METHODS AND SYSTEMS FOR THE MASS PRODUCTION OF RADIOACTIVE MATERIALS

Abstract: Systems and methods mass produce radioactive materials and structures, such as stents. A three-dimensional array of targets is disposed in a beam path of an electron beam emitted from a source thereof (e.g., a linear accelerator). A bremsstrahlung converter is interposed between the array of targets and the source of the electron beam so as to convert the electron beam to an irradiating beam which contains both electrons and photons. The electrons in the irradiating beam may be divergently directed away from the beam path (e.g., by magnetic sweepers) and along a divergent path so that the targets are irradiated predominantly by photons. Furthermore, the electron beam can be conditioned (focused) by means of magnetic stirring coils positioned upstream of the converter. The bremsstrahlung converter is most preferably provided by a plurality of individual converters which differ from one another in terms of their thickness and/or high Z material. One or more of these individual converters may thus be interposed in the beam path as may be desired in dependence upon the targets to be irradiated.

Description: METHODS AND SYSTEMS FOR THE MASS PRODUCTION OF RADIOACTIVE MATERIALS FIELD OF THE INVENTION The present invention relates generally to the production of radionuclides by irradiation with intense electron beams so as to form radioactive materials suitable for therapeutic and/or diagnostic medical purposes. In preferred embodiments, the present invention relates to systems and methods whereby radioactive structures, for example, stents, may be mass-produced by intense electron beam irradiation. BACKGROUND AND SUMMARY OF THE INVENTION In my prior U.S. Patent Application Serial Nos. 08/963,068 and 08/962,834 each filed on November 3, 1997 (the entire content of each such prior filed application being expressly incorporated hereinto by reference), there are disclosed techniques and methods for the production of point-of-use radionuclides by irradiation with intense electron beams so as to form radioactive materials suitable for therapeutic and/or diagnostic medical purposes and/or industrial applications. Broadly, the systems and methods of the present invention involve the mass production of radioactive structure, and especially the mass production of radioactive stents. More specifically, a three-dimensional array of targets formed of, or containing, radioactivatable material, is disposed in a beam path of an electron beam emitted from a source thereof (e.g., a linear accelerator). A bremsstrahlung converter is interposed between the array of targets and the source of the electron beam so as to convert the electron beam to an irradiating beam which contains both electrons and photons. The electrons in the irradiating beam may be divergently directed away from the beam path (e.g., by magnetic sweepers) and along a divergent path so that the targets are irradiated predominantly by photons. Furthermore, the electron beam can be conditioned (focused) by means of magnetic stirring coils positioned upstream of the converter. The bremsstrahlung converter is provided by a plurality of individual converters which differ from one another in terms of their thickness and/or material. One or more of these individual converters may thus be interposed in the beam path as may be desired in dependence upon the targets to be irradiated. The targets are most preferably translated relative to the beam path by a driven translator assembly. In addition, the targets preferably have a high negative voltage (e.g., between about -200 to about -5000 volts) applied thereto. These and other aspects and advantages of the present invention will become more clear after careful consideration is given to the following detailed description thereof. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Reference will hereinafter be made to the accompanying drawing FIGURE which depicts in schematic fashion one preferred system for the mass production of radioactive stents in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION The invention will be discussed below in relation to the mass production of radioactive stents. It should be understood, however, that reference to stents is to a particularly preferred embodiment of the invention and is nonlimiting with respect thereto. Thus, the present invention is well suited for the mass production of virtually any structure which is formed of, or contains, a radioactivatable material. The targets may therefore be in the form of stents or other radioactivatable structures, or may be capsules formed of a non- radioactivatable material which contain material to be activated (e.g., radioactivatable metal powders, radioactivatable liquid metals or liquid suspensions of radioactivatable metals). Suffice it to say here that those of ordinary skill in the art will appreciate the particular form that the "targets" may have and embraced by the scope of the present invention. A particularly preferred system 10 which embodies the present invention is depicted in the accompanying FIGURE. As shown, the system 10 includes a standard linear accelerator 12 which emits an electron beam 12-1. The linear accelerator 12 is most preferably is capable of generating an electron beam energy of between about 15 MeV to about 50 MeV and an average electron beam current of between about 0.05 to about 10 mA, most preferably about 1 rriA. The electron beam 12-1 impinges upon a beam converter assembly 13 which includes a bremsstrahlung converter 14 formed of a relatively high Z (atomic number) material, such as tantalum or tungsten. The term "high Z" as employed herein and in the accompanying claims is meant to refer to a material having an atomic number of greater than