referring now to fig1 , 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 fig2 , 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 fig3 , 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 fig4 , 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 fig3 ) 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 fig5 , 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 fig4 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 fig7 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 fig7 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 fig8 , 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 fig9 , 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 fig1 , 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 fig1 and fig5 , 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 fig1 , essentially rotating the rectilinear scanning pattern of fig1 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 fig1 , the magnetic beam former 32 ( shown in fig1 ) 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 fig1 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 fig1 , 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 fig6 , 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 fig1 , 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 fig1 , 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 fig1 , 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 fig1 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 fig1 , 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 fig1 , 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 fig2 , 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 fig2 , 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 fig2 , 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 fig4 , 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 .
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 .