Abstract:
The primary collimator for a radiotherapy apparatus can be made up of several layers, each comprising several apertures, and each layer being moveable so as to select a specific aperture to build up the primary collimator shape. In this way, the shape of the primary collimator can be tailored and/or the beam filters incorporated into the primary collimator assembly. This saves space in the radiation head whilst also allowing filters to be easily interchanged.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to linear accelerators, especially (but not exclusively) those for use in medical applications such as radiotherapy. 
       BACKGROUND ART 
       [0002]    A linear accelerator (“linac”) generally consists of an electron gun that accelerates electrons to relativistic speeds, an optional target onto which the electron beam is directed in order to produce an x-ray beam, and guidance apparatus to shape and direct the resulting electron or x-ray beam as required. 
         [0003]    The guidance apparatus for a linear accelerator intended for medical use generally comprises a primary collimator, to limit the beam into a generally conical shape, one or more of a range of filters to adjust the energies present in the beam and/or to adjust the distribution of those energies, and various secondary collimators such as block collimators and multi-leaf collimators. The primary collimator and any filters aim to create a uniform generic wide-aperture x-ray or electron beam, which is then shaped as required for a specific treatment by the secondary collimation. 
         [0004]    The filters that are available for use in such apparatus usually include sections of solid material (such as Nickel) which have an x-ray absorption spectrum corresponding to an energy which needs to be removed from the x-ray beam, flattening filters which have a varying thickness (or other property) across the field of the beam so as to alleviate irregularities in the beam intensity across that field, and (for electron beams) filters having a material and a thickness able to condition the beam and/or preserve the vacuum within the linear accelerator. 
         [0005]    At present, such beam modification filters have to be positioned between the primary collimator wheel and the secondary collimation device. They are usually placed in or on a rotating carousel, which is permitted to rotate freely in a manner that does not interfere with the collimator structure. The filters will however produce scattered X-radiation (as will any object placed in the beam), so further shielding needs to be put in place around the filters to prevent unwanted leakage radiation escaping from the head. This in turn increases the mass of the head and hence the mechanical load on the head support arm. 
         [0006]    Current linac construction is therefore tailored to the needs of each customer, by placing into each linac machine during construction the combination of filters and beam modifiers that are needed in order to allow the beam energy options that the customer has chosen. 
         [0007]    U.S. Pat. No. 4,198,570 discloses a system in which the electron target and beam modification components are contained within a primary collimator. The source, collimator and filter all remain in the same unitary assembly, and there seems to be no ability to interchange different filters. 
         [0008]    US2011/0075815 discloses a system where the beam filters for a radiotherapeutic device are on a wheel that can rotate through an electron beam. The rotating plate can also be translated along an axis to allow the positioning of a light field generator for beam verification. 
         [0009]    WO2009/138753 discloses a linear accelerator able to provide both a therapeutic (MV) radiation beam and an investigative (kV) radiation beam. A pair of primary collimators are provided, on a common sliding substrate so that one or the other can be located in the path of the electron beam produced by the accelerator. Each has an associated target, so that one produces MV x-radiation and the other produces bremsstrahlung x-radiation at kV energies. The latter has an associated electron absorber located within the primary collimator. Further interchangeable filters are provided on a carousel after the primary collimator, to condition the x-ray beams for use. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention therefore provides radiotherapy apparatus comprising a linear accelerator for producing a beam of electrons, optionally an x-ray target for producing a beam of ionising radiation from the electron beam, a primary collimator for delimiting the thus-produced beam to a first extent, at least one variable-geometry collimator for delimiting the beam to a second and lesser extent, wherein the primary collimator consists of a plurality of layers arranged transversely to the beam, each layer having a plurality of apertures and being independently movable so as to bring a selected one of the apertures into the path of the beam, thereby to define a complete collimator shape made up of an aperture from each layer. 