about 30, more preferably greater than about 70, for example between about 70 and about 92. As is well known the converter 14 coverts a substantial amount of the electron energy of the electron beam 12-1 into a beam 15 comprised of a mixture of electrons (identified by the straight lines in the beam 15) and photons (identified the wavy lines in the beam 15) which is directed toward a three-dimensional array of stent targets 16. The converter 14 is connected operatively to a converter adjustment assembly 18 which is capable of mechanically (e.g., via suitable mechanical couplings (not shown) that may be manually or motor driven) inserting and withdrawing the converter 14 into and out of the path of the electron beam 12-1 , respectively. In this regard, the adjustment assembly 18 is operatively coupled to a plurality of converters 14 of different thicknesses (as measured linearly parallel to the path of the electron beam 12-1 ) and/or different high Z materials. Thus, by selectively moving (substituting) one or more of the converters 14 into the path of the electron beam 12-1 , the desired beam 15 characteristics may be caused to emanate therefrom toward the stent targets 16. The adjustment assembly 18 and the converters 14 coupled thereto are in thermal communication with a cooling assembly 20 which is capable of circulating a cooling fluid (e.g., water) through the adjustment assembly and thereby providing necessary thermal cooling of the individual converters 14. The use of a relatively thin bremsstrahlung converter (e.g., less than about 2 mm thick) is most preferably employed for a single or a relatively few number of stent targets 16. The use of relatively thicker bremsstrahlung converters (e.g., from about 2 mm to about 6 mm thick formed of a tungsten material) may be employed for multiple stent targets (e.g., numbering in the hundreds) which may be directly exposed to the beam 15 or encapsulated by a low Z material. Those skilled in the art will recognize that thin high Z bremsstrahlung converts will position the high energy photon conversion into a narrow cone and it is only within this cone that the effect desired - namely, activation - will occur. (See for example, Hubbell, J. of App. Phys., 30, 981-984 (1959), the entire content of which is incorporated expressly hereinto by reference). It is noted that, according to Hubbell, the thicker bremsstrahlung converters destroy the nature of the forward cone and redistribute the high energy photons over a much wider angle. Thus, when the targets for activation are selected, they should be disposed within this new distribution in an optimum manner. The optimum manner of such distribution can be designed by Monte Carlo modeling methods (MCNP-4A, Los Alamos Laboratory, incorporated hereinto by reference). The Monte Carlo computer code may be used to calculate the distribution of energies over wider and wider cones. One may then calculate the energy flux and cross section products using prior art cross sections and computer codes. (See, for example, Weeks et al, Med. Phys. 13, 762-764 (1986), the entire content of which is expressly incorporated hereinto by reference.) The stent targets 16 are disposed in a three dimensional array (it being understood of course and only two dimensions of the array are visible in the accompanying FIGURE) which, as noted above, is most usefully positioned within the cone of energy flux which best matches or overlaps the nuclear reaction resonance energy window as devised by the Monte Carlo modeling. The array of stent targets 16 is also such that permutation of the stent targets 16 is allowed. Thus, the stent targets 16 are preferably affixed to stent support brackets 16-1 which project from a translator system 22. In this regard, the translator system 22 includes a motor drive (not shown) which selectively moves (translates) the stent targets 16 as may be desired relative to the beam 15. The particular structure of the stents that may be radioactivated according to the present invention is not critical. Thus, the stents may be configured to suit the particular therapeutic need (e.g., see U.S. Patent No. 5,059,166 to Fischell et al, the entire content of which is expressly incorporated hereinto by reference). A beam dump 24 is positioned behind the array of stent targets 16 so as to stop the beam 15. The beam dump is of a conventional variety and may include a relatively large mass of material which stops the photon and electron beam. Most preferably, the beam dump most advantageously includes a forward section formed of a relatively low Z material (e.g., aluminum) so as to be insubstantially affected by the beam 15. A suitable heat sump may be positioned rearwardly of the forward section so as to transfer heat generated thereat. Alternatively, the beam dump 24 may be in the form of a water-holding container having an ion chamber submersed therein to measure radiation by recording ionization in the chamber with an electrometer. The thickness of the beam dump depends on the electron energy and material. For example, the thickness of a water beam dump may be on the order of about six (6) feet for a 20 MeV beam, but may be on the order of only about one (1 ) foot for a lead beam dump. The system 10 also most preferably includes a pair of sweeper magnets 26 positioned laterally of the beam 15 emitted from the converter 14. The sweeper magnets are of sufficient flux intensity so as to direct electrons in the beam 15 out of the straight cone path and direct them to secondary beam dumps 28 positioned below the plane of the FIGURE. The secondary beam dumps 28 may be similar to the primary beam dump 24 described previously, but may be of a greatly reduced size owing to the smaller penetrating ability of the electrons that will impinge thereon. For