Abstract:
An ion radiation therapy machine provides a steerable beam for treating a tumor within the patient where the exposure spot of the beam is controlled in width and/or length to effect a flexible trade-off between treatment speed, accuracy, and uniformity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 12/439,466 filed Feb. 27, 2009 which claims the benefit of PCT Application PCT/US2008/055096, filed Feb. 27, 2008, and U.S. Provisional Application 60/891,859, filed Feb. 27, 2007, the disclosures of which are all incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with United States government support awarded by the following agency: NIH CA088960. The United States government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to radiotherapy systems using ions (such as protons) for the treatment of cancer and the like and, in particular, to a system providing improved treatment speed and accuracy. 
         [0004]    External beam radiation therapy may treat a tumor within the patient by directing high-energy radiation in one or more beams toward the tumor. Recent advanced external beam radiation systems, for example, as manufactured by Tomotherapy, Inc., treat a tumor with multiple x-ray fan beams directed at the patient over an angular range of 360°. Each of the beams is comprised of individually modulated beamlets whose intensities can be controlled so that the combined effect of the beamlets, over the range of angles, allows an arbitrarily complex treatment area to be defined. 
         [0005]    X-rays deposit energy in tissue along the entire path between the x-ray source and the exit point in the patient. While judicious selection of the angles and intensities of the x-ray beamlets can minimize radiation applied to healthy tissue outside of the tumor, inevitability of irradiating healthy tissue along the path to the tumor has suggested the use of ions such as protons as a substitute for x-ray radiation. Unlike x-rays, protons may be controlled to stop within the tissue, reducing or eliminating exit dose through healthy tissue on the far side of the tumor. Further, the dose deposited by a proton beam is not uniform along the entrance path of the beam, but rises substantially to a “Bragg peak” near a point where the proton beam stops within the tissue. The placement of Bragg peaks inside the tumor allows for improved sparing of normal tissue for proton treatments relative to x-ray treatments. 
         [0006]    Current proton therapy systems adopt one of two general approaches. In the first approach, the proton beam is expanded to subtend the entire tumor and the energy of the protons, and hence their stopping point in the tissue, is spread in range, to roughly match the tumor depth. Precise shaping of the exposure volume is provided by a specially constructed range correction compensator which provides additional range shifting to conform the distal edge of the beam to the distal edge of the tumor. This treatment approach essentially treats the entire tumor at once and, thus, is fast and yet less precise and requires the construction of a special compensator. 
         [0007]    In a second approach, the proton beam remains narrowly collimated in a “pencil beam” and is steered in angle and adjusted in range to deposit the dose as a small spot within the patient. The spot is moved through the tumor in successive exposures until an arbitrary tumor volume has been irradiated. This approach is potentially very accurate, but because the tumor is treated in successive exposures, is slower than the first approach. Further the small spot sizes create the risk of uneven dose placement or “cold spots” should there be patient movement between exposures. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a radiotherapy system usable with ion beams that uses multiple beam spot sizes to effect a precise trade-off between treatment speed and accuracy. For example, spatially larger beams may be used to treat large relatively homogenous portions of the treatment area with smaller beams providing precise delineation of edges and small dose features. 
         [0009]    Specifically, the present invention provides an ion therapy machine that includes an ion source for producing an ion beam. The machine further provides a means for varying the lateral width of the ion beam (perpendicular to the propagation axis of the ion beam) as a function of a control signal and a means for steering the ion beam to different portions of a patient according to a control signal. A beam controller following a stored radiation plan communicates control signals to the means for varying the lateral width and the means for steering the ion beam to apply ion beams of different widths to different portions of the patient according to the treatment plan. 
         [0010]    Thus, it is an object of one embodiment of the invention to aggregate treatment areas that may receive similar irradiation through the use of variable sized treatment beam, thus improving treatment speed or uniformity. 
         [0011]    It is another object of one embodiment of the invention to allow precise tailoring of the beam spot sizes to different portions of the treatment volume to allow a flexible trade-off between treatment speed and treatment accuracy. 
         [0012]    It is yet another object of one embodiment of the invention to provide a method of desensitizing the treatment to patient movement such as may create cold spots. The larger treatment beams naturally eliminate cold spots within the treatment beam and so it may not be necessary to employ longer treatment times to provide superior averaging of patient motion. 
