Purpose: To experimentally characterize ferromagnetic materials for use in an innovative Thermo-brachytherapy (TB) seed, serving as a heat and radiation source for concurrent administration of radiation therapy and hyperthermia. The combination of two modalities has been confirmed very effective in the treatment of prostate cancer without an increase in complications. Measuring the quantity of relative permeability as a function of temperature will be utilized through the power-versus-temperature method for calculation of temperature distribution around the seed in a prototype treatment planning system.
Methods: We measure μ_r of small (<1cm-long, ~1mm diameter) cylindrical TB seed samples with the same geometrical size and shape as the one used for prostate Brachytherapy seed implant. The nonlinear effects, demagnetization, and magnetic saturation effects in a finite-sized cylinder reduce the value of μ_r compared to those measured with a standard toroid sample (effective length of a toroid is infinite). The inductance, parasitic resistance, and relative permeability of the sample were derived from impedance measurements for a solenoid that was a custom-wound for the sample. The results are compared to the theoretical values and simulations from COMSOL Multiphysics for consistency.
Results: We measured μ_r for ferrite and Ni-Cu samples. The Curie temperature for the ferrite and Ni-Cu cores are ~55 ºC and ~ 45 ºC correspondingly. Our values are closest to theoretical results for a long solenoid corrected for the circular wire shape, mutual inductance between the coil loops, wire self-inductance, the gradient resistance inside the wire, and diameter-to-length ratio.
Conclusion: The ferrite core has a much steeper permeability fall-off with temperature. Rapid Curie transition, decoupling of the magnetic material properties from electrical properties of the coating, and higher value for permeability make ferrite samples much better candidates for the TB seed. These results can help us to identify a ferrite composition with ideal power generation.