example, the thickness of water for a 20 MeV beam is about 15 cm, but is only about 6 cm for an aluminum beam dump. In this manner, the sweeper magnets 26 and corresponding secondary beam dumps 28 ensure that a high proportion of photons will travel on to the stent targets 16 so as to activate the same. In order to reduce radiation damage to the material forming the stent targets 16, a high negative voltage (e.g., between about -200 to about -5000 volts) is applied to the stent targets 16. In order to accomplish this function, the system 10 may include a high voltage power supply 30 which is coupled through the common stent support 32 to which the stent support brackets 16-1 are physically attached. The support 32 is insulated electrically from the translator 22. Magnetic scanning coils 34 may optionally be provided laterally of the electron beam 12-1. The coils 34 are provided in order to allow the electron beam 12-1 to be scanned on the converter 14 so as to enable more efficient cooling of the converter 20 and/or a wider geometric spread of the resulting photon beam 15 emitted therefrom. By providing a wider geometric spread of the photon beam 15, a greater amount of stent targets 16 per unit time may be activated thereby significantly reducing (or eliminating entirely) the complexity of movement required to be performed by the translator 22. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A method for the mass production of radioactive materails comprising the steps of: (a) creating an electron beam along a predetermined beam path; (b) positioning a three dimensional array of radioactivatable targets in the beam path; and (c) interposing a bremsstrahlung converter in the beam path in advance of said targets. 2. The method of claim 1 , which includes varying at least one of the thickness and material of the converter in dependence upon the targets to be irradiated. 3. The method of claim 1 , which includes translating the array of targets relative to the beam path. 4. The method of claim 1 , which wherein step (c) includes providing a plurality of bremsstrahlung converters which differ from one another in at least one of thickness and material, and interposing at least one of the converters in the beam path in dependence upon the targets to be irradiated. 5. The method of claim 1 , which includes (d) positioning a beam dump in the beam path rearwardly of the array of targets. 6. The method of claim 1 or 3, which includes applying a negative voltage to the array of targets. 7. The method of claim 6, wherein the negative voltage applied to the array of targets is between about -200 to about -5000 volts. 8. The method of claim 1 , which includes positioning a sweeper magnet downstream of the bremsstrahlung converter and laterally adjacent to the beam path so as to cause a substantial number of electrons in the beam emitted from the converter to be diverted from the beam path along a divergent path. 9. The method of claim 8, which includes positioning secondary beam dump along the divergent path. 10. The method of claim 8, wherein a pair of sweeper magnets are employed. 11. The method of claim 1 , further comprising positioning a magnetic scanning coil laterally of the beam path in advance of the converter. 12. The method of any one of the preceding claims 1-11 , wherein the targets include stents. 13. The method of any one of the preceding claims 1-11 , wherein the targets include capsules which contain radioactivatable material. 14. A system for the mass production of radioactive materials comprising: an electron beam generator for generating an electron beam along a beam path; a three dimensional array of radioactivatable targets positioned in the beam path; and a bremsstrahlung converter interposed in the beam path between said electron beam generator and said array of targets. 15. A system as in claim 14, further comprising a converter adjustment assembly which includes a plurality of bremsstrahlung converters which differ from one another in terms of thickness and/or material, wherein said converter adjustment assembly is capable of selecting interposing at least one of said converters between said electron beam generator and said array of targets. 16. The system of claim 14, which further comprises a translator coupled operatively to said array of targets for translating said targets relative to said beam path. 17. The system of claim 14, which comprises a beam dump disposed in the beam path rearwardly of said array of targets. 18. The system of claim 14, which includes a voltage generator coupled operatively to the array of targets for applying a negative voltage to said targets. 19. The system of claim 18, wherein said voltage generator applies a voltage of between about -200 to about -5000 volts to said array of targets. 20. The system of claim 12, which includes a sweeper magnet positioned downstream of said converter and laterally adjacent to the beam path so as to cause a substantial number of electrons in the beam emitted from the converter to be diverted from the beam path along a divergent path. 21. The system of claim 18, which includes a secondary beam dump positioned along said divergent path. 22. The system of claim 18, comprising a pair of said sweeper magnets. 23. The system of claim 12, further comprising a magnetic scanning coil laterally positioned relative to said beam path in advance of said converter. 24. The system of any one of claims 14-23, wherein the targets include stents. 25. The system of any one of claims 14-23, wherein the targets include capsules which contain radioactivatable material.

Inventors: WEEKS KENNETH J, WEEKS, KENNETH, J.
Applicants: UNIV DUKE, WEEKS KENNETH J, DUKE UNIVERSITY, WEEKS, KENNETH, J.
Application year: 1999
Main classification: A61N
IPC codes: A61F2, A61N5, G21G1