         [0013]    The invention may further include a means for varying an extent of the beams along their axis of travel (axial extent) when the beams are steered to different portions of the patient according to a control signal, and the beam controller may execute the stored radiation plan to communicate control signals to the means for varying an axial extent of the beam, to apply ion beams of different axial extents to different portions of the patient. 
         [0014]    It is thus another object of one embodiment of the invention to change the axial extent of the beam to obtain similar benefits to those provided by the control of beam width. 
         [0015]    The means for varying the lateral width of the beam (perpendicular to the beam axis) may be at least one focusing magnet set. 
         [0016]    It is thus another object of one embodiment of the invention to allow control of beam size with reduced neutron generation as compared to scatter foils and the like. 
         [0017]    It is another object of one embodiment of the invention to provide an efficient use of the energy of the proton beam by controlling beam width without the need to block portions of the beam. 
         [0018]    A pair of successive quadrupole magnets may be used for focusing. 
         [0019]    It is thus another object of one embodiment of the invention to provide a simple and reliable focusing structure for changing beam width. 
         [0020]    The width of the ion beam may be adjusted by varying a separation of the quadrupole magnets or by varying the strength of at least one of the quadrupole magnets. 
         [0021]    It is thus another object of one embodiment of the invention to provide a flexible beam width control allowing mechanical or electrical control methods. 
         [0022]    Alternatively, the means for varying the lateral width of the beam may be a set of selectable different scattering foils movable into and out of the ion beam. 
         [0023]    It is thus an object of one embodiment of the invention to provide a simple beam width control mechanism. 
         [0024]    In one embodiment, the means for varying the lateral width of the beam may control the focusing drive signals of a dielectric wall accelerator. 
         [0025]    It is thus another object of one embodiment of the invention to provide a system that will work with next generation ion sources. 
         [0026]    The means for varying a lateral width may be a mechanical collimator. 
         [0027]    It is thus an object of one embodiment of the invention to provide a system that may work with the current generation multi-leaf collimators or the like. 
         [0028]    The means for varying the longitudinal and lateral extent of the beam may be a set of selectable mechanical scattering foils or ridge filters that may be moved into or out of the beam. 
         [0029]    Thus it is an object of one embodiment of the invention to provide a simple method of controlling the axial extent of the beam. 
         [0030]    The ion therapy machine may further include a radiation planning system receiving a dose plan for the patient and providing the radiation plan to the beam controller such that the radiation plan applies wider beams to portions of the patient with lower gradients in the dose plan and narrower beams to portions of the patient with higher dose gradients in the dose plan. 
         [0031]    It is thus an object of one embodiment of the invention to provide a radiation planning technique adapted to exploit variable resolution ion beams. 
         [0032]    Alternatively, the radiation plan may apply wider beams to portions of the patient removed from the edge of a tumor and narrower beams to portions of the patient at the edges of the tumor. 
         [0033]    It is thus an object of one embodiment of the invention to provide a planning system that selects beam spot sizes based on relative location of the beams in the tumor. 
         [0034]    The radiation planning system may provide a radiation plan to the beam controller to steer the beams to place Bragg peaks of the beams at distal edges of a tumor. 
         [0035]    It is thus another object of one embodiment of the invention to provide a simple method of positioning the beams within the tumor volume. 
