Abstract:
An intensity modulator for controlling the intensity of ions, such as protons, controllably block a portion of sub-areas of an area beam to control the average intensity within that sub-area. A fan beam is then created by a focusing process that reforms the area beam while blurring intensity variations in each sub-area to a corresponding beamlet in the fan beam of uniform intensity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application 60/891,859, filed Feb. 27, 2007 and PCT Application PCT/US2008/055104 filed Feb. 27, 2008, the disclosures of which are 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 has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to radiation therapy systems using ions, such as protons, for the treatment of cancer and the like and, in particular, to a system providing improved modulation of a ion beam. 
         [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]    Unlike x-rays, protons may be controlled to stop within the tissue, 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 at a “Bragg peak” near a point where a proton stops within the tissue. Proton therapy is described generally in U.S. Pat. No. 5,668,371 entitled “Method and Apparatus for Proton Therapy” issued Sep. 16, 1997, assigned to the assignee of the present invention and hereby incorporated by reference. 
         [0007]    In distinction from x-rays, with protons it is possible to separately control intensity (i.e., the average number of protons per time over an area) and energy (i.e., the speed of the protons). Control of the intensity of protons determines the dose delivered by the protons to the tissue whereas control of the energy of the protons determines the depth in the tissue at which the dose is concentrated. In the above reference patent application, the intensity of the protons within different “beamlets” of a fan beam are controlled by changing the time during which blocking shutters are placed in the path of each beamlet versus the time the blocking shutters are removed from the path of the beamlets. By “duty cycle” modulating, the shutter intensity variations may be obtained. 
         [0008]    A similar approach may be adopted in proton therapy systems that use a steerable pencil beam (rather than a fan beam) of protons. In this case the “dwell time” of the pencil beam at a particular location before it is moved determines the intensity of protons delivered to that location. 
         [0009]    Both of these approaches will be termed “time accumulation” approaches as they rely on changing the length of time the tissue is exposed (and thus the average intensity of the beam) to control the dose. A drawback to such time accumulation systems is that higher average intensities require correspondingly increased exposure times. As a practical matter this increases treatment times. Designing fast acting shutters or pencil beam scanning systems, that might offset these increased treatment times, can be difficult or expensive. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The present inventors have developed a shutter system that controls the instantaneous intensity of the ion beam and thus that need not entirely, or at all, rely on time accumulation to vary average intensity. Instead, in the invention, a set of shutters each blocks different latitudinally separate beamlets of an area beam by varying longitudinal amounts. The area beam is then refocused to a fan beam and this refocusing process blurs the image of the shutter to provide for uniform proton intensity within the beamlet area. The result is a set of beamlets having uniform ion intensity within the beamlet and whose instantaneous ion intensities (as opposed to average intensities) may be continuously varied. Multiple adjacent shutters are used to provide similar control on adjacent beamlets. 
         [0011]    Specifically, the present invention provides an intensity modulator for ions, the intensity modulator having an area beam generator producing an area beam of protons. The area beam is received by an intensity modulator occluding the area of beam with a set of latitudinally adjacent ion-blocking shutters controllably extended to different longitudinal distances according to a desired intensity of each beamlet of protons. The desired intensity is obtained by controlling the portion of the area beam occluded by a given shutter. The partially occluded area beam is then collapsed in the longitudinal direction by a lens system to form a fan beam directed toward the patient. 
         [0012]    It is thus one object of the invention to provide for continuous control of instantaneous beamlet intensity within a fan beam. 
         [0013]    The intensity modulator may be combined with an energy modulator having a set of latitudinally adjacent ion attenuating wedges controllably extended to different longitudinal distances according to a desired energy of a beamlet of protons defined by a portion of the area beam occludable by a given wedge. 
         [0014]    It is thus another object of the invention to provide a system that can provide both independent instantaneous control of intensity and energy. 
         [0015]    Each wedge may be matched to a corresponding second wedge providing mirror movement to the first wedge to, in combination, present a uniform thickness of material within the fan beam. 