         [0011]    The x-ray target can be permanently present so that the thus-produced beam is always an x-ray beam, or it can be omitted completely such that the thus-produced beam is always an electron beam, or is can be present within the machine and movable so as to allow the thus-produced beam to be an electron beam or an x-ray beam according to the choice made by the operator. In the latter circumstance, the x-ray target can be movable between a position in which it is located in the electron beam so as to substantially absorb the electron beam and produce an x-ray beam, and a location in which it is located substantially outside the electron beam. 
         [0012]    The layers making up the plurality of layers can be movable by rotation, or by translation. It will usually be easier if either all layers are movable exclusively by rotation, or all layers are movable exclusively by translation, but an arrangement in which some are movable by rotation and some are movable by translation can be envisaged. 
         [0013]    One or more filters for the beam can be placed within the apertures. In this way, a filter can be moved into or out of the beam easily by appropriate selection or de-selection of that aperture that contains it. Ideally, several of the apertures contain a filter; these may be in the same layer (so as to allow a choice of filters) or in different layers (so as to allow filters to be combined). The filters can be of a different nature, such as to allow the desired effect to be produced or to cater for the different types of beam (electron or x-ray). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; 
           [0015]      FIG. 1  illustrates the general layout of a typical known form of linear accelerator 
           [0016]      FIG. 2  illustrates a primary collimator according to the present invention; and 
           [0017]      FIGS. 3 ,  4 ,  5  and  6  show alternative embodiments of the multi-layer collimator. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0018]      FIG. 1  shows the linear accelerator  10  of WO2009/138753. An electron gun  12  (illustrated schematically) such as a linear accelerator is enclosed within a vacuum chamber having an external wall  14 . This wall  14  has an aperture  16  which is covered by a sliding carrier  18  that includes a Tungsten/Copper layered x-ray target and an electron window. In one position, the carrier  18  is moved so that the target covers the aperture  16 . In another position, the carrier  18  is moved so that the electron window covers the aperture. Thus, in either position the vacuum chamber  14  remains sealed, but the electron beam  46  produced by the electron gun  12  will either pass through the electron window, or will interact with the x-ray target to produce an x-ray beam and be absorbed in the process. In this way, a choice of an x-ray or an electron beam is available for therapeutic use. 
         [0019]    The present invention could of course be applied to a device having either an x-ray target only (and hence being unable to produce an electron beam) or an electron window only (and hence being unable to produce an x-ray beam). However, most commercial radiotherapy devices are capable of both these beam types. 
         [0020]    Immediately outside the chamber  14  is a primary collimator set  20 . This set  20  includes a first primary collimator  22  and a second primary collimator  24  into which has been inserted a carbon absorber  26  held in place with Aluminium support struts. The set  20  is indexable between two positions, akin to the sliding carrier  18 , so that one primary collimator of the two is presented in front of the aperture  16 . 
         [0021]    Beneath the primary collimator set  20 , there is a motorised filter carousel  28 . This is mounted on an axle offset to one side beneath the aperture  16  and includes a plurality of filter recesses. A first filter recess  30  is (in this case) empty although is could alternatively contain a conventional flattening filter. A second filter recess contains a so-called “bow-tie” filter  32 . Bowtie filters are used in CT (computed tomography) scanning for a variety of reasons, including to equalise the signal to noise ratio and to eliminate certain image artifacts etc. Generally, a bow-tie filter is used to compensate the X-ray attenuation for the different thickness regions in the patient, so that uniform X-ray intensity is produced at the detector. It allows a greater intensity to pass in a central region of the beam, progressively attenuating the beam more towards the outer edges. 
         [0022]    Below the bow-tie filter  32 , there is an ion chamber  34  and a set of collimators generally indicated as  36 . This can include elements such as multi-leaf collimators  38 , block collimators  40 , and the like, operating in one or more planes transverse to the beam. 
         [0023]    Below the collimators there will usually be a patient  42  supported on a patient table. Below the patient table is a flat panel scintillator detector  44  (as described above), mounted on an automated imager arm (not shown) which can extend the flat panel detector  44  into place or withdraw it, as required. 
         [0024]    Generally, the entire radiation head is mounted so as to be rotatable around a horizontal axis I, taking the flat panel detector  44  with it. The patient  42  is supported on the patient table so that the axis is within the patient. The intersection of the axis with the centre of the beam produced by the radiation head is usually referred to as the “isocentre”. It is usual for the patient table to be motorised so that the patient  42  can be positioned as required with the tumour site at or close to the isocentre. 
