Abstract:
An apparatus and method to deliver intensity modulated radiation therapy by irradiating a treatment volume with rotation of the radiation beam. The system includes a collimation device comprising a two-dimensional array of pivoting attenuating leaves, which are temporarily placed into the radiation beam path as the gantry rotates around the patient. The leaves are independently movable between a first position and a second position. The radiation beam intensity is modulated by controlling the time that each leaf is present to attenuate the beam.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. Provisional Patent Application No. 60/657,013, filed on Feb. 28, 2005, the entire content of which is incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     Intensity modulated radiation therapy (“IMRT”) involves changing the size, shape, and intensity of a radiation beam to conform to the size, shape, and location of a patient&#39;s tumor. IMRT precisely targets the tumor while sparing surrounding normal tissue. Generally, IMRT is delivered using a radiation therapy system equipped with a radiation source and a linear accelerator (“linac”). The radiation beam exiting the linac travels through a multi-leaf collimator (“MLC”) to change the size, shape, and intensity of the beam.  
         [0003]     Prior to the development of the linac, radiation therapy machines used Cobalt-60 (“Co-60”) as the radiation source. Co-60 radiation therapy machines use simple reliable technology that has been available since the 1950s. The Co-60 radioactive source is mounted in the radiation therapy machine. To deliver a dose to the patient, the radiation beam from the Co-60 source is collimated by jaws in a manner similar to that of the linac. Field shaping blocks may also be used to further shape the beam. Co-60 characteristics are stable and predictable, but generally require longer treatment times than the linac. Because the energy of Co-60 radiation is generally lower than that of a linac, Co-60 radiation therapy machines are normally only used to treat relatively shallow diseases such as those of the head and neck. Co-60 radiation therapy machines have not been retrofitted with a MLC, and MLC-based IMRT is not currently available with Co-60 radiation therapy machines.  
       SUMMARY  
       [0004]     Development of the MLC has enhanced the delivery of radiation therapy treatment. The MLC defines the radiation beam to focus on the size, shape, and location of the patient&#39;s tumor. The MLC also modulates the radiation beam to minimize the amount of radiation delivered to normal tissues and organs surrounding the tumor.  
         [0005]     The MLC includes a plurality of radiation blockers, called leaves. Each individual leaf in a MLC can be positioned independently allowing the user to create an infinite number of irregularly shaped fields. The radiation beams are directed between the ends of opposing arrays of the radiation blocking leaves, thereby shaping the radiation beam to closely match the shape of the tumor or target area, while shielding the normal tissues and organs from the direct radiation beam.  
         [0006]     Other methods and apparatus for modulating the radiation beam include utilizing a plurality of radiation beam segments. Such methods and apparatus utilize attenuating leaves, or shutters, in a rack positioned in the path of the radiation beam before the beam enters the patient. The tumor is exposed to radiation in slices, each slice being selectively segmented by the leaves. However, a disadvantage of this method and apparatus results from the fact that only two slices of tissue volume may be treated with one rotation of the gantry. Although the slices may be of arbitrary thickness, greater resolution is accomplished by selecting slices for treatment that are as thin as possible. As the thickness of the treatment slice decreases, the time it takes to treat the patient increases because more treatment slices are required in order to treat the entire target volume.  
         [0007]     Most modem linacs include a conventional MLC not originally designed for IMRT. Conventional MLC techniques provide limited dose conformality, based on the MLC design, number of beams and treatment time. IMRT machines require stringent position and speed tolerances that are limited by current collimator technology. Accordingly, there is a need for an improved collimator system and method for use on dynamic IMRT machines that improves dose conformality and resolution.  
         [0008]     The present invention provides for multi-slice collimation radiation therapy within Co-60 or linac-based IMRT machines. It is a system and method for delivering IMRT by irradiating a treatment volume (containing multiple patient CT slices) with a single rotation of the radiation beam. The system includes a collimation device comprising a two-dimensional array of pivoting leaves, which are temporally placed into the radiation beam path as the gantry rotates around the patient. The two-dimensional array of pivoting leaves is positioned within a two-dimensional array of binary modulated slit apertures. The leaves are independently movable between a first state and a second state. The intensity of the radiation beam is modulated by controlling the time that each leaf is present in the slit aperture to attenuate the beam. In the present invention, multiple slices are irradiated simultaneously in a single gantry rotation.  
         [0009]     In one embodiment, the invention provides a radiation therapy treatment system comprising a collimator including a plurality of leaf assemblies. Each of the leaf assemblies includes a support structure, a plurality of members supported by and extending from the support structure, and a plurality of leaves, each leaf supported by one of the members and adapted to move between a first position and a second position upon actuation. The arrangement of the leaf assemblies provides a two-dimensional array, the collimator operable to deliver radiation to a patient in a plurality of slices.  