         [0016]    It is thus another object of the invention to provide for energy modulation that is uniform within the cross-section of the beamlet. 
         [0017]    The lens system may be a pair of quadrupole lenses aligned in rotation with respect to the other about a common axis. 
         [0018]    It is thus an object of the invention to provide for a simple lens system to produce a fan beam that minimizes the productions of neutrons. 
         [0019]    The beam generator may be a scattering foil receiving a pencil beam to spread it into an area beam. 
         [0020]    Thus it is an object of the invention to provide a simple method of converting a pencil proton beam obtained from a cyclotron or synchrotron into an area beam for modulation. 
         [0021]    The invention may provide a gantry holding the modulator for rotation of the fan beam with respect to the patient about a longitudinal axis while controllably varying the longitudinal distances of the shutters. 
         [0022]    It is thus another object of the invention to provide a treatment technique that works well with the continuous modulation that may be obtained by moving shutters, the treatment intensity sinograms used in such orbiting treatment being generally smoothly continuous to comport with achievable modulation with the shutters. 
         [0023]    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 
         [0024]      FIG. 1  is a simplified representation of a proton therapy machine suitable for use with the present invention and having a rotating gantry for directing a fan beam of protons toward a patient support at a range of angles, the beamlets of the fan beam controlled by a modulator; 
           [0025]      FIG. 2  is an exploded isometric representation of the modulator showing the various stages of intensity of the area beam and energy modulation of a fan beam; and 
           [0026]      FIG. 3  is a side elevational view of one modulation element of an energy modulator of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0027]    Referring now to  FIG. 1 , a proton therapy machine  10  may include a gantry  12  having a modulator  14  that may orbit  16  about a patient (not shown) on a patient support table  18 . 
         [0028]    The modulator  14  receives a source of protons from a proton source conduit  22  that may receive a pencil beam of protons from a synchrotron, cyclotron or the like. The pencil beam of protons may be curved through the gantry  12  by means of bending magnets  23  to direct the pencil beam along axis  21  toward the patient support table  18  at all positions of the gantry  12  within the orbit  16 . 
         [0029]    During treatment, the pencil beam of protons is received by the modulator  14  which converts the pencil beam into a fan beam  20  and individually modulates beamlets  24  within the fan beam  20  in both energy and intensity. The energy and intensity of the beamlets  24  is under the control of a control computer  25  receiving control sinograms  26  providing data indicating desired intensities and energies of each individual beamlet  24  as a function of an angle of the gantry  12  within the orbit  16 . 
         [0030]    Referring now to  FIG. 2 , the modulator  14  which rotates with the gantry  12 , receives a pencil beam  30  along axis  21  at a scattering foil  32  or the like which spreads the pencil beam  30  into an area beam  34 . The area beam  34  may be collimated to provide a generally rectangular cross-sectional area extending latitudinally  36  and longitudinally  38 . 
         [0031]    After collimation, the area beam  34  may be received by an intensity modulator  40  that provides for a set of latitudinally adjacent and longitudinally extending proton-opaque shutters  42 . Each of the shutters  42  may, for example, be a rectagular block of ion blocking material (for example a dense metal) having its longest dimension aligned with the longitudinal direction and its latitudinal width defining the width of a beamlet  24 . The shutters  42  may slide against each other at abutting latitudinal edges. 
         [0032]    Each shutter  42  may be connected to an electronic or pneumatic actuator  44  controlled by the control computer  25  to move a distal end of the shutters  42  to different longitudinal distances within the area beam  34  while maintaining the proximal end of the shutters  42  outside of the area beam  34 . As depicted, the actuators  44  are represented as motors (for example servomotors or stepper motors) connected to the shutters  42  by a machine screw mechanism. It will be understood that other well-known actuator systems including, for example, linear motors, pneumatic cylinders, or standard rotary motors with pulley or rack systems may be used in an open or closed loop fashion, the latter providing sensors such as optical or LVDT sensors, to close the feedback loop. 
