Finite element method aided design of a beam shaping hyperthermia microwave applicator

Phase 3 clinical trials have demonstrated the efficacy of hyperthermia as an adjuvant to radiotherapy for the treatment of small superficial tumours. Adequate heating of larger tumours however remains a technically challenging problem due to the inhomogeneous nature of tissue properties in large tumours. A dynamic bolus attachment for external single-element waveguide heating applicators is being developed in order to improve heat delivery to large tumours. The prototype of the bolus consists of an ellipsoid shaped, saline-filled patch, centrally located, and a planar array of 25 smaller saline-fixed patches. The central patch is designed to flatten out the Gaussian beam profile produced by typical waveguide applicators. The small patches are designed to allow for spatial and temporal control of the heating pattern. This is accomplished by changing the thickness of each patch by adding or removing saline. A three-dimensional finite element model of microwave propagation through arbitrary structures has been developed to predict the heating patterns resulting from a bolus design. The model also includes heat transfer calculations of steady-state temperature profiles. Simulations of several patch configurations placed between a BSD MA120 waveguide applicator and layers of skin, fat and muscle tissue have allowed suggested improvements to the initial design by increasing the salinity of the small patches. Future simulations with the model will allow for further improvements to the bolus design. The model is also designed to allow us to study other factors in hyperthermia treatment, such as the effect of nearby bone and fat tissue on heating patterns.