         [0010]     In another embodiment, the invention provides a radiation therapy treatment system comprising a gantry, a collimator, and a controller. The collimator is supported by the gantry and includes a plurality of leaf assemblies. Each leaf assembly includes a support structure, a plurality of members supported by and extending from the support structure, a plurality of leaves, each leaf supported by one of the members and adapted to move upon actuation, and a plurality of actuators, each actuator supported by one of the members and coupled to one of the leaves. The arrangement of the leaf assemblies provides a two-dimensional array and the controller is operable to selectively instruct the actuators to move the leaves to provide temporal and spatial modulation of a radiation beam.  
         [0011]     In another embodiment, the invention provides a method of delivering radiation therapy treatment to a patient. The method comprises the acts of positioning the patient in a radiation therapy treatment system including a radiation source and a gantry operable to rotate around the patient, temporally and spatially modulating a radiation beam generated by the radiation source, and delivering a plurality of slices of radiation to the patient during a single rotation of the gantry.  
         [0012]     In yet another embodiment, the invention includes a radiation therapy treatment system comprising a collimator and means for delivering a plurality of slices of radiation to a patient, the delivery means supported by the collimator.  
         [0013]     The present invention makes clinical Co-60 IMRT possible, by using or incorporating IMRT into lower cost Co-60 radiation therapy machines. The present invention could be retro-fitted to existing Co-60 radiation therapy machines or linac-based radiation therapy machines.  
         [0014]     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a perspective view of a radiation therapy treatment system.  
         [0016]      FIG. 2  is a perspective view of a multi-leaf collimator according to one embodiment of the invention that can be used in the radiation therapy treatment system illustrated in  FIG. 1 .  
         [0017]      FIG. 3  is a side view of the multi-leaf collimator of  FIG. 2 .  
         [0018]     FIGS.  4 A-B illustrate views of a leaf of the multi-leaf collimator of  FIG. 2 .  
         [0019]     FIGS.  5 A-C schematically illustrate an array of leaves of the multi-leaf collimator of  FIGS. 2 and 6 - 8 .  
         [0020]      FIG. 6  is a side view of another construction of the multi-leaf collimator of  FIG. 2 .  
         [0021]      FIG. 7  is a side view of another construction of the multi-leaf collimator of  FIG. 2 .  
         [0022]      FIG. 8  is a side view of another construction of the multi-leaf collimator of  FIG. 2 .  
         [0023]      FIG. 9  schematically illustrates a method for radiation delivery using the multi-leaf collimator of  FIG. 2 .  
     
    
     DETAILED DESCRIPTION  
       [0024]     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 herein of “including,” “comprising,” or “having” and variations thereof 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. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.  
         [0025]     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.  
         [0026]     In addition, it should be understood that embodiments of the invention include both 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.  
         [0027]      FIG. 1  illustrates a radiation therapy treatment system  10  that can provide radiation therapy to a patient  14 . Often, radiation is directed to a specific area or part of the patient  14 , which is referred to and illustrated as a target  18 . The radiation therapy treatment system  10  includes a gantry  22 . Though the gantry  22  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 or partial ring gantry could be used. Any other framework capable of positioning the radiation source at various rotational and/or axial positions relative to the patient may also be employed.  
         [0028]     The gantry  22  supports a radiation source  26 . The radiation source  26  can include a linear accelerator  30  or a Co-60 source operable to generate a beam  34  of photon radiation. Other radiation sources may also be employed; for example, a Cobalt-60 radiation source, or any other source capable of delivering radiation of therapeutic or diagnostic benefit to the patient. The radiation source  26  can also include a modulation device  38  operable to modify or modulate the radiation beam  34 . The modulation device  38  provides modulation needed to vary or modulate the intensity of photon radiation delivered to the patient  14 . Such modulation is sometimes referred to as intensity modulated radiation therapy (“IMRT”).  
         [0029]     The modulation device  38  can include a collimation device  42  as illustrated in  FIG. 2 . The collimation device  42  includes a set of jaws  46  that define and adjust the size of an aperture  48  through which the radiation beam  34  may pass. The jaws  46  include an upper jaw  50  and a lower jaw  52 . The upper jaw  50  and the lower jaw  52  are moveable to adjust the size of the aperture  48 . The lower jaw  52  can also adjust the width of the beam  34  for a single slice.  
         [0030]     The collimation device  42  also includes a plurality of leaf assemblies  54  operable to move between first and second positions to provide intensity modulation. Each of the leaf assemblies  54  includes a leaf  58 , which can move between the first and second positions. The plurality of leaf assemblies  54  and jaws  46  modulate the intensity, size, and shape of the radiation beam  34  before the radiation beam  34  reaches the target  18 . The jaws  46  are independently controlled by an actuator  62  (such as a motor, a pneumatic cylinder, a relay, a hydraulic cylinder, and the like) so that the size of the aperture  48  can be adjusted according to the treatment parameters. The actuator  62  can be controlled by a computer  66 , controller, or similar device.  
         [0031]     Generally, the thickness of the radiation beam slice is defined by the space between adjacent leaf assemblies  54 . The slice thickness can be modified by closing the lower jaws  52  to a position that would define a narrower slice thickness than the space between adjacent leaf assemblies  54 . The plurality of leaf assemblies  54  define a plurality of radiation beam slices and allow a plurality of the radiation beam slices to penetrate the target  18 . One technique for delivering multi-slice radiation therapy is that each leaf assembly  54  would provide a treatment slice only until the area treated by an adjacent leaf assembly  54  is reached. For example, as the gantry  22  rotates around the target  18 , the leaf assemblies  54  are sequentially activated to deliver its treatment slice. Alternatively, all (or some as prescribed) of the leaf assemblies  54  can be activated to deliver the treatment slice. Another technique for delivering multi-slice radiation therapy is defined for extended targets  18 . Each leaf assembly  54  delivers a radiation slice to treat the entire target  18  volume as discussed below with respect to  FIG. 9 .  
         [0032]     The radiation therapy treatment system  10  can also include a detector  70  ( FIG. 1 ) operable to detect the radiation beam  34 , a couch  74  to support the patient  14 , and a drive system  78  operable to manipulate the location of the couch  74  based on instructions provided by the computer  66 . The linear accelerator  30  or a Co-60 source and the detector  70  can also operate as a computed tomography (CT) system to generate CT images of the patient  14 . The linear accelerator  30  emits the radiation beam  34  toward the target  18  in the patient  14 . The target  18  and surrounding tissues absorb some of the radiation.  
         [0033]     The CT images can be acquired with a radiation beam  34  that has a fan-shaped geometry, a multi-slice geometry, or a cone-beam geometry. In addition, the CT images can be acquired with the linear accelerator  30  or Co-60 source delivering megavoltage energies or kilovoltage energies. It is also noted that the acquired CT images can be acquired from the radiation therapy treatment system  10  or other image acquisition devices, such as other CT scanners, MRI systems, and PET systems. For example, previously acquired CT images for the patient  14  can include identified regions of interest and/or regions at risk. Newly acquired CT images for the patient  14  can be registered with the previously acquired CT images to assist in identifying the regions of interest and/or regions at risk in the new CT images.  
         [0034]     In some embodiments, the radiation therapy treatment system  10  can include an x-ray source and a CT image detector. The x-ray source and the CT image detector operate in a similar manner as the linear accelerator  30  or a Co-60 source and the detector  70  to acquire image data. The image data is transmitted to the computer  66  where it is processed to generate cross-sectional images or “slices” of the patient&#39;s body tissues and organs.  
         [0035]     The computer  66 , illustrated in  FIG. 2 , includes an operating system  82  for running various software programs and/or communication applications. In particular, the computer  66  can include a software program or programs  86  that facilitate communication between the computer  66  and the radiation therapy treatment system  10  or other devices assisting in the radiation treatment process such as a laser positioning system or other computers. The computer  66  can include suitable input/output devices adapted to be accessed by medical personnel or technicians. The computer  66  can include typical hardware such as a processor, I/O interfaces, and storage devices or memory. The computer  66  can also include input devices such as a keyboard and a mouse. The computer  66  can further include output devices, such as a monitor. In addition, the computer  66  can include peripherals, such as a printer and a scanner.  
         [0036]     According to one embodiment of the present invention, the collimation device  42 , as illustrated in  FIG. 2 , includes a plurality of leaf assemblies  54  or leaf banks. The space between adjacent leaf assemblies  54  defines a path  90  (see  FIG. 3 ) for the radiation beam  34 . Only one leaf assembly  54  is described herein, however, it is noted that the description applies to each of the leaf assemblies  54 . The leaf assembly  54  includes a support structure or frame  94  adapted to support a plurality of leaves  58 . The leaves  58  are positioned generally adjacent to one another, but could include a space between adjacent leaves  58 . In addition, the sides of the leaves  58  can be angled or tapered, for example, in a tongue and groove arrangement, to reduce potential radiation leakage between adjacent leaves. The collimator  42  and/or leaf assembly  54  can include a primary collimator  60  having radiation attenuation material (see  FIG. 3 ) adapted to attenuate the radiation beam  34  and define the slice thickness. The radiation attenuation material can comprise tungsten, tungsten alloy or other suitable material. The radiation attenuation material  60  is positioned above the leaves  58  and can be positioned closer to the radiation source  26  than illustrated in  FIG. 3 . It is noted that the attenuation material  60  is arranged to diverge with respect to the radiation source  26 . For example, the attenuation material  60  generally follows the divergence of the leaf assemblies as illustrated in  FIG. 2 .  