         [0036]    These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]      FIG. 1  is a perspective view in partial phantom of an ion therapy system suitable for use with the present invention having a synchrotron ion source providing ions to multiple gantry units; 
           [0038]      FIG. 2  is a cross-section along line  2 - 2  of  FIG. 1  showing the path of the ion beam into a gantry to be directed into a patient after passage through a modulation assembly; 
           [0039]      FIG. 3  is a block diagram of a first embodiment of the modulation assembly of  FIG. 2 ; 
           [0040]      FIGS. 4   a  and  4   b  are elevational views of one embodiment of an ion range shifter assembly using counter-translating wedges, showing two positions of the wedges that provide different amounts of blocking material in the path of the ion beam to control the average ion energy; 
           [0041]      FIG. 5  is a perspective view of two rotating disks holding different scattering foils and ridge filters respectively, to control beam width and beam axial extent; 
           [0042]      FIG. 6  is an elevational cross-section of two ridge filters of the disk of  FIG. 5  such as provide different axial extents of an ion beam; 
           [0043]      FIG. 7  is a schematic representation of a dose map for a patient, the dose map having treatment zones and showing different width beams superimposed on the dose map, and further showing the axial and lateral profiles of those beams; 
           [0044]      FIG. 8  is a figure similar to that of  FIG. 3  showing an alternative embodiment of the modulation assembly using quadrupole magnets for beam widening; 
           [0045]      FIG. 9  is a flowchart of a treatment planning program that may work with the present invention to determine desirable beam resolution in the treatment of a patient; 
           [0046]      FIG. 10  is a detail of the flowchart of  FIG. 9  providing a step of locating the position of ion beams according to dose gradient; 
           [0047]      FIG. 11  is a representation of a non-uniform dose map, its gradient along one axis, and a positioning of a Bragg peak of ion beams of different resolutions based on those gradients; and 
           [0048]      FIG. 12  is a plan view of a multi-leaf collimator that may be operated to effectively control beam widths and beam locations. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0049]    Referring now to  FIGS. 1 and 2 , an ion therapy system  10  may include a cyclotron or synchrotron  12  or other ion source providing a pencil beam  14  of ions that may be directed to a gantry unit  16 . The pencil beam  14  may be received at the gantry unit  16  along an axis  22  into an axial portion of a rotating arm  20  rotating about the axis  22 . The rotating arm  20  incorporates guiding magnet assemblies of a type known in the art to bend the pencil beam  14  radially away from the axis  22  then parallel to the axis and spaced from the axis  22  to be received by a treatment head  26 . The treatment head  26  orbits about the axis  22  with rotation of the rotating arm  20  and incorporates magnets bending the ion pencil beam  14  back toward the axis  22  to intersect the axis perpendicularly. 
         [0050]    As will be described in more detail below, the treatment head  26  may include a modulation assembly  30  to produce a variable resolution treatment beam  24 . A patient  32  may be positioned on a support table  34  extending along the axis  22  so that the variable resolution treatment beam  24  may irradiate the patient  32  at a variety of angles  36  about the axis  22 . A cylindrical neutron shield  40  having a bore for receiving the table  34  and the rotating arm  20  may surround the gantry unit  16  to block generated neutrons. 
         [0051]    In one embodiment, a second rotating arm (not shown) may rotate with or independently of the rotating arm  20  to support an x-ray source  42  and x-ray detector  44  opposed across the axis  22  to illuminate the patient  32  at a range of angles to provide CT imaging capabilities according techniques well-known in the art. 
         [0052]    Referring now to  FIG. 3 , the modulation assembly  30  produces the variable resolution treatment beam  24  by controlling the size, energy, and angle of the variable resolution treatment beam  24  to steer a variably sized treatment spot  54  through different locations within the patient  32 . Specifically, the modulation assembly  30  includes a global range shifter  46  controlling the average energy of the ions in the pencil beam  14 , a beam steering yoke  48  steering the pencil beam  14  in angle in one or two dimensions, a beam axial-extent controller  50  controlling a range of energies of the pencil beam  14 , and a beam width controller  52  controlling a lateral width of the pencil beam in one or two dimensions. As used herein, “lateral” will refer to a direction generally perpendicular to a propagation axis of the pencil beam  14  and axial will refer to a direction generally aligned with a propagation axis of the pencil beam  14 . 
         [0053]    Each of the global range shifter  46 , the beam steering yoke  48 , the beam axial-extent controller  50 , and the beam width controller  52 , provides for electrical connections to a controller  65  that may control each of these elements electrically according to a stored a radiation plan  63 . The controller  65  may communicate with a computer terminal  67  for use by a physician in preparing the radiation plan  63  according to techniques that will be described further below. 
         [0054]    Referring now to  FIG. 4 , the global range shifter  46 , in one embodiment, provides a first wedge  56  and second wedge  58  in the form of identical right triangles of lateral thickness (perpendicular to the plane of the triangles) equal to at least the lateral thickness of the pencil beam  14 . The wedges  56  and  58  are mounted on opposite outer sides of a laterally extending belt  64  with the outer surfaces of the belt attached to corresponding bases of the wedges  56  and  58 . As attached, one wedge  58  is rotated with respect to the other wedge, once about an axis aligned with the attached base and once about an axis perpendicular to the attached base. 
