THE EFFECT OF CATHETERS AND COATINGS ON THE PERFORMANCE OF PALLADIUM-NICKEL THERMOSEEDS - EVALUATION AND DESIGN OF IMPLANTATION TECHNIQUES

In the development of materials for self-regulating thermoseeds much e ffort is put in improvement of the self-regulating temperature control mechanism of the seeds. The catheters and coatings which are needed t o implant the seeds or to guarantee biocompatibility, generally impair the optimized performance of the ferromagnetic seeds. The influence o f various coatings on the performance of PdNi seeds has been investiga ted by means of one-dimensional modelling and calorimetric experiments . Implantation using thin walled catheters is acceptable provided that the catheters are filled with water to assure good thermal coupling. Air layers inside catheters should be avoided as they reduce the sharp gradient of the heat production at the Curie temperature significantl y. An alternative for the application of catheters is to insert the se eds into metallic needles. The effect of shielding by the metal needle can be minimized by driving the seed into its saturated state using a high magnetic field strength. The thermal interaction between the see d and surrounding tissue can also be enhanced by placing PdNi, e.g. tu bular, on the outside of the catheter or brachytherapy needle. An addi tional advantage of this new design is an increase in the heat product ion and the quality of temperature control due to an increase in the a mount of PdNi. For permanent implantation seeds can be coated with an inert metal, ceramics or plastic. The performance of the seeds is not affected by any of the coatings if certain conditions are met. For pla stic coatings the thickness of the coating has to be very thin, prefer ably less than or equal to 20 mu m, to avoid thermal isolation.