Purpose: Combining hyperthermia and brachytherapy functionalities into one seed has several advantages over the existing approach of delivering the two modalities through separate implants, in particular, decreasing associated patient’s trauma. The addition of heat to radiation enhances direct cell destruction, prevents DNA repair through multiple pathways, and enhances tissue radiosensitivity. Furthermore, hyperthermia can reduce the immune-suppressed state of the tumors and increase immunotherapy effectiveness. The purpose of this study is to identify optimal hyperthermia-only (HT) and thermo-brachytherapy seed configurations that generate the most heating power.
Methods: To find the power generated by the seed, its magnetic properties had to be measured. The permeability of ferrite material was measured in an RL circuit using thin wire winding around the core prototype. Subsequently, power was calculated by using the power versus temperature model. Finally, to ensure that the optimized seed can provide the desired temperature distribution throughout the tissue, we modeled seed arrangement in realistic prostate tissue in COMSOL Multiphysics package for various blood perfusion rates.
Results: We identified soft ferrite as the best candidate for the core material because of its high permeability, high Curie temperature (52℃), and sharp Curie transition. Demagnetization significantly decreased the relative permeability from its intrinsic value of about 5000 to 10. Power generation sharply dropped as shell thickness deviated from its optimal value, especially for the HT-only seeds. Gold was chosen for the shell because of its biocompatibility and high electrical conductivity, providing 30% more power than titanium. The resulting temperature distribution in the simulation has >90% volume coverage.
Conclusion: Optimizing the HT seed properties can significantly increase heat generation. Our simulations show a sharp drop in magnetic permeability of the ferrite core at Curie temperature, which is essential for temperature regulation. The blood perfusion rate is non-homogenous throughout the prostate, affecting the temperature distribution.