         [0037]     As shown more clearly in  FIG. 3 , the leaf assembly  54  includes a wall or a member  98  extending in a generally downward direction from the frame  94 . The member  98  includes a first end  102  and a second end  106 . The member  98 , near the first end  102 , includes a plurality of apertures  110  (see  FIG. 2 ), each adapted to receive at least a portion of one of the leaves  58  (described below). The member  98 , near the second end  106 , includes a lip  114  adapted to extend in a direction substantially perpendicular to the member  98 . The lip  114  extends the length of the member  98  and includes a plurality of slits  118 . Each slit  118  is adapted to receive at least a portion of one of the leaves  58 . The frame  94 , the member  98 , and the lip  114  can be comprised of steel or other suitable material.  
         [0038]     The leaf assembly  54  can also include a plurality of leaf guides  122  coupled to the frame  94  and the member  98 . The leaf guides  122  are arranged near the first end  106  and along the length of the member  98  with a space between adjacent leaf guides  122 . Each leaf guide  122  includes a recess  126  adapted to receive at least a portion of one of the leaves  58 . The leaf guide  122  includes a shaft  130  adapted to receive a leaf  58  such that the leaf  58  pivots about the shaft  130 .  
         [0039]     Each leaf  58  is adapted to pivot about its respective shaft  130  from a first position  134  to a second position  138 . For example, a leaf  58  can pivot from the first position  134  (a closed position) to the second position  138  (an open position) or to a position between the first position  134  and the second position  138 . The leaves  58  can be comprised of tungsten, a tungsten alloy, or other suitable material.  
         [0040]     The leaf  58  is biased toward the first position  134 , e.g., a closed position, with a biasing device  142 , such as a spring or other elastic device. The biasing device  142  is coupled to the frame  94  and extends in a direction generally parallel with the member  98 . The biasing device  142  exerts a generally downward force on the leaf  58  that causes the leaf  58  to pivot about the shaft  130  to the first position  134  (a closed position). The positioning of the leaf  58  is controlled by an independent actuator  146  (such as a motor, a pneumatic cylinder, a solenoid, a hydraulic cylinder, and the like) to permit or block the passage of radiation. In this construction, the actuator  146  is a pneumatic cylinder having a piston moved by air pressure. In other constructions, the actuator  146  can include electromechanical or hydraulic means to move the leaf  58 .  
         [0041]     As illustrated in  FIGS. 2 and 3 , the actuator  146  includes a piston  148 , which moves in a direction generally parallel with the member  98  to apply an upwardly directed force to the leaf  58 . This force compresses the biasing device  142  and moves the leaf  58  in a direction toward the second position  138  or a position between the first position  134  and the second position  138 . The upward force of the piston  148  on the leaf  58  causes the path  90  to open and allow the radiation beam  34  through the path  90 . The radiation beam  34  is modulated based on the position of each of the leaves  58  in the collimation device  42 .  
         [0042]     Each of the actuators  146  is controlled by the computer  66 . The computer  66  receives input, via the software program  86 , regarding the patient&#39;s treatment plan, which includes the prescription for the radiation dose. The computer  66  processes the treatment plan to determine the position and timing of each of the leaves  58  and corresponding actuators  146  as the gantry  22  rotates around the patient  14 . For example, in the construction illustrated in  FIGS. 2 and 3 , the computer  66  instructs one of the actuators  146  to move a certain distance and to remain in a certain position for a predetermined period of time such that the corresponding leaf  58  moves from the first position  134  to a second position  138  to allow passage of the radiation beam  34  through the respective path  90  based on the prescribed dose and gantry location.  
         [0043]     With reference to  FIGS. 3, 4A , and  4 B, the leaf  58  includes a first portion  150 , which includes an axis  158  that extends generally parallel with the member  98  when the leaf  58  is positioned on the shaft  130  of the leaf guide  122  and in the second position  138 . The first portion  150  includes a first end  162  and a second end  166  (see  FIG. 4B ). The first end  162  includes an aperture  170  adapted to receive the shaft  130  of the leaf guide  122  and an extension  174  that extends in a direction generally perpendicular to the axis  158  of the first portion  150 .  