         [0025]    Most radiation heads in use are simpler than that described above, and include (for example) the alternative primary collimators, one with an electron window and one without, according to whether the radiation head is in an x-ray production mode or an electron beam production mode. Typically, all include the carousel  28  for inserting one or more filters into the beam such as a bow-tie or a flattening filter. 
         [0026]    The idea of the present invention, as illustrated in  FIG. 2 , is to create a “mix and match” beam modification device which contains multiple elements necessary for both collimation and energy modification, within the same structure. To achieve this, a primary collimator is provided which is split into at least two different sections (in this case, three), each of which is mounted onto a wheel (see  FIGS. 3 to 6 ) or alternative means for moving the sections, such as sliding layers ( FIG. 2 ), so that several alternative collimation sections can be moved into the path of the beam. In this way it is easier to switch between a greater number of different beam configurations compared with the existing system. 
         [0027]    Thus,  FIG. 2  illustrates schematically the essential parts of a primary collimator according to the present invention, to replace the primary collimator set  20  and optionally the motorised filter carousel  28  of  FIG. 1 . An upper sliding section A, an middle sliding section B, and a lower sliding section C are all provided on appropriate mounts (not shown), and are able to slide laterally relative to each other and independently of each other. Each sliding section has (in this case) three collimating apertures  50 ,  52 ,  54  which are shaped as truncated conical sections, sized so that, together, a collimating aperture from each of the upper, middle and lower sliding sections collectively define a collimator shape that is the desired shape of a primary collimator. 
         [0028]    Thus, the upper sliding section A has three apertures  50 A,  52 A and  54 A, the middle sliding section B has three apertures  50 B,  52 B and  54 B, and the lower sliding section C has three apertures  50 C,  52 C and  54 C. As illustrated, the apertures  52 A,  52 B and  52 C are in line with a beam path  56 , but the sliding sections can be adjusted so as to collimate the beam  56  using any one of the three upper apertures  50 A,  52 A and  54 A, plus any one of the three middle apertures  50 B,  52 B and  54 B, and any one of the three lower apertures  50 C,  52 C and  54 C. 
         [0029]    The apertures are all conically-sided, with the sizes increasing progressively from the upper apertures to the lower apertures so that, when aligned, they together define a smooth conical shape that is the familiar circular-section cone, typical of primary collimators. In this example, all the upper apertures are identical in shape, as are all the middle apertures and all the lower apertures, but this is not necessarily so and the sliding sections could include apertures having alternative shapes or alternative surface treatments. Likewise, the three sliding sections all have the same number of apertures, but if it proved necessary then one or more sliding section could be provided with a greater or lesser number than the others. 
         [0030]    Thus, the sliding sections are supported so that one collimating aperture from each can be located in the path of a beam to, together, define a primary collimator. Further, the sliding sections can be moved so that a chosen collimating aperture from each sliding section is brought into register with the beam path and contributes toward the primary collimator shape. 
         [0031]    Each aperture of each sliding section is also provided with a beam modifying element which may be one of a beam conditioning filter, a flattening filter, a bow-tie filter, no filter at all (i.e. empty), or any other form of beam modifying element. Conditioning filters usually comprise a sufficient thickness of an element that has an x-ray absorption peak at an energy (or frequency) that corresponds to an energy that needs to be removed or attenuated from the beam. Nickel is commonly used for this purpose. Flattening filters seek to attenuate the beam to a greater extent in its centre section and to a lesser extent at its margins, as the “raw” beam produced by the accelerator often has a greater intensity in its centre section. Thus, a flattening filter creates a more even beam whose images are easier to interpret with the human eye. A bow-tie filter does the opposite, creating a beam that is stronger in its centre section where most of the attenuation in the patient takes place, and whose images are thus more susceptible to CT reconstruction. Each sliding section does however have at least one empty aperture (i.e. no filter at all) to allow electron beam therapy or filter-free X-ray therapy to be performed. 
         [0032]    In this example, the apertures are filled as follows: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Aperture 50 
                 Aperture 52 
                 Aperture 54 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Upper 
                 Empty 
                 Conditioning filter 1 
                 Conditioning filter 2 
               
