Abstract:
A system for modulating a fan beam for radiation treatment employs shutters that may move rapidly into and out of different beamlets of a fan beam, the shutters having a systematic weighting so that a limited number of shutters may obtain a far greater number of regularly spaced energy reductions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application 60/891,859, filed Feb. 27, 2007, PCT Application PCT/US2008/055162, filed Feb. 27, 2008, the disclosures of which are incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     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 
     The present invention relates to radiotherapy systems, such as those using ions (such as protons), for the treatment of cancer and, in particular, to a system providing improved treatment speed and accuracy. 
     External beam radiation therapy may treat a tumor within a patient by directing high-energy radiation in one or more beams toward the tumor. Highly sophisticated 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 a complex area to be treated. 
     X-rays deposit energy 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 beamlets of x-ray beamlets can minimize radiation applied to healthy tissue outside of the tumor, the inevitability of x-ray irradiation of healthy tissue along the path to the tumor has led to the investigation of ions, such as protons, as a substitute for x-rays. 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 to a “Bragg peak” near a point where the proton beamstops 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. 
     Current proton therapy systems adopt one of two general approaches. In the first approach, termed the “spread out Bragg peak” (SOBP) approach, the range of energies in the proton beam is expand so that their Bragg peaks extend over a range roughly matching the tumor depth. Precise shaping of this volume is provided by a specially constructed correction range compensator which provides additional range shifting to warp the distal edge of the Bragg peaks to the distal edge of the tumor. This treatment approach can treat the entire tumor at once and therefore is fast. But it is difficult to conform the dose to the tumor volume and the construction of a special range compensator is required. 
     In a second approach, termed the “magnetic spot scanning” (MSS) approach, the proton beam remains narrowly collimated in a “pencil beam” and is steered in angle and range to deposit the dose as a series of small spots within the patient. The spots are located to cover 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 many successive exposures, this approach is much slower than the SOBP approach. Further the small spot sizes create the risk of uneven dose placement or “cold spots” between the treatment spots, something that is exacerbated if there is any patient movement between exposures. 
     SUMMARY OF THE INVENTION 
     The present invention provides a treatment system that uses a fan beam of ions composed of “beamlets” each of which may be separately modulated. In this way the benefits of parallel treatment of different portions of the tumor of SOBP is combined with the benefit of precise control of small portions of the beam of MSS. The modulator uses a set of arrays of energy reducing modulation elements, the arrays subtending the fan beam, and the modulation elements lined up within the beamlets of the fan beam. The modulation elements maybe separately inserted or removed from the beam. 
     By assigning different weights to each of these modulation elements, a number of regular steps of energy reduction can be obtained that greatly exceed the number of elements. For example, if each element is given a weighting according to a binary power (e.g. 1, 2, 4, 8), then eight shutters may obtain 256 separate energy reduction levels. Unlike movable wedge systems, this “binary shutter system” can jump rapidly between different levels of energy reduction and provide highly repeatable levels without sophisticated feedback control systems. The same modulation system may be used for photon radiation as well, but now the intensity of the beam is reduced, not the energy. 
     Specifically, then, the present invention provides a modulator for therapeutic radiation having an inlet receiving a fan beam of radiation traveling along an axis and having a cross-sectional area whose greatest dimension extends along a plane. A set of arrays of modulation elements are positioned side by side along the plane within the cross-sectional area and the attenuation elements of the arrays are aligned along the axis. Each modulation element is movable between at least one extended position within the cross-sectional area and a retracted position outside of the cross-sectional area. 
     A set of actuators communicates with each attenuation element to independently actuate the attenuation elements to move the attenuation elements between two discrete positions, one position out of the fan beam and at least one position in the fan beam. Different attenuation elements presents the beam with variable thickness that is finely controlled. The attenuating thickness provide different reductions in radiation energy so that a series of regular increments of energy reduction can be provided in different portions of the fan beam by selection of different combinations of the attenuation elements for actuation. If the radiation happens to be photons then the variable thickness reduces the intensity of the photons at a specific energy bin. 
     It is thus an object of one embodiment of the invention to provide an improved fan beam modulator for radiation that can provide high-speed, precise control of adjacent beamlets within the fan. 
     The actuators may independently actuate the modulation elements to move the elements between only two states, a retracted position outside of the beam and an extended position fully covering the beam. 
     It is thus another object of one embodiment of the invention to provide a binary shutter system greatly simplifying the control of the actuators. 
     The energy reduction provided by different attenuation elements may be related according to a binary power sequence. 
     It is thus an object of one embodiment of the invention to provide a simple modulation sequence providing uniform increments. 
     Two sets of modulation elements may be positioned in opposition across the plane. 
     It is thus another object of one embodiment of the invention to provide simultaneous control of two adjacent fan beams. 
     The attenuation elements of an array may be of uniform material and have different thicknesses within the cross-sectional area when in the extended position providing different reductions in ion beam energy or photon beam intensity. 
     It is thus an object of one embodiment of the invention to provide for simplified construction with well-characterized homogenous materials. 
     The modulating elements of different thickness may be ordered within the beam to create jumps in thicknesses deviating from an ordering according to thickness. 
     It is thus an object of one embodiment of the invention to provide for improved spacing between actuators allowing direct drive of the attenuating elements by the actuators. 
     Alternatively or in addition the modulating elements of the array may have different densities providing different reductions in ion beam energy or photon beam intensity. 
     It is thus an object of one embodiment of the invention to provide a more compact shutter system. 
     The actuators of an array may in combination block the radiation to provide for intensity modulation of the radiation when the radiation is an ion beam. 
     Thus it is an object of one embodiment of the invention to provide a modulator that may perform both range shifting and modulation of intensity of heavy ion beams. 
     These particular objects 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 FIGURES 
         FIG. 1  is a simplified representation of the elements of a prior art radiation therapy system using the SOBP approach described above; 
         FIG. 2  is a figure similar to that of  FIG. 1  showing the elements of a prior art radiation therapy system using the MSS approach described above; 
         FIG. 3  is a figure similar to that of  FIGS. 1 and 2  showing the elements of a fan beam system of the present invention employing a fan beam with individually modulated beamlets and a rocking exposure pattern; 
         FIG. 4  is a perspective view of an ion therapy machine incorporating the elements of  FIG. 3  providing constrained rotation of the fan beam; 
         FIG. 5  is a cross-section taken along line  5 - 5  of  FIG. 4  showing the range of motion of a center axis of the fan beam in the present invention with respect to stationary neutron shield; 
         FIG. 6  is a top plan view in phantom of the system of  FIG. 5  showing positioning of a patient to be pre-scanned with a tomography ring and then treated using the present invention; 
         FIGS. 7   a  and  7   b  are simplified representations of cross-sectional dose patterns for treatment of a tumor generated with a 360° scan and generated with a 150° scan per one embodiment of the present invention showing the latter scan&#39;s superior protection of sensitive distal tissue; 
         FIG. 8  is a perspective view of a “semi-helical” scanning pattern that may be implemented with the present invention; 
         FIG. 9  is a top plan view of the helical scan of  FIG. 8  showing overlap of the scans that provides for “re-painting” reducing hot spots/cold spots; 
         FIG. 10  is a figure similar to that of  FIG. 9  showing an alternative rectilinear scan system; 
         FIG. 11  is a figure similar to that of  FIGS. 9 and 10  showing an alternative rectilinear scan that may be superior for motion gating; 
         FIG. 12  is a perspective view of a magnetic beam former using two sequential and aligned quadrupole magnet systems and showing a mechanism for adjusting the separation of those magnet systems to adjust the resulting fan beam; 
         FIG. 13  is a simplified cross-sectional view along  13 - 13  through one quadrupole magnet of  FIG. 12  showing the magnet orientations; 
         FIG. 14  is a magnetic field map of the quadrupole of  FIG. 13 ; 
         FIG. 15  is an aligned top plan and side elevation view of the beam of  FIG. 12  showing the effective operation of the quadrupole magnets as both converging and diverging lenses in different axes; 
         FIG. 16  is a perspective view of the elements of  FIG. 3  showing the two quadrupole magnets and a binary shutter system that may be used to generate and modulate the fan beam in the present invention; 
         FIG. 17  is a side view of the binary shutter system showing a side-by-side arrangement of arrays of attenuation elements providing shutters; 
         FIG. 18  is a side elevational view of one attenuation element showing its actuator for moving the attenuation element between a retracted position outside of the beam and an extended position within the beam; 
         FIG. 19  is a simplified representation of one array of binary-weighted attenuation elements fully extended to block the beam; 
         FIG. 20  is a figure similar to that of  FIG. 19  showing selected retraction of the attenuation elements such as it may provide controlled energy reduction in the beam; and 
         FIG. 21  is an alternative embodiment showing the use of two modulation systems face to face to provide for two independently modulated adjacent fan beams. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a conventional ion radiation therapy system  10  employing the SOBP approach described above provides an ion source  12  producing a pencil beam  14  of ions traveling along an axis  20 . 