         [0044]     The first portion  150  includes an upper surface  178  generally oriented to be parallel with the axis  150  and a lower surface  182  generally oriented to be parallel with the axis  150 . The lower surface  182  is in contact or substantial contact with the member  98  when the leaf  58  is in the second position  138 . The lower surface  182  includes a length  186  and extends beyond the lip  114  of the member  98 . The length  186  of the lower surface  182  of each of the leaves  58  can vary with respect to one another. The upper surface  178  includes a length  194  generally less than the length  186  of the lower surface  182  of the leaf  58 . The length  194  of the upper surface  178  of each leaf  58  can vary with respect to one another. The first portion  150  further includes a surface  198  that connects the upper surface  178  and the lower surface  182 . The surface  198  is oriented at an angle  202  with respect to the axis  158 . The angle  202  can be in the range of about 10 degrees to about 75 degrees. The surface  198  includes a member  206  that extends outwardly from at least a portion of the surface  198 . The member  206  is adapted to be received in the slit  118  of the lip  114  of an adjacent leaf assembly  54  when the leaf  58  is in the first position  134 .  
         [0045]     The first portion  150  of the leaf  58  also includes a front surface  210  and a rear surface  214 . The leaf  58  includes a front member  218  coupled to the front surface  210  and a rear member  222  coupled to the rear surface  214 . The front member  218  is further described herein with respect to the front surface  210 , and the rear member  222  is not further discussed, however, it is noted that the rear member  222  is similarly arranged with respect to the rear surface  214 .  
         [0046]     The front member  218  includes an upper surface  226  generally coplanar with respect to the upper surface  178  of the first portion  150  of the leaf  58 . For example, the upper surface  226  of the front member  218  is generally parallel with respect to the axis  158  of the leaf  58 . The front member  218  also includes a lower surface  230  generally coplanar with respect to the lower surface  182  of the first portion  150  of the leaf  58 . For example, the lower surface  230  of the front member  218  is generally parallel with respect to the axis  158  of the leaf  58 . The front member  218  further includes a surface  234  that connects the upper surface  226  and the lower surface  230  and is generally coplanar with respect to the surface  198  of the first portion  150  of the leaf  58 . In other constructions of the leaf  58 , the upper surface  226 , the lower surface  230 , and the surface  234  of the front member  218  can extend beyond respective upper surface  178 , lower surface  182 , and surface  198  of the first portion  150  such that the upper surface  226 , the lower surface  230 , and the surface  234  of the front member  218  are not coplanar with respect to the upper surface  178 , lower surface  182 , and surface  198  of the first portion  150 , respectively.  
         [0047]     As illustrated in  FIGS. 4A and 4B , the leaf  58  appears to include three components, the first portion  150 , the front member  218 , and the rear member  222 , however, in some constructions, the leaf  58  is constructed as a single component. The leaf  58  is comprised of tungsten, a tungsten alloy, or other suitable material.  
         [0048]     The plurality of leaf assemblies  54  create an array  250  of radiation paths  90  (beamlets) as illustrated in FIGS.  5 A-C. The array  250  of radiation paths  90  provide temporal and spatial modulation of the radiation source  26 . The array  250  of paths  90  can deliver radiation treatment to the patient  14  in a plurality of slices. A single slice  254  is defined by the paths  90  of the adjacent leaves  58  of a single leaf assembly  54 . The FIGS.  5 A-C illustrate a single slice  254  as a vertical column and a leaf assembly  54  as a column in the array  250 . Specifically,  FIG. 5A  illustrates an array of leaf assemblies  54  with each leaf  58  in the first position  134  (i.e., closed).  FIG. 5B  illustrates an array of leaf assemblies  54  with each leaf  58  in the second position  138  (i.e., open).  FIG. 5C  illustrates an array of leaf assemblies  54  with some leaves  58  in the first position  134  and some of the leaves  58  in the second position  138 .  
         [0049]      FIG. 6  illustrates another independent construction of the collimation device  42 . The collimation device  42  includes a plurality of leaf assemblies  270  or leaf banks. The space between adjacent leaf assemblies  270  defines a path  274  for the radiation beam  34 . Only one leaf assembly  270  is described herein, however, it is noted that the description applies to each of the leaf assemblies  270 . The leaf assembly  270  includes a support structure or frame  278  adapted to support a plurality of leaves  282 . The leaves  282  are positioned generally adjacent to one another, but a space could separate adjacent leaves  282 .  
         [0050]     The leaf assembly  270  includes a wall or a member  286  extending in a generally downward direction from the frame  278 . The member  286  includes a first end  290  and a second end  294 . The member  286 , near the first end  290 , includes an actuator  298  adapted to move the leaf  282  from a first position  302  (e.g., an open position) to a second position  306  (e.g., a closed position) or a position between the first position  302  and the second position  306 . The frame  278  and the member  286  can be comprised of steel or other suitable material.  
         [0051]     The leaf  282  includes an axis  310  that extends generally parallel with the member  286 . The leaf  282  includes a first surface  314  generally oriented to be parallel with the axis  310  and the member  286  when the leaf  282  is in the first position  302 . The first surface  314  is in contact or substantial contact with the member  286  while the leaf  282  is in the first position  302 . The first surface  314  includes a length  318  that extends beyond second end  294  of the member  286 . The length  318  of the first surface  314  of each of the leaves  282  can vary with respect to one another. The leaf  282  also includes a second surface  322  generally oriented to be parallel with the axis  310  and member  286  and is opposite the first surface  314 . The second surface  322  includes a length  326  generally less than the length  318  of the first surface  314  of the leaf  282 . The length  326  of the second surface  322  of each leaf  282  can vary with respect to one another. The second surface  322  is adapted to make contact with or substantial contact with a member  286  of an adjacent leaf assembly  270 . The leaf  282  further includes a third surface  330  that connects the first surface  314  and the second surface  322 . The third surface  330  is oriented at an angle  334  with respect to the axis  310 .  
         [0052]     The leaf  282  is biased toward the second position  306 , e.g., a closed position or the second surface  322  in substantial contact with an adjacent member  286 , with a biasing device  338 , such as a spring or other elastic device. The positioning of the leaf  282  is controlled by the actuator  298 . In this construction, the actuator  298  is a pneumatic cylinder having a piston moved by air pressure. In other constructions, the actuator  298  can include electromechanical or hydraulic means to move the leaf  282 . As illustrated in  FIG. 6 , the actuator  298  includes a piston  442 , which moves in a direction generally parallel with the member  286  to apply a downwardly directed force to the leaf  282 , which compresses the biasing device  338  and moves the leaf  282  in a direction toward the first position  302  or a position between the first position  302  and the second position  306 . The downward force of the piston  442  on the leaf  282  causes the path  274  to open and allow the radiation beam  34  through the path  274 . The radiation beam  34  is modulated based on the position of each of the leaves  282  in this construction of the collimation device  42 .  
         [0053]     Each of the actuators  298  is controlled by the computer  66 . The computer  66  receives input, via the software program  86 , regarding the patient&#39;s treatment plan, which includes the prescription for the radiation dose. The computer  66  processes the treatment plan to determine the position and timing of each of the leaves  282  and corresponding actuators  298  as the gantry  22  rotates around the patient  14 . For example, in the construction illustrated in  FIG. 6 , the computer  66  instructs one of the actuators  298  to move a certain distance along the axis  310  and to remain in a certain position for a predetermined period of time such that the corresponding leaf  282  moves from the second position  306  to a first position  302  to attenuate the radiation beam  34  traveling through the respective path  274  based on the prescribed dose and gantry location.  
         [0054]     The plurality of leaf assemblies  270  creates an array  250  of radiation paths  274  (beamlets) as illustrated in FIGS.  5 A-C. The array  250  of radiation paths  274  provides temporal and spatial modulation of the radiation source  26 . The array  250  of paths  274  can deliver radiation treatment to the patient  14  in a plurality of slices. A single slice  254  includes a path  274  from each leaf  282  at the same respective position of each leaf assembly  270 . For example, a single slice  254  is defined by the paths  274  of the first leaf  282  of each leaf assembly  270 . FIGS.  5 A-C illustrate a single slice  254  as a horizontal row and a leaf assembly  270  as a column in the array  250 . Specifically,  FIG. 5A  illustrates an array of leaf assemblies  270  with each leaf  282  in the second position  306 .  FIG. 5B  illustrates an array of leaf assemblies  270  with each leaf  282  in the first position  302 .  FIG. 5C  illustrates an array of leaf assemblies  270  with some leaves  282  in the first position  302  and some of the leaves  282  in the second position  306 .  
         [0055]      FIGS. 7 and 8  illustrate another construction of the collimation device  42 . In this construction, the collimation device  42  includes a plurality of leaf assemblies  450 . The space between adjacent leaf assemblies  450  defines a path  454  for the radiation beam  34 . Each leaf assembly  450  or leaf bank includes a support structure or frame  458  adapted to support a plurality of leaves  462 . The leaves  462  are positioned generally adjacent to one another, but a space could separate adjacent leaves  462 .  
         [0056]     The leaf assembly  450  includes a wall or a member  466  extending in a generally downward direction from the frame  458 . The member  466  includes a first end  470  and a second end  474 . The member  466 , near the second end  474 , includes a lip  478  adapted to extend in a direction substantially perpendicular to the member  466 . As illustrated in  FIG. 8 , the lip  478  can extend the length of the member  466  and, in some constructions, includes a plurality of slits  482 . Each slit  482  is adapted to receive at least a portion of one of the leaves  462 . The frame  458 , the member  466 , and the lip  478  can be comprised of steel or other suitable material.  
         [0057]     The leaf assembly  450  includes a shaft  486  coupled to the frame  458 . Each leaf  462  is adapted to pivot about its respective shaft  486  from a first position  490  to a second position  494 . For example, a leaf  462  can pivot from the first position  490  (a closed position) to the second position  494  (an open position) or to a position between the first position  490  and the second position  494 . The leaves  462  can be comprised of tungsten or other suitable material.  
         [0058]     In some constructions, the leaf assembly  450  can include a plurality of leaf guides  498  ( FIG. 8 ) coupled to the frame  458  and the member  466 . The leaf guides  498  are arranged near the first end  470  and along the length of the member  466  with a space between adjacent leaf guides  498 . Each leaf guide  498  includes a recess  502  adapted to receive at least a portion of one of the leaves  462 . The leaf guide  498  is adapted to support and guide the position of the leaf  462  as it moves from a first position  490  to a second position  494  or any position between the first position  490  and the second position  494 .  
         [0059]     The leaf  462  includes an axis  506  that extends generally parallel with the member  466  when the leaf is positioned in the second position  494 . The leaf  462  includes a first surface  510  generally oriented to be parallel with the axis  506  and the member  466  when the leaf is positioned in the second position  494 . The first surface  510  is in contact or substantial contact with the member  466 . The first surface  510  extends beyond the second end  474  of the member  466 . The leaf  462  also includes a second surface  514  generally oriented to be parallel with the first surface  510 . The length of the first surface  510  and the second surface  514  of each leaf  462  can vary with respect to one another. The leaf  462  further includes a third surface  518  that connects the first surface  510  and the second surface  514 . The third surface  518  is oriented at an angle  522  with respect to the axis  506 .  
         [0060]     In some constructions, the third surface  518  can include a member  526  or second leaf guide that extends outwardly from at least a portion of the surface  518 . The member  526  is adapted to be received in the slit  502  of the lip  482  of an adjacent leaf assembly  450  when the leaf  462  is in the first position  490 . The member  526  is adapted to support and guide the position of the leaf  462  as it moves from a first position  490  to a second position  494  or any position between the first position  490  and the second position  494 .  
         [0061]     The leaf  462  is biased toward the first position  490 , e.g., a closed position, with a biasing device  530 , such as a spring or other elastic device. The positioning of the leaf  462  is controlled by an independent actuator  534 . The actuator  534  in this construction includes a pull-type solenoid  538  and can include more than one pull-type solenoid. The solenoid  538  includes a link  542  or plunger coupled to the leaf  462 . The link  542  can be coupled to the leaf  462  near the first surface  510  of the leaf  462 . The pull-type solenoid  538  operates to apply a force or pull the leaf  462  toward the second position  494  (e.g., toward the member  466 ) with the link  542  when a magnetic field is generated. The generation of the magnetic field causes the leaf  462  to open the path  454  for passage of the radiation beam  34 . When the magnetic field is deactivated, the biasing device  530  applies a force to the leaf  462  and pushes the leaf  462  toward the first position  490  (e.g., toward the member  466  of the adjacent leaf assembly  450 ). The radiation beam  34  is modulated based on the position of each leaf  462 .  
         [0062]     Each of the actuators  534  is controlled by the computer  66 . The computer  66  receives input, via the software program  86 , regarding the patient&#39;s treatment plan, which includes the prescription for the radiation dose. The computer  66  processes the treatment plan to determine the position and timing of each of the leaves  462  and corresponding actuators  534  as the gantry  22  rotates around the patient  14 . For example, in the construction illustrated in  FIGS. 7 and 8 , the computer  66  instructs one of the solenoids  538  to generate a magnetic field or to deactivate the magnetic field for a predetermined period of time such that the corresponding leaf  462  moves from the first position  490  to the second position  494  to attenuate the radiation beam  34  traveling through the respective paths  454  based on the prescribed dose and gantry location.  
         [0063]     The plurality of leaf assemblies  450  creates an array  250  of radiation paths  454  (beamlets) as illustrated in FIGS.  5 A-C. The array  250  of leaf assemblies  450  can deliver radiation treatment to the patient  14  in a plurality of slices  254 . The array  250  of radiation paths  454  provide temporal and spatial modulation of the radiation source  26 . A single slice  254  includes a path  454  from each leaf  462  at the same respective position of each leaf assembly  450 . For example, a single slice  254  is defined by the paths  454  of the first leaf  462  of each leaf assembly  450 . The FIGS.  5 A-C illustrate a single slice  254  as a horizontal row and a leaf assembly  450  as a column in the array  250 . Specifically,  FIG. 5A  illustrates an array of leaf assemblies  450  with each leaf  462  in the first position  490 .  FIG. 5B  illustrates an array of leaf assemblies  450  with each leaf  462  in the second position  494 .  FIG. 5C  illustrates an array of leaf assemblies  450  with some leaves  462  in the first position  490  and some of the leaves  462  in the second position  494 .  
         [0064]     In some constructions of the collimation device  42 , a sensor  558  or a plurality of sensors can be employed with each of the leaves  58 ,  282 , and  462  of the respective leaf assemblies  54 ,  270 , and  450 . The sensor  558  can detect the deflection amount of the biasing device  142 ,  338 , and  530  to determine whether the leaf  58 ,  282 , and  462  is open, closed, opening, or closing. The sensor  558  could also be positioned to detect movement of the piston and/or actuator to determine whether the leaf  58 ,  282 , and  462  is open, closed, opening, or closing.  
         [0065]      FIG. 9  schematically illustrates a method for radiation delivery using the collimation device  42  of the present invention. The collimation device  42  includes a plurality of leaf assemblies  550  (each leaf assembly  550  represented by one of the shaded boxes in each row), each leaf assembly  550  including a plurality of leaves  554 . The leaf assemblies  550  are oriented in a plurality of rows and, the leaf assembly  550  of one row is offset with respect to a leaf assembly  550  of an adjacent row. The leaf assemblies  550  can be in the form of the leaf assemblies  54 ,  270 , or  450  as described above. Likewise, the leaves  554  can be in the form of the leaves  58 ,  282 , or  462  as described above. Each leaf assembly  550  defines a radiation path with a different divergence from the radiation source.  
         [0066]     The distance (S) between leaf assemblies  550  at the axis of gantry rotation, the radiation slice width (w) and the distance of couch travel per gantry rotation (D) may be adjusted to achieve optimal radiation delivery with minimal junctioning effects between the slices. Pitch is the couch travel distance for a complete gantry rotation relative to the axial beam width at the axis of rotation. By selecting an appropriate spacing between slices (S) and slice width (w), radiation can be delivered for extended distances with a real pitch of one and a virtual pitch of less than one, if the distance D is set equal to w. This method for radiation delivery can minimize dose ripple since this avoids junctioning effects produced by the different divergences of the leaf assemblies  550 . For example, for a three slice collimator, if the couch distance (D) traveled per rotation is set equal to the selected slice width (w), and the slice spacing (S) is chosen as defined by equation 1, 
 
 S= 2⅔× w   Equation 1 
 
 then a delivery with a virtual pitch of ⅓ will be created (i.e., each position in the patient will be irradiated by each of the three collimated slices). This applies to slices beyond the distal leaf assembly  550 . 
 
         [0067]     The embodiment illustrated in  FIG. 9  can be used with extended target distances. Each leaf assembly  550  provides a treatment slice that penetrates the entire target volume. In this embodiment, the lower jaws  52  move to define which leaf assemblies  550  are allowed to irradiate the patient  14 , and the selected leaf assemblies  550  deliver the radiation slices to the target  18  as the gantry  22  rotates.  
         [0068]     The collimation device  42  in any one of the various constructions described above can be used with a Co-60 radiation therapy machine to provide intensity modulation radiation therapy. Also, linac-based IMRT machines could use lower doses or shorter treatment times. It also allows for the use of overlapping fields (shift patient) or offset units to treat larger volumes. One-hundred-eighty degree collimator rotation may be used for better resolution in the plane of rotation. A tongue and groove connection between the leaves may be incorporated to reduce leakage between the leaves. Also, multi-slice collimation radiation therapy may be a viable option as a dedicated unit for specific sites (e.g. head and neck), representing a significant fraction of a radiology department&#39;s workload.  
         [0069]     Various features and advantages of the invention are set forth in the following claims.