               
                 Section (A) 
               
               
                 Middle 
                 Empty 
                 Flattening filter 1 
                 Flattening filter 2 
               
               
                 Section (B) 
               
               
                 Lower 
                 Empty 
                 Bow-tie filter 
                 Conditioning filter 3 
               
               
                 Section (C) 
               
               
                   
               
             
          
         
       
     
         [0033]    Thus, the filters can be provided in a wide range of combinations. For example, conditioning filter  1  plus flattening filter  2  can be provided by moving the sliding sections so as to align apertures  52 A,  54 B and  50 C. Alternatively, conditioning filters  1  and  3  plus flattening filter  2  can be provided by moving the sliding sections so as to align apertures  52 A,  54 B and  54 C. Existing arrangements can often limit the number and/or nature of filters that can be provided in combination, and the present invention overcomes this. 
         [0034]    The structure in which these different beam modification elements are contained is inherently designed to contain scattered X-radiation. Such shielding is normally necessary around a primary collimator in any case, so this adds little or no additional weight to the structure. However, the filter carousel  28  is no longer needed and can be omitted in its entirety, together with the shielding previously provided to cater for the scattered X-radiation that it created. Thus, overall the design of the present invention offers a beam collimator/modifier structure with an overall depth and a weight that is less than if the collimator is to be separated from the beam modification filters. The depth of the radiation head in which the beam collimator/modifier structure is contained needs to be accommodated between the gantry arm and the patient, and a lower-profile head is advantageous. A reduction in the weight of the head also improves the mechanical accuracy of the positioning of the head, as the head must be supported at the end of the support arm in the manner of a cantilever. 
         [0035]      FIGS. 3 to 6  show that there a number of different ways this type of beam collimation and profile adjustment (i.e. energy make up of the beam or intensity distribution across the beam) can be arranged for spatial efficiency. Variables include width and depth of collimator layers, movement of axis of rotation and the types of filter placed in each layer (apertures shown blank in some cases, simply to illustrate the aperture variations). It should also be noted that the wheels may be made of more than one type of material. For example, a suitable matrix material of necessary structural rigidity could be used for the main body of each layer, with a higher attenuation material sleeve being inserted into the matrix material for the purposes of collimating the X-rays. For electron collimation, this sleeve material can be made even thinner due to the poorer material-penetrating properties of electrons. In this way, cost, weight, and machining time can be reduced. For the rotating layers, a central shaft can be used for those systems where the rotational axis is aligned between layers. Alternatively, bearings or cog teeth around the outside of the wheel can be provided. 
         [0036]    Thus,  FIG. 3  shows a system  60  with three layers  62 ,  64 ,  66 , all of the same thickness and all journalled on a common shaft axis  68 . Each layer comprises a pair of apertures (A and B), one empty and one containing a filter of various types. Thus, apertures  62 A,  64 A and  66 A are all empty. Aperture  62 B contains a Nickel-based beam modifying filter, aperture  64 B contains a flattening filter, and aperture  66 B contains a bowtie filter. Thus, any combination can be provided of the flattening filter, bowtie filter, or neither, with and without the beam modifying filter, by appropriate rotation of the layers  62 ,  64 ,  66  to align the appropriate apertures with the beam path  70  and build up the primary collimator. 
         [0037]      FIG. 4  shows a system  80  in which the width of the layers  82 ,  84 ,  86  increases progressively along the beam path  90 . The layers all share a common rotation axis  88 . The uppermost layer  82  (i.e. that closest to the source) has smaller apertures and the lowest layer  86  has wider apertures, due to the conical nature of the primary collimator shape that needs to be built up. Accordingly, providing the same depth of material around the apertures will mean that the upper layer can be less wide than the lower layer, reducing material usage and weight. 
         [0038]      FIG. 5  shows a system  100  in which this is taken one step further. The width of the upper layer  102  is reduced still further, by placing the apertures closer to the rotation axis of that layer. This means that, in order to locate the apertures along the same beam axis  110 , each layer needs its own rotation axis. Accordingly, the upper layer  102  rotates about a first axis  112 , the middle layer  104  rotates about a second axis  114 , and the lower layer  106  rotates about a third axis  116 . The axes  112 ,  114 ,  166  are spaced progressively further from the beam axis  110  so as to place the progressively larger apertures in alignment as shown. 
         [0039]      FIG. 6  shows a system  120  in which the three layers  122 ,  124 ,  126  have different depths along the beam axis  130 , tailored to the depths of the filters that are accommodated within the apertures concerned. These layers are shown with varying widths and journalled on a common rotation axis  128  as in  FIG. 4 , but they could of course have the same widths as shown in  FIG. 3  or individual rotation axes as shown in  FIG. 5 . Indeed, the variations shown in  FIGS. 3 to 6  can be combined as desired. 
         [0040]    It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention.