Brain Tumor Hyperthermia with Static and Moving Seeds

Thermodynamic studies are presented for both static and moving ferromagnetic "seeds" imbedded in biological media. These studies were performed in support of the development of a system which delivers localized hyperthermia to deep-seated brain tumors. In this system, a magnetic "seed" of approximately 5 mm dimension (length and diameter) is remotely repositioned within the brain by an externally produced magnetic field. The seed is inductively heated and repositioned throughout the tumor volume. An induction heating system was built for experimental use with tissue phantoms and animals. The maximum level of direct tissue heating produced by this system was measured in vivo in three animals. An upper limit on the power absorption was placed at 0.46 mW cm^{ -3}, a factor of 10^{-4 } below the power density produced in ferromagnetic seeds by the same system. Measurements were made of the temporal and spatial dependence of the temperature rise in the vicinity of a statically placed 6 mm diameter nickel sphere, in vivo in four pigs, and in one which was euthanized. These results were compared to a theroetical model which was based on a point source solution to the thermal diffusion equation and estimates of blood flow rates, tissue thermal conductivity and seed power absorption were found using a parameter estimation algorithm. Studies were also made of the temperature gradients produced by a heated iron ellipsoid of 4.8 mm diameter and 9.6 mm length in a brain tissue phantom. Temperature measurements were made both with the seed statically imbedded in the tissue phantom and with the phantom moving at a constant velocity of 0.11 mm s^{-1 } with respect to the seed. These static and moving data were compared to obtain an estimate for the thermal field and convective cooling of a moving seed. In addition, an exploratory study was performed in which the dependence of seed heating efficiency on material and geometry were tested. A "hybrid" seed was developed consisting of a permanent magnet core surrounded by a non -magnetic spacing material and a 0.5 mm thick ferromagnetic outer sleeve. This seed was designed to accommodate potentially conflicting magnetic force and induction heating requirements.