         [0033]    When all the shutters  42  are fully extended into the beam  34 , they wholly block protons of the area beam  34 . When all of the shutters  42  are wholly retracted, they allow unimpeded passage of the area beam  34 . Normally the shutters  42  will partially block portions of the area beam  34  as determined by their extended length controlled by the actuator  44 . In this latter case, the average intensity of the protons within an area  43  potentially occluded by a given shutter  42  will vary continuously depending on the percentage of this area blocked by the shutter  42  and thus the amount the shutter  42  has been extended into the area beam  34  by its actuator  44 . The average intensity within this area  43  results from two regions of discontinuous intensity: one region  45   a  fully blocked by the shutter  42  and the other region  45   b  not blocked by the shutter  42 . Thus, the intensity within this area  43  is not uniform. 
         [0034]    The area beam  34 ′ as modulated by the modulator  40  is then received by a lens array  46  comprised of two quadrupole magnet  48   a  and  48   b  of a type known in the art. Each quadrupole magnet  48   a  and  48   b  is aligned along the common axis  21  and aligned in rotation with respect to the other so that the first quadrupole magnet  48   a  has opposed north poles along an axis  50   a  and the second quadrupole magnet  48   b , beneath the first quadrupole magnet  48   a , has a corresponding axis  50   b  aligned with axis  50   a.    
         [0035]    The effect of this lens array  46  is that the area beam  34  is reformed into a fan beam  52 . The fan beam  52  also extends along axis  21  but has a larger latitudinal dimension than the area beam  34 ′ and a much narrower longitudinal dimension than the area beam  34 ′. As a result, each of the longitudinally extended areas  43  controlled by each shutter  42  in the intensity modulator  40  are compressed severely in the longitudinal direction. This compression creates the fan beam  20  of multiple controllable beamlets  24  each having an area  54  corresponding generally to one of the areas of  43 . 
         [0036]    The focusing effect of the lens array  46  also results in the discontinuous intensities of regions of  45   a  and  45   b  of areas  43  produced by the intensity modulator  40  being blurred so that the intensities of the beamlets  24  within the areas  54  are substantially uniform. The instantaneous intensity of the beamlets  24  in areas  54  thus will be equal to the average intensity of the beam in area  43  multiplied by the area of area  43  and divided by the area of area  54 . This results from a substantially equal flux of protons passing through areas  43  and  54 . 
         [0037]    The lens system may be implemented by other elements including gratings and/or scattering foils and collimation plates to provide a blurring and collimation of the area beam into a fan beam (?). 
         [0038]    Each of the beamlets  24  defined by an area  54  is then received by an energy modulator  60 . For clarity, only one energy modulation element of the energy modulator  60  for one beamlet  24  corresponding to a particular area  54  is depicted as also shown in  FIG. 3 . Each element of the energy modulator  60  provides for two opposed wedges  62  and  64  overlapping within the area  54  with their narrowest portions (measured along axis  21 ) directed toward each other. 
         [0039]    Referring to  FIG. 3 , each wedge  62  and  64  may provide a right triangle of radiation attenuating material, with one wedge inverted with respect to the other along the longitudinal axis and rotated by 180° along the axis of the beamlet  24  so that their hypotenuses slide along each other and their bases remain parallel. In this way, a thickness of material of the combined wedges  62  and  64  within the fan beam  52  is constant throughout the area  54 . The wedges  62  and  64  are each connected to actuators  66  which work to move the wedges  62  and  64  in opposite directions, both moving out of and into the beamlet  24  in synchrony so that the total thickness of the wedge material may be controlled. The wedges  62  and  64  serve only to slow the protons rather than block them completely and thus provide for energy modulation or range control of the protons indicated by arrows  68 . 
         [0040]    The actuators  66  also connect to the control computer  25  so that both the intensity and the energy of each beamlet within the fan beam  52  may be independently controlled during treatment. 
         [0041]    It will be understood that these wedges need not be shaped like a wedge (necessary for uniform wedge material) but may be, for example, constructed of materials with variable attenuation to act like a wedge while being shaped differently. 
         [0042]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to 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.