         [0055]    When the belt  64  is moved by motor actuator  66  the wedges  56  and  58  move in opposite directions, with the angled hypotenuses of the wedges  56  and  58  being maintained generally parallel to each other. It will be understood that in this configuration that when the pencil beam  14  passes through both of the wedges  56  and  58  it will pass through a constant amount of wedge material over the entire lateral extent of the pencil beam  14 , providing uniform energy attenuation of the photons of the pencil beam  14 . In a first position of the wedges  56  and  58 , shown in  FIG. 4   a , with the wedges  56  and  58  fully overlapping in an axial direction, the combined material of the wedges  56  and  58  forms an equivalent rectangular bolus  68  having a first height  70 . In a second position of the wedges  56  and  58 , shown in  FIG. 4   b , with the wedges  56  and  58  axially separated by a full amount still allowing them to overlap in the area of the pencil beam  14 , the equivalent rectangular bolus  68  has a second height  70 ′ less than the first height  70 . The height of the equivalent bolus  68  controls the average energy of the protons in the pencil beam  14  and thus movement of the wedges  56  and  58  allows control of the depth of the treatment spot  54  within the patient. The motor actuator  66  may be, for example, a stepper or servomotor as is understood in the art. 
         [0056]    Referring again to  FIG. 3 , after the pencil beam  14  has passed through the global range shifter  46 , the pencil beam  14  is received by a beam steering yoke  48  which may, for example, be a set of electromagnetic coils or opposed electrostatic plates well known for steering charged particles in one or two lateral dimensions. The beam steering yoke  48  allows the pencil beam  14  to be steered at an angle from an axis  60  perpendicular to the axis  22  about which the beam rotates. In this way the treatment spot  54  to be moved to an arbitrary lateral location within the patient  32 . Together these beam steering yokes  48  and the range shifter  46  allow the treatment spot  54  to be moved to arbitrary locations within the patient  32 . 
         [0057]    Referring now to  FIGS. 3 and 5 , the size of the treatment spot  54 , in terms of axial length, is controlled by the beam axial-extent controller  50  which varies the energies of the ions in the pencil beam  14  to create one of a number of predefined energy ranges. In one embodiment, the beam axial-extent controller  50  uses a disk  73  extending in a lateral plane and rotatable by motor  72  about an axis parallel to the axis of the pencil beam  14 , to bring various apertures  76  in the periphery of the disk into alignment with the pencil beam  14  as the disk is rotated. Each of the apertures  76  may be fitted with a different ridge filter  78  providing for a different spread of energies and thus a different axial length  75  of the treatment spot  54 . 
         [0058]    Referring to  FIG. 6 , a first axial ridge filter  78 , for example in a first aperture  76 , may have a set of triangular ridges  80  whose peaks provide a first axial thickness to reduce ions&#39; energies to provide an average stopping point  82  in the patient  32 , and troughs having reduced thickness and allowing increased proton energy to provide an average stopping point  84  in the patient  32 . The difference between these two stopping points  82  and  84  represents the axial length  75   a  of the treatment spot  54 . 
         [0059]    For comparison, a second ridge filter  78 ′ in a different aperture  76 , may have a similar profile but with ridges of lesser amplitude whose peaks provide a first stopping point  82 ′ and whose troughs provide a second stopping point  84 ′ that are closer together to produce an axial length  75   b  that is substantially shorter than the axial length  75   a . A number of different filters  78  may provide for a range of different axial lengths  75  for the treatment spot  54 . 
         [0060]    Referring still to  FIGS. 3 and 5 , the beam width controller  52  may be a similar disk  91  positioned below disk  73  and axially aligned therewith and rotatable by motor  95  to bring various apertures  90  in the periphery of the disk  91  into alignment with the pencil beam  14 . In this case, the apertures  90  may be fitted with different scattering foils  92  such as cause a lateral spreading of the pencil beam  14  by various amounts according to the material and thickness of the scattering foil to control the lateral width  94  of the treatment spot  54 . 
         [0061]    Referring now to  FIG. 7 , a radiation plan  63  describing the positioning of the multiple treatment spots  54  and their sizes may be developed with reference to a dose map  100  prepared by a physician using planning software to convert the dose map  100  to a radiation plan  63 . The dose map  100  may be prepared, for example, using a graphics terminal with the physician viewing one or more CT images of the patient to define desired doses in different zones within the volume of the patient. 
         [0062]    A simple dose map  100  follows the outline of a tumor  99  and provides a desired uniform dose within that outline. The present invention may provide a radiation plan  63  that uses multiple treatment spots  54   a - 54   f  to deliver the desired dose. Generally the axial length of the treatment spot  54  will affect the profile of the dose within the treatment spot  54 . Thus, for example, a small treatment spot  54   e  will have an axial profile  102  exhibiting a well-defined Bragg peak with a sharp distal fall off whereas a large treatment spot  54   f  will exhibit an axial profile  104  with a more gradual falloff being the aggregate of Bragg peaks for multiple protons of different energies. For this reason, smaller treatment spots  54  may preferentially be used near the distal edge of the tumor or at other points of high dose gradient. 
         [0063]    The lateral width of the treatment spot  54  will also affect the lateral profile of the dose within the treatment spot  54 . In this case the lateral falloff is not determined by the Bragg peak but simply by beam spreading after collimation. 
         [0064]    Intuitively, it will be understood from  FIG. 7  that a large treatment spot  54   f  may be advantageously placed roughly centered within the tumor  99  and smaller treatment spots  54   a - 54   e  may be used close to the distal edge of the tumor  99  to take advantage of the sharper Bragg peak available from those smaller spots. As the gantry is rotated and axis  60  of the pencil beam  14  moves about the tumor  99 , different edges of the tumor  99  become the distal edge allowing this approach to be repeated for the entire tumor  99  to provide sharp demarcation of the outline of the tumor  99 . 
         [0065]    This general observation may be exploited more precisely by a radiation treatment planning system implemented by program  110  executed in the terminal  67  to prepare a radiation plan  63 . Referring now to  FIGS. 7 and 9 , the treatment plan may begin by receiving a dose map  100  as indicated by process block  112  generally describing a spatial extent of a portion of the patient  32  where an ion dose will be applied. In contrast to the dose map  100  of  FIG. 7 , the dose map  100  more generally will include multiple zones within a dose map  100  describing variations in the intensity of the doses within those zones. 
         [0066]    At process block  114 , a first set of beams, for example, producing large treatment spots  54   f  may be fit to the dose map  100 . This fitting determines both an intensity of the different treatment spots  54  and the location of the beam treatment spot  54 . One method for locating the treatment spot  54  tries to fit as many of the treatment spots  54  into the tumor area of the dose map  100  as can be done with controlled overlapping or extending outside of the tumor  99 . The intensities may then be determined by an iterative process, for example “simulated annealing”, considering multiple exposures for different gantry angles. 
         [0067]    Once the intensity of the large treatment spot  54  is determined, then at process block  130  smaller treatment spots  54  (for example treatment spot  54   a - e ) are positioned on the dose map  100  in gaps between the larger treatment spots  54   f . These gaps may be identified simply by creating a difference map indicating differences between the dose implemented by the large treatment spots  54   f  and the desired dose of the dose map  100 , and placing the smaller treatment spots  54   a - e  according to the difference map. The intensities and positions of the optimized larger treatment spots  54   f  are held fixed and only the intensities of the new smaller treatment spots  54   a - e  are optimized iteratively. Alternatively, the intensities and positions of the optimized larger treatment spots  54   f  may be used as a starting position for renewed optimization of both the larger treatment spots  54   f  and the new smaller treatment spots  54   a - e.    
         [0068]    As illustrated by process block  132 , this process may be repeated for yet smaller treatment spots  54   g  shown in  FIG. 7 . 
         [0069]    Referring now to  FIGS. 10 and 11 , an alternative method of locating the treatment spots  54 , as indicated by process block  116 , determines a gradient  122  of the dose map  100  being the spatial derivative of the dose  120  along a particular treatment axis (e.g. aligned with axis  60  for each treatment fraction). For simplicity, the dose map  120  may be discretized into two or more dose levels as shown by discretized dose map  120 ′ and a discretized gradient  122 ′ developed (indicating generally gradient sign). 
         [0070]    For example, the dose map  100  may include a first central zone  119  of lower dose  121  and an outer peripheral zone  118  of higher dose  123 . Discretized derivative values  122 ′ along axis  60  may provide for two positive going transitions  123 , a negative going transitions  124 , a positive going transition  123 , and two negative going transitions  124  (from left to right) following the discretized gradient  122 ′. These transitions  123  and  124  may be used to align the Bragg peak  126  of treatment spots  54  to provide a location of those beam spots for intensity optimization according to the following rules: 
         [0071]    (a) place a Bragg peak  126  along the ray of a given proton beam at points where the dose gradient drops below a user-defined negative threshold (A) (or in the case of the discretized gradient  122 ′, where there are negative transitions); 
         [0072]    (b). place a Bragg peak  126  along the ray of a given proton beam at points where the dose gradient exceeds below a user-defined negative threshold (B) (or in the case of the discretized gradient  122 ′, where there are positive transitions) after there has been at least one peak placed per (a) above. 
