to the spacing of the simulation grid. A correction method using an analytical solution was implemented in quasar to correct the calculated power distribution for the size of the electrode [35]. This feature allows adequate modelling of irregular implants; electrodes do not need to be aligned with the rectangular grid.…”

“…Validation of treatment planning software is very important to guarantee reliable results of simulations and therefore extensive validation studies have been performed for this software package [35,[49][50][51][52]. The quasi-static implementation of Maxwell's equations in quasar has been validated by comparing simulations and analytical solutions for absorbed power in tissue [35].…”

Magnetic resonance imaging in radiotherapy treatment planning

“…Lulu et al 1990;Weaver et al 1990;Warszawski et al 1992;Schupak et al 1995. At our department, in collaboration with the Daniel den Hoed Kliniek, Rotterdam and the Academisch Medisch Centrum, Amsterdam, a new MRI based treatment planning system for the treatment of brain tumours with interstitial hyperthermia is being developed Kotte et al 1996a;De Bree et al 1996;Lagendijk et al 1995;Van der Koijk et al 1996. This sytem uses vascular information to optimize implants.…”