     The pencil beam  14  may be received by a foil  17  scattering the pencil beam into a cone beam  18  having a circular cross-section  21 . The energy of the ions in the cone beam  18  is then received by a rotating wedge propeller placing a material of varying thickness in the cone beam  18  and acting as a range shifter  16  continuously changing the energy and thus range of penetration of the ions into tissue. 
     The cone beam  18  then passes through a collimator  24  approximating the outline of the tumor and a compensator  22  tailor-made for the particular tumor being treated after which the cone beam  18  is received by the patient  26  to produce a treatment pattern  28 . As noted, this treatment approach simultaneously treats the entire volume of the tumor and is therefore relatively quick, but requires custom built collimators  24  and compensators  22  and also produces a treatment pattern  28  with imperfect conformance to an arbitrary tumor volume. 
     Referring to  FIG. 2 , a radiation therapy system  10 ′ for implementing the MSS approach, described above, receives a pencil beam  14  from an ion source  12  and passes it through a range shifter  16 , for example, a set of movable plastic blocks of different thicknesses. The range shifted pencil beam  14  passes next to a magnetic steering yoke  19  which steers the pencil beam  14  to different spots  30  within the patient  26 . Multiple spots  30  together create the treatment pattern  28 . This system produces good conformance of the treatment pattern  28  to an arbitrary tumor, but the sequential process is slow. 
     Referring now to  FIG. 3 , the radiation therapy system  10 ″ of the present invention employs an ion source  12  producing a pencil beam  14 . In a preferred embodiment, the pencil beam  14  is received by a magnetic beam former  32  converting the pencil beam  14  into a fan beam  34  by magnetic deflection rather than scattering and thus minimizing the generation of neutrons. 
     The fan beam  34  is next received by a binary shutter system  36  which individually modulates the range and the intensity of the individual beamlets  38  of the fan beam  34 , the beamlets  38  being adjacent sectors of that fan beam  34 . The modulated fan beam  34  may be moved in a partial arc  40  with respect to the patient  26  to provide for complex treatment patterns  28  taking advantage both of multiple angles of treatment and the ability to individually control the intensity and range of the beamlets  38 . 
     Referring now to  FIG. 4 , the structure of the radiation therapy system  10 ″ may provide, for example, an axial proton beam conduit  42  receiving the pencil beam  14  of protons, for example, from a remote cyclotron or synchrotron (not shown). 
     Beam steering magnets of a type well known in the art (not shown) may bend to the pencil beam  14  to follow a “crank arm” path of a gantry  44  having a radially extending segment  47  passing on a line of radius from an axis  46  of the entering pencil beam  14  and an axial segment  48  parallel to the axis  46  but spaced from the axis  46  as attached to the end of the radially extending segment  47 . The distal end of the axial segment  48  holds a gantry head  50  (whose elements are shown generally in  FIG. 3 ) and which directs a fan beam  34  toward a patient support  52 , the latter generally aligned with the axis  46 . 
