Abstract:
An x-ray target pedestal assembly and a method of protecting the x-ray target from breaking down as a result of the extreme heat that is produced when an electron beam is aimed at the target to produce x-rays. The target is submerged in cooling fluid and is rotated by a constant flow of the cooling fluid over and around the target in order to dissipate heat. The fluid is guided by integrated flow diverters in the target cover. The target may also be protectively coated either in its entirety or along the electron beam path in order to further protect it from the heat of the electron beam impact or from breakdown as a result of attack of free radicals or other chemically reactive components of the cooling fluid which are produced in the extreme target environment.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to external radiotherapy treatment systems and methods. More specifically, the invention relates to an x-ray target pedestal assembly and to a method of preserving the target when it is exposed to an electron beam to produce x-ray radiation. 
       BACKGROUND OF THE INVENTION 
       [0002]    Medical equipment for radiation therapy treats tumorous tissue with high-energy radiation. One source of therapeutic radiation is high-energy x-rays. To produce high-energy x-rays, an electron beam is emitted from a linear accelerator (linac) and is aimed at a solid target. The target is bombarded with the electrons from the beam, and high-energy photons (x-rays) are produced as a result of the interaction between the fast moving electrons and the atomic structure of the target. The deceleration of electrons caused by the interaction of the electrons with the material of the target creates the x-rays in a process known as bremsstrahlung. The resulting x-rays are then collimated and directed to the treatment area. 
       SUMMARY OF THE INVENTION 
       [0003]    The target is usually made from a refractory metal with a very high atomic number and density and with a high melting point. Suitable target materials include tungsten, molybdenum, and rhenium. The intensity of x-ray radiation produced is a function of properties of the material including the atomic number of the target material. The target can be integral to the linac, or it may be removable if it is housed outside of the linac&#39;s vacuum environment. The target may be either rotating or stationary and may be a variety of shapes. The x-ray spectrum produced will vary with the thickness of the target and with the energy of the electron beam, as electrons progressively lose energy as they pass through the target. 
         [0004]    Not all of the electron beam&#39;s energy is converted into x-rays. Large amounts of thermal energy are created from the interaction of the high-energy electrons with the target. As such, a cooling mechanism is typically used to preserve the target material by protecting the target from heat stresses and preventing the target from reaching its melting point. Rotating targets are often used so that the electron beam does not consistently contact the same portion of the target. 
         [0005]    The target may be cooled by any number of coolants, or cooling media, as keeping the target submerged in a coolant boosts the heat transfer away from the target. Water is the most benign coolant that can be subjected to the radiation of the target environment. However, incident radiation causes oxidation, breaking down water molecules to produce free radical hydrogen and oxygen atoms that will attack and degrade the target material. The target material will break down or become unstable over time as a result of the extreme environment created by the interaction of the electrons with the cooling fluid. As the target material breaks down, it becomes thinner and its photon conversion capacity is diminished. Therefore, target life is limited, and the target must be replaced from time to time. 
         [0006]    The present invention relates to a radiation therapy x-ray target pedestal assembly designed to preserve the life of the x-ray target and a method for protectively coating the target to extend the target life. An electron beam contacts the target to generate x-rays, and the target rotates around a central axis to dissipate the resulting heat. The target is submerged in a cooling fluid, and the cooling fluid flow is directed to and around the target by integrated flow diverters in the target cover, which guide the flow such that the target passively rotates, distributing the heat around the target. The target may be protected from chemical corrosion if a protective coating is applied to the target, either completely encasing the target or protecting a portion of the target. The target pedestal assembly allows the target to rotate freely about a replaceable rotational pin at its central axis, guides coolant flow to and around the target, and allows for fluid cooling of the target by maintaining consistent rotation of the target and consequently extends target life. 
         [0007]    In one embodiment, the invention provides target pedestal assembly for a radiation delivery device. The target pedestal assembly comprises a pedestal weldment coupled to a linear accelerator of the radiation delivery device, the pedestal weldment including a first recessed area and a first channel adjacent to the first recessed area, the channel including a first flow diverter at a first end of the channel and a second flow diverter at a second end of the channel; a cover coupled to the pedestal weldment, the cover including a second recessed area complementary to the first recessed area and a second channel complementary to the first channel; and a target rotationally coupled to the pedestal weldment and the cover and positioned between the first recessed area and the second recessed area, a portion of the target positioned between the first channel and the second channel, the target being protected with a material sufficient to reduce chemical reactivity between the target and cooling medium flowing in the channel. 
         [0008]    In another embodiment, the invention provides a target pedestal assembly for a radiation delivery device. The target pedestal assembly comprises a housing coupled to a linear accelerator of the radiation delivery device, the housing defining a cavity and a channel adjacent to the cavity, the channel including a first flow diverter at a first end of the channel and a second flow diverter at a second end of the channel; a target positioned within the cavity and rotationally coupled to the housing, a portion of the target positioned within the channel, the target including a serrated edge arranged around a perimeter of the target; and a cooling medium positioned within the cavity and the channel, the target being protected with a material sufficient to reduce chemical reactivity between the target and the cooling medium. 
         [0009]    In yet another embodiment, the invention provides a target pedestal assembly for a radiation delivery device. The target pedestal assembly comprises a housing including a cavity and a cooling medium flowing within the cavity; and a target comprising tungsten and positioned within the cavity, the target submerged in the cooling medium, the target being protected with a material sufficient to reduce chemical reactivity between the target and the cooling medium. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a perspective view of a radiation therapy treatment system. 
           [0011]      FIG. 2  is a perspective view of a multi-leaf collimator that can be used in the radiation therapy treatment system illustrated in  FIG. 1 . 
           [0012]      FIG. 3  is a perspective view of the expanded target pedestal assembly. 
           [0013]      FIG. 4  is a perspective view of the target pedestal assembly. 
           [0014]      FIG. 5  is a perspective view of a pedestal weldment of the target pedestal assembly illustrated in  FIG. 4 . 
           [0015]      FIG. 6  is a top view of the pedestal weldment of the target pedestal assembly illustrated in  FIG. 4 . 
           [0016]      FIG. 7  is a perspective view of a cover of the target pedestal assembly illustrated in  FIG. 4 . 
           [0017]      FIG. 8  is a perspective view of a target of the target pedestal assembly illustrated in  FIG. 4 . 
           [0018]      FIG. 9  is a perspective view of a pin of the target pedestal assembly illustrated in  FIG. 3 . 
           [0019]      FIG. 10  is a cross-sectional view of the target pedestal assembly illustrated in  FIG. 4 . 
           [0020]      FIG. 11  is an enlarged cross-sectional view of a portion of the target pedestal assembly illustrated in  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. 
         [0022]    Although directional references, such as upper, lower, downward, upward, rearward, bottom, front, rear, etc., may be made herein in describing the drawings, these references are made relative to the drawings (as normally viewed) for convenience. These directions are not intended to be taken literally or limit the present invention in any form. In addition, terms such as “first,” “second,” and “third” are used herein for purposes of description and are not intended to indicate or imply relative importance or significance. 
         [0023]    In addition, it should be understood that embodiments of the invention include hardware, software, and electronic components or modules that, for purposes of discussion, may be illustrated and described as if the majority of the components were implemented solely in hardware. However, one of ordinary skill in the art, and based on a reading of this detailed description, would recognize that, in at least one embodiment, the electronic based aspects of the invention may be implemented in software. As such, it should be noted that a plurality of hardware and software based devices, as well as a plurality of different structural components may be utilized to implement the invention. Furthermore, and as described in subsequent paragraphs, the specific mechanical configurations illustrated in the drawings are intended to exemplify embodiments of the invention and that other alternative mechanical configurations are possible. 
         [0024]      FIG. 1  illustrates a radiation therapy treatment system  10  that can provide radiation therapy to a patient  14 . The radiation therapy treatment can include photon-based radiation therapy, brachytherapy, electron beam therapy, proton, neutron, or particle therapy, or other types of treatment therapy. The radiation therapy treatment system  10  includes a gantry  18 . The gantry  18  can support a radiation module  22 , which can include a radiation source  24  and a linear accelerator  26  (a.k.a. “a linac”) operable to generate a beam  30  of radiation. Though the gantry shown in the drawings is a ring gantry, i.e., it extends through a full 360° arc to create a complete ring or circle, other types of mounting arrangements may also be employed. For example, a C-type, partial ring gantry, or robotic arm could be used. Any other framework capable of positioning the radiation module  22  at various rotational and/or axial positions relative to the patient  14  may also be employed. In addition, the radiation source  24  may travel in a path that does not follow the shape of the gantry  18 . For example, the radiation source  24  may travel in a non-circular path even though the illustrated gantry  18  is generally circular-shaped. The gantry  18  of the illustrated embodiment defines a gantry aperture  32  into which the patient  14  moves during treatment. 
         [0025]    The radiation module  22  can also include a modulation device  34  operable to modify or modulate the radiation beam  30 . The modulation device  34  provides the modulation of the radiation beam  30  and directs the radiation beam  30  toward the patient  14 . Specifically, the radiation beam  30  is directed toward a portion  38  of the patient. Broadly speaking, the portion may include the entire body, but is generally smaller than the entire body and can be defined by a two-dimensional area and/or a three-dimensional volume. A portion or area desired to receive the radiation, which may be referred to as a target or target region, is an example of a region of interest. Such modulation is sometimes referred to as intensity modulated radiation therapy (“IMRT”). Another type of region of interest is a region at risk. If a portion includes a region at risk, the radiation beam is preferably diverted from the region at risk. 
         [0026]    The system  10  can also include a patient support, shown as a couch  82 , operable to support at least a portion of the patient  14  during treatment and a drive system  86  operable to manipulate the location of the couch  82  based on instructions provided by the computer  74 . The couch  82  moves along at least one axis  84  in the x, y, or z directions. In other embodiments of the invention, the patient support can be a device that is adapted to support any portion of the patient&#39;s body. The patient support is not limited to having to support the entire patient&#39;s body. The drive system  86  can be controlled by the computer  74 . 
         [0027]    The computer  74 , illustrated in  FIGS. 1 and 2 , includes an operating system for running various software programs and/or a communications application. In particular, the computer  74  can include a software program(s)  90  that operates to communicate with the radiation therapy treatment system  10 . 
         [0028]      FIGS. 3-4  illustrate a target pedestal assembly  102  according to one embodiment of the present invention. The pedestal assembly  102  allows for consistent rotation of a fluid-cooled x-ray target. The target pedestal assembly  102  allows for improved rotation of the target and provides an environment for the target that is conducive to the dissipation of heat away from the target. The pedestal assembly  102  can be composed of metal and can vary in shape. The target pedestal assembly  102  allows for the implementation of several target-cooling techniques, including guided fluid flow, consistent rotation of the target, and protection of the target from extreme heat and attack from chemically active components in the cooling medium or fluid. Such chemically active components may include decomposition products such as free radicals. Moreover, the cooling medium or fluid may be in liquid or gaseous phases, or a combination thereof. 
         [0029]    The target pedestal assembly  102  includes an internal snap ring  106 , an electron filter  110 , a pedestal subassembly or housing  114 , a target  118 , a rotational pin  122 , a target cover  126 , and a plurality of fasteners (e.g., screws)  130  to attach the target cover  126  to the pedestal weldment  114 . 
         [0030]    The internal snap ring  106  is constructed of a suitable material, such as stainless steel, that resists corrosion and is of the type used for pieces of similar application. The internal snap ring  106  holds the electron filter  110  in place within the pedestal weldment  114 . The snap ring  106  is removable and replaceable. Because the snap ring  106  is removable, the electron filter  110  is replaceable. 
         [0031]    The pedestal subassembly  114 , further illustrated in  FIGS. 5-6 , can be brazed, welded, bolted or coupled together by suitable fasteners. The pedestal subassembly  114  includes a base  134  having a plurality of apertures  138  that allow for its attachment as part of the greater radiation therapy system  10 . The pedestal weldment  114  also includes a raised portion  142  extending from the base  134  and generally centered within the base  134 . The raised portion  142  includes a generally planar first surface  146  having a plurality of apertures  150  each adapted to receive one of the fasteners  130 . The raised portion  142  also includes a second generally planar surface  154  defining a recess  156  formed within and non-co-planar with the first surface  146  and configured to receive the target  118 . The recess  156  includes an aperture  160  and a raised portion  164  generally surrounding the aperture  160 . The aperture  160  is offset from a center point of the raised portion  142 . The aperture  160  is configured to receive the pin  122  such that the target  118  is rotationally secured within the recess  156  with the rotational pin  122 . 
         [0032]    The raised portion  142  also includes a third generally planar surface  168  at an elevation lower than the second surface  154  and defining a channel  172  formed within the first surface  146 . A portion of the channel  172  is positioned adjacent to the recess  156  and includes a first end  176  and a second end  180 . A portion of the target  118  is positioned over the channel  172  with a gap existing between the target  118  and the third surface  168  due to the third surface  168  being at a lower elevation than the second surface  154 . The first end  176  and the second end  180  of the channel  172  include a ramped surface  184  extending gradually to a lower elevation than the third surface  168 . 
         [0033]    The target  118 , further illustrated in  FIGS. 6 and 8 , can include a serrated perimeter  188  and an aperture  192  at its center to accommodate the rotational pin  122 . Although  FIGS. 6 and 8  illustrate the target  118  having a serrated perimeter, the target  118  can include a smooth or other suitable shaped perimeter. The target  118  is comprised of a refractory metal with a high melting point such as tungsten, molybdenum, rhenium, or any suitable alloys, of the type that produce x-rays when impacted by a high energy electron beam  30 . In one embodiment, the target  118  is a solid, disc-shaped piece of metal at least partially covered with a protective agent that has good thermoconductivity and little or no chemical reactivity, especially with the cooling fluid such as water or its components. The target  118  can be protected with various materials including diamond, chromium nitride, titanium nitride, or iridium. 
         [0034]    The protective agent can be applied to the target  118  via several processes such as by physical vapor deposition. In one embodiment, a tungsten target is coated with chromium nitride through a physical vapor deposition process so that the coating is atomically bonded to the target material. 
         [0035]    The target  118  also can be completely encased in a chemically resistant protective coating. The casing material may be bonded (e.g., brazing, spin-welding, other types of welding, plating, and explosive bonding) to the target  118 . Various alloys could be used to encase the target, as well as chromium nitride, titanium nitride, or iridium, but in any embodiment, the casing material should be able to handle high amounts of radiation and should have little chemical reactivity to the cooling fluid and its decomposition products. When the casing material is brazed to the target  118 , extra care must be taken to ensure that there is good thermal contact between the target  118  and the casing to make sure that heat is efficiently transferred from the target through the casing to the coolant. 
         [0036]    Another way to protect the target is to braze a protective surface to a portion of the target  118  in order to spread out the heat generated at the point where the electron beam  30  impacts the target  118 . The protective surface could be applied to the target  118  in an annulus or ring shape, for example, covering the path of the electron beam  30  about the rotating target  118 . The protective surface on the target  118  may be applied outside the beam path. The protective surface can be made of any number of alloys, such as a zirconium alloy, that is capable of withstanding a significant amount of radiation. In some embodiments, both the top and the bottom surfaces of the target  118  may include a protective surface. In addition, some or all of the serrated edges of the target  118  can include a protective surface. 
         [0037]    Coating, encasing, brazing, or otherwise covering the target  118  with a protective material facilitates steady rotation of the target. Even as the protective material degrades along the electron beam path  30 , the protective material stays intact around the rotational pin  122 , which is where rotational support occurs. Thus, the protective coating on the target  118  helps to keep rotation steady by maintaining the diameter of the aperture  192 , spreading the heat evenly across the target  118 , and prolonging target life. 
         [0038]    The rotational pin  122 , further illustrated in  FIG. 9 , is configured to be received in the aperture  192  of the target  118  and the aperture  160  of the recess  156  to rotationally secure the target  118  to the pedestal weldment  114 . The pin  122  includes a beveled edge at a first end and a second end thereof. The rotational pin  122  is replaceable, making possible refurbishment of the target pedestal assembly  102 . The pin  122  has a high lubricity, which allows for high target rotational speeds with minimal wear of the target aperture  192 . In one embodiment, the pin  122  is comprised of tungsten carbide, making it resistant to oxidation, thermal effects, and mechanical wear and making it superior to a stainless steel pin, which would mechanically wear due to continuous contact with, for example, a tungsten target. 
         [0039]    The pin  122  includes a predetermined length to maintain a minimum gap between the raised portion  164  and raised portion  216 . The gap prevents the target  118  from being pinched thereby slowing or eliminating rotation. The rotational pin  122  also protects the target  118  from the target cover  126  by supporting the target within the target cavity  202 . The rotational pin  122  does not pass through the aperture  212  in the target cover  126 . By keeping the pin  122  in the cover, it prevents the linac window from being punctured by the pin  122  and releasing the vacuum. Moreover, the target cover  126  gets hot when the electron beam is operating, and the cover may bow downward toward the target  118 . The pin  122  supports the target cover such that even with the bowing the target cover does not contact the target  126  and high rotational speeds can be maintained. 
         [0040]    The cover  126 , further illustrated in  FIG. 7 , includes a first generally planar surface  196  having a plurality of apertures  200  each adapted to receive one of the fasteners  130  to connect the target cover  126  to the pedestal weldment  114 . The cover  126  also includes a second generally planar surface  204  defining a recess  208  formed within the first surface  196  and configured to receive the target  118 . The recess  208  includes an aperture  212  and a raised portion  216  generally surrounding the aperture  212 . The aperture  212  is offset from a center point of the cover  126  and is substantially aligned with the aperture  160  on the pedestal weldment  114  when the cover  126  is attached thereto. The aperture  212  is configured to receive one end of the pin  122  such that the target  118  is rotationally secured within the recess  208  with the rotational pin  122 . The recess  208  of the cover  126  and the recess  156  of the pedestal weldment  114  form a cavity  202  (illustrated in  FIGS. 4 and 10 ) in which the target  118  is configured to rotate when the cover  126  is connected to the pedestal weldment  114 . 
         [0041]    The cover  126  also includes a third generally planar surface  220  at an elevation lower than the second surface  204  and defining a channel  224  formed within the first surface  196 . A portion of the channel  224  is positioned adjacent to the recess  208  and includes a first end  228  and a second end  232 . A portion of the target  118  is positioned over the channel  224  with a gap existing between the target  118  and the third surface  220  due to the third surface  220  being at a lower elevation than the second surface  204 . The first end  228  and the second end  232  of the channel  224  include a ramped surface  236  extending gradually to a higher elevation than the third surface  220 . The cover  126  also includes a window  240  positioned within the channel  224  and between the first end  228  and the second end  232 . 
         [0042]    As discussed above, the cover  126  is connected to the pedestal weldment  114  with the plurality of fasteners  130 . As illustrated in  FIGS. 10-11 , the fasteners  130 , such as 100 degree flat head screws, and counter sink do not break through the cavity  202  and thus cannot impede target rotation.  FIG. 10  further illustrates a side view of the cavity  202 , the target  118 , and the pin  122 . The rotational pin  122  does not extend through the target cover  126 . 
         [0043]    As illustrated in  FIG. 7 , the ramped surfaces  236  at the first end  228  and the second end  232  form integrated flow diverters which direct the flow of cooling fluid (e.g., water) to and around the target  118  and support a generally straight tangential, parallel flow of cooling fluid across the target  118 . The integrated flow diverters guide the flow of cooling fluid to and around the target  118 . The flow diverters push the fluid flow to the perimeter of the target  118 , engaging the target&#39;s serrated edges for better rotation, while at the same time not reducing the coolant flow across the target surface that is being impacted by the electron beam  30 . The target  118  can achieve higher rotational speeds, such as about 3800-4100 RPM, when the cover  126  with integrated flow diverters are used as compared to speeds of only 1800 RPM with a partial target cover lacking the flow diverters. 
         [0044]    The use of the target cover  126  with integrated flow diverters allows for flattening of the coolant flow while keeping consistent flow across the top and bottom surfaces of the target  118 . The ramped surfaces  236  guide the cooling fluid as it either enters or exits the target cavity  202 . Water is circulated through the flow diverters and over the target  118 . The water or other coolant fluid cools the target  118  because the heat generated by the electron beam contacting the target is transferred from the target to the flowing water. The ducted, parallel fluid flow with the flow diverters reduces out-of-plane torque on the target  118 , which aids in consistent rotation of the target, as out-of-plane torque can impede rotation. The parallel movement of the fluid reduces the tendency of the target  118  to tip and jam, thereby allowing for high rotational speed. 
         [0045]    To produce x-rays as part of the radiation therapy system  10 , the linear accelerator  26  produces a high-energy electron beam  30  that moves through a linac exit window. The electron beam  30  is aimed at the target  118  to produce x-ray radiation. The target  118  can be integral to the linac  26 , or it may be housed outside the linac&#39;s vacuum environment. In one construction, the target  118  is external to the linac  26  and is supported by the target pedestal assembly  102 . 
         [0046]    In operation and according to one embodiment, the electron beam  30  impacts the target near the serrated edge of the target  118 . The target  118  is at least partially positioned between the fluid channel  172 ,  224  such that the serrated edge of the target  118  engages the flow of cooling fluid across the target  118  like paddle wheels, resulting in the passive rotation of the target  118 . Rotation is passive because it is achieved by a viscous drag of the coolant on the target  118  surface and/or by momentum transfer of the coolant on the serrated edge. The target  118  can rotate in order to spread out the electron beam pulses, prevent overlap of those pulses, and allow for adequate dissipation of heat to preserve the target  118 . As the target  118  rotates, the electron beam  30  contacts the target  118  in a circular path rather than at only a singular impact point, as would be the case with a stationary target. The target cover  126  directs the flow of cooling fluid through the channel  172 ,  224  such that the target  118  is submerged in the cooling fluid. The electron beam  30  passes through the fluid. The fluid moves in direct contact with the target  118  with a flow that is substantially parallel to the target  118  and provides a protected environment to allow for consistent rotation of the target  118 . This directed, controlled flow allows for high target rotational speed of the target  118  and for heat transfer away from the target  118 . 
         [0047]    Still other embodiments are within the scope of the invention. For example, water is not the only coolant that may be used in the target-cooling system. Moreover, having described several embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the claims. A number of modifications may be made to the present invention without departing from the inventive concept therein. 
         [0048]    Various features and advantages of the invention are set forth in the following claims.