Patent Publication Number: US-7582886-B2

Title: Gantry for medical particle therapy facility

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to a medical cancer therapy facility and, more particularly, to a medical particle delivery system having a compact gantry design. 
   It has been known in the art to use a particle accelerator, such as a synchrotron, and a gantry arrangement to deliver a beam of particles, such as protons, from a single source to one of a plurality of patient treatment stations for cancer therapy. In such systems, the cancerous tumor will be hit and destroyed by particles in a precise way with a localized energy deposition. Thus, the number of ion interactions on the way to the tumor through the healthy body cells is dramatically smaller than by any other radiation method. A position of the center of the tumor inside the body defines a value of the particle energy. The transverse beam raster is defined by the transverse size of the tumor with respect to the beam, while the width of the tumor defines the beam energy range. The energy deposition is localized around the “Brag” peak of the “implanted particles” and remaining energy is lost due to particle interaction with the tumor cells. 
   Such cancer treatment facilities are widely known throughout the world. For example, U.S. Pat. No. 4,870,287 to Cole et al. discloses a multi-station proton beam therapy system for selectively generating and transporting proton beams from a single proton source and accelerator to one of a plurality of patient treatment stations each having a rotatable gantry for delivering the proton beams at different angles to the patients. A duoplasmatron ion source generates the protons which are then injected into an accelerator at 1.7 MeV. The accelerator is a synchrotron containing ring dipoles, zero-gradient dipoles with edge focusing, vertical trim dipoles, horizontal trim dipoles, trim quadrupoles and extraction Lambertson magnets. 
   The beam delivery portion of the Cole et al. system includes a switchyard and gantry arrangement. The switchyard utilizes switching magnets that selectively direct the proton beam to the desired patient treatment station. Each patient treatment station includes a gantry having an arrangement of bending dipole magnets and focusing quadrupole magnets. The gantry is fully rotatable about a given axis so that the proton beam may be delivered at any desired angle to the patient. 
   The gantry of typical particle beam cancer therapy systems accepts a particle beam of a required energy from the accelerator and projects it with a high precision toward a cancerous tumor within a patient. The beam from the gantry must be angularly adjustable so that the beam can be directed into the patient from above and all sides. Because of these requirements, the gantry of a conventional particle beam cancer therapy facility is typically the most expensive piece of equipment of the treatment facility and its magnets are generally very large and heavy. 
   For example, the proton-carbon medical therapy facility described by R. Fuchs and P. Emde in “The Heavy Ion Gantry of the HICAT Facility” includes an isocentric gantry system for delivery of protons, Helium, Carbon and Oxygen ions to patients. The gantry system has a total weight of 630 tons and the required beam line elements for transporting and delivering fully stripped Carbon and Oxygen ions with 430 MeV/nucleon kinetic energy have a total weight of 135 tons. The rotating part of the isocentric gantry system weighs about 570 tons due to its role to safely transport and precisely delivers ions to the patients. 
   Advances in particle accelerator design have resulted in accelerators utilizing smaller and less complex magnet arrangements. For example, a nonscaling fixed field alternating gradient (FFAG) accelerator has recently been developed which utilizes fixed field magnets, as opposed to much larger and more complex variable magnetic field coil magnets. Such advances, however, have heretofore not been applied to the gantry design of typical cancer therapy facilities. 
   Accordingly, it would be desirable to improve upon the prior art medical cancer therapy facilities by providing a simpler, less expensive and more compact gantry design utilizing some of the advances made in the field of particle accelerators. 
   SUMMARY OF THE INVENTION 
   The present invention is a particle therapy gantry for delivering a particle beam to a patient. The gantry generally includes a beam tube having a curvature defining a particle beam path and a plurality of fixed field magnets sequentially arranged along the beam tube for guiding the particle beam along the particle path. 
   In a preferred embodiment, each of the fixed field magnets is a combined function magnet performing a first function of bending the particle beam along the particle path and a second function of focusing or defocusing the particle beam. Also, the magnets are preferably arranged in triplets, wherein each triplet has two focusing magnets and one defocusing magnet disposed between the focusing magnets. The focusing magnets perform the combined function of bending the particle beam and focusing the particle beam and the defocusing magnet performs the combined function of bending the particle beam and defocusing the particle beam. The defocusing magnets are preferably positive bending magnets for bending the particle beam along an arc defined by a positive center of curvature and the focusing magnets are preferably negative bending magnets for bending the particle beam along an arc defined by a negative center of curvature, wherein the positive and negative centers of curvature are oriented on opposite sides of the beam pipe. 
   In one embodiment, the fixed field magnets are permanent magnets including a ferromagnetic core having a curvature defined by a center of curvature and forming a beam tube receiving cavity having the beam tube supported therein. The core is shaped to provide a magnetic field in the beam tube which grows stronger in a direction toward the core center of curvature. In an alternative embodiment, the fixed field magnets include superconducting coils adjacent the beam tube for providing the combined function. 
   In either case, the beam tube of the gantry preferably includes a particle beam entry point, a transition point, a particle beam exit point, a first curved particle beam path arc length extending between the entry point and the transition point and a second curved particle beam path arc length extending between the transition point and the exit point. The first arc length bends about ninety degrees and the second arc length bends about one hundred eighty degrees in a direction opposite the first arc length. Two half-triplets are preferably disposed in juxtaposed orientation at the beam tube transition point and a half-triplet is preferably disposed at each of the beam tube entry point and the beam tube exit point. Each of the half-triplets includes a defocusing magnet and a focusing magnet. 
   The present invention further involves a method for delivering a particle beam to a patient through a gantry. The method generally includes the steps of bending the particle beam with a plurality of fixed field magnets sequentially arranged along a beam tube of the gantry, wherein the particle beam travels in the beam tube, and alternately focusing and defocusing the particle beam traveling in the beam tube with alternately arranged combined function focusing and defocusing fixed field magnets. 
   In a preferred embodiment, the combined function fixed field magnets are arranged in triplets, wherein each triplet includes two focusing magnets and one defocusing magnet disposed between the focusing magnets. The focusing magnets perform the combined function of bending the particle beam and focusing the particle beam and the defocusing magnet performs the combined function of bending the particle beam and defocusing the particle beam. The defocusing magnets are preferably positive bending magnets for bending the particle beam along an arc defined by a positive center of curvature and the focusing magnets are preferably negative bending magnets for bending the particle beam along an arc defined by a negative center of curvature, wherein the positive and negative centers of curvature are oriented on opposite sides of the beam pipe. 
   The gantry of the present invention may be utilized in a medical particle beam therapy system having a source of particles, a particle accelerator, an injector for transporting particles from the source to the accelerator, one or more patient treatment stations including rotatable gantries of the present invention for delivering a particle beam to a patient and a beam transport system for transporting the accelerated beam from the accelerator to the patient treatment station. 
   The preferred embodiments of the particle beam gantry of the present invention, as well as other objects, features and advantages of this invention, will be apparent from the following detailed description, which is to be read in conjunction with the accompanying drawings. The scope of the invention will be pointed out in the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top plan view of a typical medical particle delivery therapy facility. 
       FIG. 2  is a side view of the arrangement of the gantry treatment room of the medical facility shown in  FIG. 1 . 
       FIG. 3  is a cross-sectional view of the gantry according to the present invention. 
       FIG. 4  is a graphical representation of one of the magnet triplets forming the gantry of the present invention. 
       FIG. 5  is another a graphical representation of one of the magnet triplets forming the gantry of the present invention. 
       FIG. 6  is an isometric view of one of the magnet triplets forming the gantry of the present invention. 
       FIG. 7  is a graph showing the horizontal and vertical betatron functions and the dispersion function of a magnet triplet at the reference momentum. 
       FIG. 8  is a graph showing the minimum required aperture for a combined function magnet with a defocusing gradient. 
       FIG. 9  is a graph showing the minimum required aperture for a combined function magnet with an opposite bend and focusing field. 
       FIG. 10  is a cross-sectional view of the combined function bending/defocusing magnet shown in  FIG. 5 , taken along line  10 - 10 . 
       FIG. 11  is a cross-sectional view of the combined function bending/focusing magnet shown in  FIG. 5 , taken along line  11 - 11 . 
       FIG. 12  is a cross-sectional view of a fixed field combined function magnet utilizing superconducting tapes or coils without an iron core. 
       FIG. 13  shows a cross-sectional view of a similar superconducting magnet having a super ferric core and superconducting coils surrounding the beam tube. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a typical medical particle delivery therapy facility  10 . The facility  10  generally includes an injector  12 , a particle accelerator  14 , and a beam delivery network  16  including a rotatable gantry treatment room  18  for delivering a beam to a patient. The beam delivery network  16  may also be designed to divert independent beams to various other applications as desired. For example, the beam delivery network  16  may be designed to deliver a beam to a beam research room  20  and a fixed beam treatment room  22 . The research room  20  may be provided for research and calibration purposes, with an entrance separate from the patient areas, while the fixed beam treatment room  22  may include separate beam lines for such therapeutic applications, such as eye treatments. 
   The beam injector module  12  can be a conventional LINAC or a tandem Van de Graaf injector with an injection kicker, which completes the task of particle injection into the accelerator  14 . In the case of proton particles, the injector typically provides proton beam pulses at 30 Hz with a pulse width varying between 25 and 100 nanoseconds at a delivered energy of 7 MeV. 
   The particle accelerator  14  can be a synchrotron, cyclotron or some other conventional design known in the prior art. The accelerator  14  accelerates particles to a desired energy level for extraction and delivery to the patient treatment rooms  18  and  22 . Variation of the extraction energy is achieved by adjusting, for example, an RF frequency within the accelerator  14 . Again for proton particles, extraction typically occurs when the kinetic energy of the particles is in the range 60 to 250 MeV. 
   The beam delivery network  16  connects the accelerator  14  to the treatment rooms  18  and  22  and the beam research room  20 . The network  16  generally includes an extraction line  26 , a switchyard  28  and a plurality of beam transport lines  30 . The switchyard  28  is typically an arrangement of switching magnets for diverting the particle beam to a desired beam line  30 . The beam transport lines  30  take the particle beam from the switchyard  28  to the different treatment rooms of the facility. 
   Referring additionally to  FIG. 2 , the rotatable gantry treatment room  18  includes a rotating gantry  24 , which is rotatable by plus or minus 200 degrees from the vertical about a point of rotation  32  to deliver a particle beam to a patient  33  at a gantry iso-center  34 . The gantry system accepts particles already accelerated to required energy. The first part  24   a  of the gantry bends particles within a quarter of a circle for 90 degrees. The second part  24   b  of the gantry bends the particles in a half of a circle and brings the particles straight towards the required direction  34 . 
   The gantry  24  is constructed as a three-dimensional structure supported on the treatment room side by a bearing  36  and, on the beam inlet side, by a bearing  38 . The gantry  24  is further preferably balanced around its rotation axis. Gantry movement can be realized by a gear motor/gear ring drive  40  that allows high precision positioning. Each gantry  24  is preferably controlled by means of an individual independent computer unit that ensures mutual braking of the main drive units, soft start and soft deceleration functions, control of the auxiliary drive units for the treatment room, and supervision of the limit switches. The gantry  24  further includes a nozzle  42  for delivering the particle beam to the patient  33 . 
   Referring now to  FIG. 3 , the optical components of the gantry  24  according to the present invention are shown. The gantry  24  generally includes a hook-shaped beam pipe  44  and a series of identical fixed-field magnet triplets  46  arranged in sequence around the beam pipe. The beam pipe  44  can be provided as a continuous pipe, or it can be assembled from a plurality of beam pipe segments welded or otherwise fastened together in a conventional manner. The beam pipe  44  and the magnet triplets  46  are enclosed in a gantry housing  47 . 
   Referring additionally to  FIGS. 4-6 , the magnet triplet  46  is considered the “unit cell” and contains a relatively long combined function bending/defocusing magnet (QD)  48  flanked by a pair of shorter combined function bending/focusing magnets (QF)  50 . The cell  46  is symmetric with respect to the center of the defocusing magnet  48 . 
   Thus, the gantry  24  is made of densely packed identical “triplet” cells  45 . Three combined function magnets make a cell. The central magnet  48  produces major bending and has a linear horizontal defocusing gradient. Two smaller identical but opposite bending magnets  50  are placed on both sides of the major bending magnet  48 . They have a linear focusing gradient. Each of the combined function magnets  48  and  50  performs two functions. The first function is to bend the particle beam along an orbital path, while the second function is to focus or defocus the particle beam as it travels around the path. 
   The defocusing magnet (QD)  48  has a strong central field and a negative gradient (horizontally defocusing) at the center, while the focusing magnets (QF)  50  have a positive gradient (horizontally focusing). Both magnets  48  and  50  are fixed field dipole-type magnets using a very strong focusing and small dispersion function. The horizontal and vertical betatron functions βx and βy and the dispersion function in the basic cell  46 , at the reference momentum, are shown in  FIG. 7 . The minimum required aperture for the two combined function magnets major bend with the defocusing gradient and the opposite bend with the focusing field are presented in  FIGS. 8 and 9 , respectively. 
   Thus, the QD and QF magnets  48  and  50  are arranged in a non-scaling, fixed field alternating gradient (FFAG) configuration. Such FFAG configurations have been used before in particle accelerators, but have heretofore never been proposed in a therapeutic particle delivery gantry of a medical facility. 
   Also, both types of magnets  48  and  50  are somewhat arc-shaped or wedge-shaped when viewed in a direction perpendicular to the path of the beam pipe  44 . Thus, each magnet  48  and  50  is defined by an axis  48   a  and  50   a , which may represent the center of curvature in the case of an arc-shaped magnet, or an intersection point of the two outside faces in the case of a wedge-shaped magnet. 
   In either case, each defocusing magnet (QD)  48  of each magnet triplet  46  is arranged along the beam pipe  44  so that its axis  48   a  falls on the same side of the beam pipe  44  as the beam pipe&#39;s center of curvature  44   a . Conversely, each flanking pair of focusing (QF) magnets  50  of each magnet triplet is arranged along the beam pipe  44  so that their axes  50   a  falls on the opposite side of the beam pipe  44  as the beam pipe&#39;s center of curvature  44   a . In this manner, each defocusing magnet (QD)  48  can be termed a “positive bending” magnet, wherein the shape and arrangement of this magnet bends the particles passing therethrough in a path generally matching the curvature of the beam pipe, as shown in  FIGS. 3-6 . Each focusing magnet (QD)  50 , on the other hand, can be termed a “negative bending” magnet, wherein the shape and arrangement of these magnets bend the particles passing therethrough in a path generally opposite to the curvature of the beam pipe. It has been found that such alternating arrangement of positive and negative bending magnets results in a particle beam having a reduced dispersion. 
   Referring now to  FIG. 10 , each defocusing magnet (QD)  48  includes a ferromagnetic core  52  made up of an upper  53  and a lower half  54  forming a dipole magnet. The upper  53  and lower halves  54  are identical in cross-section and can be solid ferromagnetic masses, as shown in  FIG. 10 , or they can be made from a series of stacked laminates. In either case, the upper core half  53  includes an angled face  53   a  and the lower core half includes an angled face  54   a . The angled faces  53   a  and  54   a  of the upper and lower core halves  53  and  54  face each other and form a beam pipe receiving cavity  56  when the core halves are assembled together to form the magnet core  52 . 
   Referring to  FIG. 11 , each focusing magnet (QF)  48  is similarly constructed. Specifically, each focusing magnet  50  includes a ferromagnetic core  58  made up of an upper  59  and a lower half  60  forming a dipole magnet. Again, the upper  59  and lower halves  60  can be solid ferromagnetic masses or they can be made from a series of stacked laminates  55 . Also, the upper core half  59  includes an angled face  59   a  and the lower core half includes an angled face  60   a . The angled faces  59   a  and  60   a  of the upper and lower core halves  59  and  60  face each other and form a beam pipe receiving cavity  62  when the core halves are assembled together to form the magnet core  58 . 
   As mentioned above, each magnet  48  and  50  is a combined function arc magnet combining the functions of bending the particle beam and focusing or defocusing the particle beam. The bending function is achieved by the curvature of the magnet, while the focusing or defocusing function is achieved by the arrangement of the magnet cores  52 ,  58 . 
   In particular, the upper  53  and the lower  54  halves of the defocusing magnet core  52  are arranged together respectively above and below the beam pipe  44  so as to provide a magnetic field in the beam pipe which grows stronger in a direction toward the center of curvature  48   a  of the core, as shown in  FIG. 10 , whereas the upper and the lower halves  59  and  60  of a focusing magnet core  58  are arranged together respectively above and below the beam pipe so as to provide a magnetic field in the beam pipe which grows weaker in a direction toward the center of curvature of the defocusing core  48   a , but which grows stronger in a direction toward the center of curvature  50   a  of its own core. 
   Thus, in a defocusing combined function magnet  48 , as shown in  FIG. 10 , a proton, or other particle, in the beam pipe  44  radially further from the core center of curvature  48   a  and the beam pipe center of curvature  44   a  (to the right in  FIG. 10 ) is subject to a weaker magnetic field and bends less, while a proton, or other particle, closer to the beam pipe center of curvature (to the left in  FIG. 10 ) sees a stronger magnetic field and bends more. This results in a more dispersed horizontal concentration of protons, but a denser vertical concentration, in the beam pipe just downstream of a defocusing combined function magnet. 
   Conversely, in a focusing combined function magnet  50 , as shown in  FIG. 11 , a proton, or other particle, in the beam pipe  44  radially further from the beam pipe center of curvature  44   a , or closer to the core center of curvature  50   a , (to the right in  FIG. 11 ) is subject to a stronger magnetic field and bends more, while a proton closer to the beam pipe center of curvature, or away from the core center of curvature, (to the left in  FIG. 11 ) sees a weaker magnetic field and bends less. This results in a greater horizontal concentration of particles, but a weaker vertical concentration of particles in the beam pipe just downstream of a focusing combined function magnet. 
   The above defocusing effect is achieved by orienting the angled surfaces  53   a  and  54   a  of the upper and lower core halves  53  and  54  of the defocusing magnet core  52  so that they form an intersection point  64  that falls on the same side of the beam pipe  44  as the beam pipe center of curvature  44   a , as shown in  FIG. 10 . A focusing magnet  50  is formed by orienting the angled surfaces  59   a  and  60   a  of the upper and lower core halves  59  and  60  of the focusing magnet core  58  so that they form an intersection point  66  that falls on the side of the beam pipe  44  opposite the beam pipe center of curvature  44   a , as shown in  FIG. 11 . In other words, the angled faces  53   a  and  54   a  of a defocusing magnet  48  meet adjacent the inner arc of the beam pipe  44 , whereas the angled faces  59   a  and  60   a  of a focusing magnet  50  meet adjacent the outer arc of the beam pipe, with respect to the center of curvature  44   a  of the beam pipe. 
   Accordingly, not only are the positive and negative bending functions alternately arranged, but also the focusing and defocusing functions of the magnets are alternately arranged. Such alternate arrangement of the positive and negative bending and the focusing and defocusing functions provides to the present invention the feature of net strong particle beam focusing in both horizontal and vertical planes. 
   At the transition point  68  of the gantry  24 , where the beam pipe  44  reverses its curvature, and/or at the beam entry point  70  and/or at the beam exit point  74 , modifications of the magnet triplet  46  can be utilized to provide the desired bending and focusing/defocusing functions. For example, a half-triplet  76  consisting of a single negative-bend focusing magnet  50  and a reduced length, positive bend defocusing magnet  48   a  can be utilized at the beam entry point  76  and/or the beam exit point  72  of the gantry to achieve the desired bend angle and focusing at these points. Similarly, at the beam pipe curvature transition point  68 , two half-triplets  76 , as described above, can be assembled together in a juxtaposed orientation to form a “straight” magnet triplet  78 . 
   For proton therapy systems, the combined function defocusing magnet  48  and the combined function focusing magnet  50  used in the gantry can be very small permanent magnets, as described above. For example, a suitable magnetic field of about 1.8 T can be achieved using defocusing magnets  48  that measure about 6 cm×8 cm×10 cm. For larger particles, such as carbon, the magnets can utilize high-temperature superconductor tapes (HTS) or superconducting Niobium-Tin coils to achieve the required greater magnetic fields of about 6 T. In either case, the magnets are still fixed-field magnets. 
     FIG. 12  shows a cross-section of a fixed field combined function magnet  80  utilizing high-temperature superconductor tapes (HTS) or superconducting Niobium-Tin coils  82  surrounding the beam tube  44 , without an iron core.  FIG. 13  shows a cross-section of a similar superconducting magnet  84  having a super ferric core  86  and superconducting coils  88  surrounding the beam tube  44 . 
   As a result of the present invention, the size of the gantry in a particle therapy facility can be dramatically reduced and the control system for a gantry treatment room can be greatly simplified. Specifically, the gantry  24  can be made about 20 meters long, from the rotation point  32  to the iso-center  34 , with a height of about 3.2 meters. The gantry  24  preferably has a free space of about 1.6 meters from the last magnet to the isocenter  34 . 
   Thus, the present gantry invention reduces the weight of the gantry system by using a non-scaling Fixed Field Alternating Gradient (FFAG) triplet structure with permanent, superconducting or high-temperature superconducting combined function magnets. This invention allows a very close control of focused ion transport through the beam line with different energies but under the fixed magnetic field. The ions are delivered to the isocentric non-scaling FFAG gantry system at the same entrance position. This invention can achieve presented goals due to a very large momentum acceptance and very strong focusing properties of the non-scaling FFAG structures. The ions with different energies transported through the system arrive at the end of it with small differences in positions (−2.5 up to +3.2 mm) easily adjusted by the raster-scanning focusing part of the gantry. 
   Although preferred embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various other changes and modifications may be affected herein by one skilled in the art without departing from the scope or spirit of the invention, and that it is intended to claim all such changes and modifications that fall within the scope of the invention.