     The fan beam  34  lies generally within a plane of rotation  54  of the gantry head  50  as the gantry head  50  moves about the patient support  52 . By aligning the axis of rotation of the gantry head  50  with the axis  46  of the entering pencil beam  14 , constant field bending magnets within the gantry  44  may channel the pencil beam  14  to the gantry head  50  at any of its angular positions. 
     Referring momentarily to  FIG. 5 , the gantry head  50  may rotate in an arc  56  about the axis  46  by an amount substantially less than 180° and in the preferred embodiment approximately 150°. As will be described further below, the present inventors have determined that this limited rotation, un-intuitively, can provide a superior dose pattern  28  when compared to a more complete 360° rotational of the gantry head  50 , such as would be preferred for intensity modulated radiation therapy using photons. 
     The limited range of arc  56  allows a massive stationary neutron stop  58  to be placed under the patient support  52  to receive neutrons generated by interaction of the ions with the patient  26  over the full range of arc  56 . The ability to use a stationary neutron stop  58 , allows the neutron stop  58  to be larger and closer to the patient  26 , allowing, for example, a form in-place concrete neutron shield. 
     Referring now to  FIGS. 4 and 6 , an x-ray tomography ring  60  may be placed adjacent to the neutron stop  58  along the axis  46  so as to provide for planning tomographic images of the patient  26  contemporaneous with the radiation treatment. The displacement of the x-ray tomography ring  60  from the plane of rotation  54  allows a full 360° of access to the patient (generally required of an x-ray tomography machine) for supporting both the detector and opposed x-ray source on opposite sides of the patient. 
     Referring now to  FIGS. 7   a  and  7   b , a simplified treatment plan may be developed to treat a tumor  62  in the patient  26  having circular cross-section. Such a plan implemented with ion beam exposure over 360° provides a central region  64  of a dose pattern  28  having a high dose value resulting from aligned Bragg peaks  67  of ion beams entering the patient  26  over a range of angles of 360° about the patient. This central region  64  is surrounded by a fringe  68  resulting from a reduced but measurable entrance dose of these proton beams. This fringe  68  can be problematic if there is radiation sensitive tissue  70 , as is often the case, directly adjacent to the tumor  62 . 
     As shown in  FIG. 7   b , a constrained rotation of the gantry head  50  and hence the fan beam  34  can substantially limit the fringe  68  while preserving good conformity between the central region  64  and the tumor  62 . The ability to stop the ions within the tissue at the Bragg peak  67  can wholly spare the radiation sensitive tissue  70 . The present inventors have determined that the limitation of the arc  56  to as little as 150° still provides close conformance of the shape of central region  64  to the tumor  62  and minimization of hot/cold spots. 
     Referring now to  FIG. 8 , the limited width of the fan beam along axes  46  makes it desirable to translate the patient support  52  along axes  46  with respect to the gantry head  50  in order to obtain treatment volumes matching the longitudinal extent of the tumor while still preserving good spatial resolution determined by the thickness of the fan beam. The table may be translated by a table translation mechanism  61  such as a motorized carriage moving the patient support  52  or the gantry head  50  or both. 
     In one embodiment of the present invention, the translation of the patient support  52  may be continuous as the gantry head  50  rocks back and forth over the treatment arc  56  in a so-called “semi-helical” scan pattern such as traces a sawtooth raster  66  along axes  46  on an imaginary cylinder  69  surrounding the axis  46 . 
     Referring now to  FIG. 9 , a sweeping of the cross-sectional area  71  of the fan beam  34  in this semi-helical scan pattern may be given a “pitch” by changing the relative speed of movement of the patient support  52  with respect to the speed of movement of the gantry head  50  in each cycle of reciprocation. The pitch determines the degree of overlap between successive sweep paths  72  of the sawtooth raster  66  moving cross-sectional area  71 , such overlap serving to reduce hotspots. The pitch shown here is greatly exaggerated and, in practice, would be reduced to a fraction of the width of the cross-sectional area  71  along axes  46 . The scanning of the cross-sectional area  71  serves also to eliminate inhomogeneities in the treatment caused by gaps between shutters used to modulate the beamlets  38  as will be described below. 
     Referring now to  FIG. 10 , alternatively a rectilinear raster  66 ′ may be adopted where the gantry head  50  is allowed to complete one half of a cycle of its reciprocation about axis  46  and then is stopped at the limits of the arc  56  to allow translation of the patient  26  along axes  46 . When movement of the patient  26  is complete the next cycle of reciprocation along arc  56  is performed. 
     Referring now to  FIG. 11  and  FIG. 5 , motion gating may be incorporated into the radiation therapy system  10 ″ of the present invention in which a sensor system  73  senses movement of the patient  26  or internal organs of the patient  26  (for example, using ECG or respiration signals) to turn the fan beam  34  from the gantry head  50  on and off to treat the patient  26  at a constant phase of periodic motion. This gating process may be improved with a rectilinear raster  66 ″ shown in  FIG. 11 , essentially rotating the rectilinear scanning pattern of  FIG. 10  so that a full range of translation of the patient support  52  is completed before moving the gantry head  50  incrementally along arc  56 . 
     Referring now to  FIG. 12 , the magnetic beam former  32  (shown in  FIG. 1 ) in a preferred embodiment may comprise two quadrupole magnet assemblies  74  and  76  receiving the pencil beam  14  (as delivered to the gantry head  50  along gantry  44 ). The pencil beam  14  is first received by a first quadrupole magnet assembly  74  and then received by the second quadrupole magnet assembly  76  downstream from the first quadrupole magnet assembly  74 . Both quadrupole magnet assemblies  74  and  76  include apertures  78  coaxially aligned along a center axis  20  of the pencil beam  14  and the fan beam  34 . 
     Referring momentarily to  FIGS. 13 and 14 , quadrupole magnets of the type used in quadrupole magnet assemblies  74  and  76  are well known in the fields of high-energy accelerator physics and electron microscopy where quadrupole magnets with relative rotations of 90° about the axis of the beam are used to help refocus a pencil beam  14  to maintain its narrow cross-section. Each quadrupole magnet assembly  74  and  76  comprises two pairs of magnets: a first pair  82   a  and  82   b  opposed across the aperture  78  along axes  79  with facing north poles, and a second pair  84   a  and  84   b  opposed across the aperture  78  along axes  79 ′ perpendicular to axes  79 . The magnets may be permanent magnets or preferably electromagnets so that the field strengths may be varied to allow the width and intensity profiles of the resultant fan beam  34  to be varied in both the convergent and divergent planes. 
     Referring again to  FIG. 12 , two quadrupole magnet assemblies  74  and  76  are aligned with respect to each other so that axes  79 ′ of quadrupole magnet assembly  74  lies in the same plane as axes  79 ′ of quadrupole magnet assembly  76  (this plane also including axis  46 ) and so that axes  79  of quadrupole magnet assembly  74  lies in the same plane as axes  79  of quadrupole magnet assembly  76 . 
     Referring to  FIGS. 6 ,  14  and  15 , the quadrupole magnet assemblies  74  and  76  produce a magnetic field  86  that tends to widen a cross-section  35  of the fan beam  34  along the plane of rotation  54  and compress it in a z-direction normal to the plane of rotation  54 . 
     As shown in  FIG. 15 , quadrupole magnet assemblies  74  and  76  act like diverging lenses when viewed in the plane of rotation  54  and converging lenses when viewed across the plane of rotation  54 . Because the forming of the pencil beam  14  into a fan beam  34  is done without scattering in a solid material, the production of neutrons is largely eliminated. 
     Note the quadrupole system will work for heavy ions of either polarity with a simple reversal of dimensions. 
     Referring again to  FIG. 12 , the quadrupole magnet assemblies  74  and  76  may be connected by controllable actuator mechanism  88  (such as a motor and rack and pinion mechanism) that may separate each of the quadrupole magnet assemblies  74  and  76  along the axis  20  according to an electrical signal and/or by mechanical adjustment. This controllable separation allows adjustment of the cross-sectional dimensions of the fan beam  34  to reduce collimation that also produces neutrons. The ability to change the cross-sectional dimensions of the fan beam  34  without collimation further allows for better utilization of the fan beam energy. The adjustment of the fan beam size may also be used for dynamic change of the beamlets  38  during treatment. 
     Referring now to  FIG. 16 , the pencil beam  14 , ultimately received by the magnetic beam former  32  (composed of quadrupole magnet assemblies  74  and  76 ) may first pass through an emergency beam stop  80  and an entrance dose monitor  81  of conventional design, the latter measuring the energy of the beam  14 . A pencil beam aperture collimator  83  may then shape the pencil beam  14  into a predictable cross-section for receipt by quadrupole magnet assembly  74 . After exiting from quadrupole magnet assembly  76  the fan beam  34  may pass through a segmented monitor measuring an energy or intensity profile of the beam  34  that may be used to further correct the energy profile of the fan beam  34  (by compensation using the binary shutter system  36  as will be described) or to correct a cross-section of the fan beam  34 , for example by controlling the field strengths of electromagnets of the quadrupole magnet assemblies  74  and  76 . The fan beam  34  is then received by a set of collimator blocks  87  sharpening the edges of the fan beam to conform with a binary shutter system  36  as will be described below. 
     Simulations have been performed modeling a 235 MeV proton beam traversing two quadrupole magnet assemblies  74  and  76  having effective lengths of 20 cm and 40 cm with transverse gradients of 22 T/m and 44 T/m respectively and a center-to-center quadrupole separation of 50 cm. The results of these simulations indicate that a proton fan beam of suitable cross-section (40×2 cm 2 ) can be generated from an entrant Gaussian beam of protons (1.5 cm FWHM) over a distance of 1.5 m. Referring now to  FIGS. 16 and 17 , the binary shutter system  36  may provide a set of attenuating arrays  90  each aligned with a separate beamlet  38  of the fan beam  34 . Each attenuating array  90  may be composed of a set of attenuating elements  92  (blade) each attenuating element  92  of a single array  90  being aligned with a particular beamlet  38 . Multiple arrays  90  are placed side by side to span the width of the fan beam  34  so that each beamlet  38  may be controlled independently by a different array  90 . 
     Referring now to  FIG. 18 , each attenuating element  92  comprises blade  94  of an energy absorbing material having a width  93  approximating the angular width of a beamlet within the plane of rotation  54  and a variable effective thickness  95  that will differ for different blades  94  as will be described. The term “effective thickness” is intended to include blades of different materials and different thickness that nevertheless operate as if they were of equal thicknesses of a single material. The blade  94  is attached to an actuator  96  that may move the blade  94  up and down along the y-axis generally perpendicular to the central axis  20  of the fan beam  34 . In a preferred embodiment, the blade  94  may be moved between two positions, one within the path of the fan beam  34  and the other completely removed from the path of the fan beam  34 . With this “binary” motion the actuator  96  may be extremely simple, for example, a pneumatic piston and cylinder (controlled by fluid pressure controlled in turn by a valve mechanism not shown) or electrical solenoid directly controlled by an electrical circuit. 
     Referring now to  FIG. 19 , a single array  90  may, for example, contain eight attenuating elements  92  having blades  94   a - 94   h . In a first embodiment, the effective thickness  95  of each blade  94   a - 94   h  along axis  20  may be according to a binary power series so, for example, blade  94   a  through  94   h  will have relative effective thicknesses  95  corresponding to successive terms in a binary power sequence (e.g.: 1, 2, 4, 8, 16 etc.). Thus, for example, blade  94   d  may be eight times as thick as the thinnest blade  94   a . In this way, as shown in  FIG. 20 , any one of 256 equal increments of attenuation may be obtained by drawing some of the blades  94  out of the beam  34  and placing some of the blades  94  into the beam. In the example of  FIG. 20 , a relative attenuation of  43  may be obtained consisting of the combined blades  94   d ,  94   a ,  94   b , and  94   f  (having attenuation&#39;s 8, 1, 2, and 32 respectively where 1 is the attenuation provided by the thinnest blade  94   a ). This “binary” sequence must be distinguished from the “binary” action of the shutters and a binary sequence need not be used for the binary shutter system  36  as will be described below. 
     This binary power series provides the simplest blade structure and actuation mechanisms but it will be understood that other power series can also be used and in fact the variations in attenuations among blades  94  need not conform to a power series but, for example, may conform to other series and may include duplicate blades  94  of a single attenuation, for example to operate at higher speed or distribute wear. For example, the blades  94  may have the relative effective thicknesses  95  of 1, 1, 3, 6, 9, 18, etc. 
     Alternatively blades  94  positionable in any of three (or more) positions with respect to the fan beam  34  (and hence capable of providing three effective attenuation levels per attenuating element  92 ) could be used providing attenuations in the series (0, 1, 2), (0, 3, 9), (0, 9, 18), (0, 27, 54) . . . . 
     It will be further understood that attenuating elements  92  need not be constructed of a uniform material in which their effective thicknesses  95  corresponds to attenuation, but may be constructed of different materials having different densities to minimize their differences in effective thickness  95  for mechanical or structural reasons. The order of the blades  94  in the fan beam  34  need not conform to their relative ranking in attenuation, and in fact in the preferred embodiment this order is buried so as to provide for suitable clearance for the attached actuators  96 . 
     In a preferred embodiment the combination of all attenuating elements  92  completely stops the fan beam  34 , and thus a proper selection of different attenuating elements  92  (short of blocking the fan beam  34 ) may be used to control range shifting of ions of the fan beam  34 , while a selection of all attenuating elements  92  (fully blocking the fan beam  34 ) may be used to control the intensity of the beam through duty-cycle modulation so that both range and intensity may be controlled with the modulator  36 . Alternatively a separate blocking element (not shown) for each beamlet  38  may be used to provide this intensity modulation. The intensity modulation or range shifting effected by the binary shutter system  36  may be augmented by other mechanisms applied to some or all of the beamlets  38 , for example those correcting the profile of the fan beam  34  or serving to offset the range shifting of all the beamlets  38  based on patient size. 
     The control of the individual blades  94  may be performed, for example, so that all of the attenuating blades  94  do not move simultaneously but are rather staggered to ensure the minimum deviation in range shifting during the transition of the blades  94 . Thus, for example, the movement of blades  94  providing greater attenuation may be alternated with movement of blades  94  providing less attenuation to reduce variations in range shifting. 
     Referring now to  FIG. 21 , two binary shutter systems  36  and  36 ′ may be opposed about the fan beam  34  effectively dividing the fan beam  34  along an x-y plane (parallel to the plane of rotation  54 ) into two separately modulated fan beams  34  and  34 ′ effectively allowing multi-slice treatment of the patient improving the speed/resolution trade-off of the treatment system. In this case the geometry of the actuators  96  and blades  94  allows all of the actuators  96  to be fully displaced out of the area of the beam  34 . 
     The binary shutter system  36  may also be used for photon modulation; the term “radiation” as used herein will include generally both photons and particles serving for treatment of tissue. 
     Referring again to  FIG. 4 , an electronic computer  100  executing a stored program may be associated with the radiation therapy system  10 ″ executing a radiation treatment plan that coordinates and controls all of the electrically controllable elements described above including but not limited to the binary shutter system  36 , the magnetic beam former  32  (including magnetic field strength of the magnets and their separation) and the movement of the gantry  44  and patient support  52  as well as receipt and control of the x-ray tomography ring  60 . This control may be done according to a stored radiation treatment plan, and in light of signals obtained from monitors  81  and  85 . Data collected by the computer  100  then provide images for the assessment of the treatment plan, as well as inputs to feedback loops confirming the proper operation of the system according to techniques known in the art of intensity modulated radiation therapy. 
     During the movement of the gantry head  50  with respect to the patient support  52 , the range and intensity of individual beamlets  38  will be modulated according to a treatment plan stored in the computer  100  and typically determined by a health care professional using an image of the tumor using the tomography ring  60 . Determination of the proper modulation of the beamlets  38  may be done by techniques analogous to those used with prior art intensity modulated radiation therapy adapted to the unique properties of ion beams. These techniques include for example Simulated Annealing and gradient based optimization techniques. 
     The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.