         [0073]    The height of the peaks  124  may also be matched to the steepness of the Bragg peaks  124  of the different sizes of treatment spots  54  which, as noted, before, tend to vary with the treatment spot  54  size. 
         [0074]    Once locations of treatment spot  54  are fixed, the intensities may be optimized as described before or by iterative techniques such as Simulated Annealing or Gradient Based Optimization Techniques beams at multiple angles. Multiple delivery angles, for example over 360 degrees, and control of the intensity of the beam spots will then build up the dose to match the dose map  100 . By selecting a beam range prior to iteration, the iteration process is much simplified. 
         [0075]    Alternatively or in addition, the above technique of locating the Bragg peaks of the treatment spots  54  may be used on an “ex ante” basis and an optimization program  117  may then be run in which the dose produced by the ex ante placement is compared to the desired dose. The deficiency in the dose is then used to place additional treatment spots  54 . In this way locations that did not receive a sufficient amount of dose from the first pass are filled in with spots that are added based on the difference. 
         [0076]    Referring now to  FIG. 8 , in an alternative embodiment of the modulation assembly  30 , axial range shifter  46  may be followed by a first and second quadrupole magnet  152  and  154  rotated along axis  60  at 90 degrees with respect to each other. The pencil beam  14  passing through the successive quadrupole magnets  152  and  154  is expanded into a diverging fan beam. The width of this diverging fan beam may be controlled by changing the separation of the quadrupole magnets  152  and  154  by a mechanical focusing assembly  158 , and/or by control of the strength of the magnets in one or both quadrupole magnets  152  and  154  by controlling an electromagnetic current according to signals from the controller  65 . 
         [0077]    The variable resolution treatment beam  24  from the quadrupole magnets  152  and  154  is then received by the beam axial-extent controller  50  and then steered by beam steering yoke  48  as described before. 
         [0078]    Referring now to  FIG. 12 , control of the beam width and its location may, in an alternate embodiment, be accomplished by a multi-leaf collimator  160  having individually controllable leaves  162  which may be moved into or out of a fan beam  164  to create apertures  166  defining beam widths  168  and, by their offset from a center of the fan beam  164 , may control the positioning of the beam within the patient  32 . Thus one mechanism may provide both for steering and beam width control, the separate control signals being combined to produce control signals for selection of particular shutters for opening and/or closing. A shutter system suitable for this use is described in U.S. Pat. No. 5,668,371 described above. Although only a single aperture  166  is shown, in the simplest embodiment, this technique may be used to produce simultaneous multiple apertures (not shown) of different widths for concurrent treatment using the same axial extent or variable axial extent provided by corresponding range shifters for each aperture, again as taught in U.S. Pat. No. 5,668,371. 
         [0079]    Generally, the invention anticipates that the source of protons may also be a dielectric wall accelerator. As is understood in the art a dielectric wall accelerator provides a linear acceleration of charged particles through the use of successively applied electrostatic fields that serve to accelerate the charged particles as they move through the dielectric wall accelerator. Energy modulation may be obtained by simply controlling the degree of acceleration of the charged particle through the switching of the electrostatic fields and their timing. The beam widths may be controlled by electronic control of focusing electrodes incorporated into the body of the dielectric wall accelerator. By deflecting the protons at the proximal end of the dielectric wall accelerator early in the acceleration process, it is believed that it should be possible to steer the proton beam. The electrodes used to control the beam width can also be used for focusing the beam spot. 
         [0080]    Dielectric wall accelerators suitable for this purpose are described for example in “Development of a Compact Radiography Accelerator Using Dielectric Wall Accelerator Technology” by Sampayan, S. et als. Proceedings of the Particle Accelerator Conference, 2005. PAC 2005. Publication Date: 16-20 May 2005 pp: 716-718 ISBN: 0-7803-8859-3. 
         [0081]    The present invention contemplates changing of the size of the treatment spot  54  in three dimensions: axially and in two perpendicular lateral directions. The present invention may also be used with beam spot control in only two dimensions: axial and one lateral dimension within a plane of rotation of the gantry head  26 . Under this control technique the patient may be treated on a slice-by-slice basis through a “rotate and step” scanning pattern or a helical scanning pattern of a type known in the art for x-ray tomography. 
         [0082]    Alternatively such a system may also combine helical scanning, for example, with variable beam widths in three dimensions including along the axis about which the head  26  is rotated. Such a system would anticipate common structure in adjacent slices to provide for treatment of these structures over a longer period during multiple slices. 
         [0083]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.