Room: AAPM ePoster Library
A recently developed innovative thermo-brachytherapy (TB) seed for LDR prostate cancer treatment requires utilization of high magnetic permeability ferrite ceramics, possessing Curie temperature matching that of the hyperthermia range (42-46 °C). We determine these characteristics experimentally for a specific formulation of the ferrite material to assess its suitability for use in TB seeds.
To measure the magnetic permeability we used a fully wound toroidal sample in the R-L circuit. Toroidal shape and high relative permeability of the sample minimized magnetic flux loss. Inductance was calculated by using the measured values for voltage difference and phase difference. Finally, permeability was inferred from inductance by using Ampere's law for toroidal shape. This procedure was repeated at various temperatures to observe a drastic change in magnetic permeability and measure the Curie temperature. To remain in the fully inductive zone (phase shift between inductance and conductance at +90°) for the toroid we keep the number of turns as a variable parameter, allowing us to find the permeability at different frequencies. The parasitic capacitance of the toroidal coil should be kept low to mitigate the effects of self-resonance.
Our theoretical modeling of magnetically-mediated hyperthermia established that ferromagnetic material should have a relative permeability in the range of thousands at ~100 kHz frequency, and Curie temperature ~50 °C. The relative permeability of the ferrite ceramics was determined to be 7000; Temperature-dependent measurements established a sharp drop in the relative permeability of the sample at the Curie temperature of 43 °C.
We developed a method for reliable measurements of temperature-dependent relative permeability and obtained promising results. Since permeability is an intrinsic material property we will use the results to characterize cylindrical cores of TB seed prototypes. Finding ferrite material with suitable properties will bring a TB seed implant closer to clinical implementation.