Abstract:
Embodiments disclose a radiotherapy apparatus comprising a source of radiation configured to emit a beam of radiation and a collimator structure configured to limit a lateral extent of the beam, the collimator structure including a primary collimator configured to shape the beam, a first collimator comprising a plurality of adjacent elongate leaves, the leaves being extendable into the beam in a first direction transverse to the beam, and a block collimator including an aperture configured to permit the beam to pass through, the block collimator being extendable into the beam in a second direction transverse to the beam and transverse to the first direction. In some embodiments, the aperture may be cone-shaped or a through-hole, which may be empty or filled with a radiotransparent material. In some embodiments, the block collimator may include a plurality of apertures, which may be of varying dimensions.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to collimation apparatus for use in radiotherapy. 
       BACKGROUND ART 
       [0002]    Radiotherapy consists of the application of harmful radiation to lesions (such as tumours) within the body. The radiation interferes with cellular processes within the lesion and alleviates the condition. A principal concern within radiotherapy is to maximise the dose that is delivered to the lesion and minimise the effect of dose that is delivered to surrounding healthy tissue. This is done in a number of ways. 
         [0003]    One is to deliver the dose in a number of “fractions”, separated over time such as on sequential days. Another is to deliver the radiation from multiple different directions, thus delivering only a partial dose to the surrounding tissue. Often, this is done by mounting the radiation source on a gantry that is rotatable around an axis and directed along a beam direction that co-incides with that axis, thus delivering radiation from (potentially) all directions within a plane transverse to the axis. Another method is to shape the cross-sectional profile of the beam using suitable collimators, such as to conform to the external cross-section of the lesion when viewed along the beam axis, or to another shape calculated to deliver a dose that will build up towards a desired three-dimensional dose distribution. Usually, some combination of all three will be employed. 
         [0004]    In relation to the collimation of the beam, most tumours can be dealt with using a combination of block collimators and a so-called “multi-leaf” collimator. A block collimator is a solid block of radiopaque material such as tungsten, which usually has a straight front edge that spans the entire width of the device&#39;s aperture, and which can be advanced and/or withdrawn across the aperture in a direction transverse to the front edge. Thus, the block collimator has the effect of adjusting the width of the aperture as needed. A pair of such collimators arranged face-to-face can thus narrow the aperture from both opposing sides. 
         [0005]    A multi-leaf collimator such as the one disclosed in our earlier application EP-A-0,314,214 comprises an array of long, narrow, deep leaves of radiopaque material that can each be extended into and out of the aperture. Arranged side-by-side, the tips of the leaves therefore define a chosen shape which can be varied at will by extending or retracting individual leaves. 
         [0006]    Both the leaves and the block collimators usually have rounded tips in order to reduce the penumbra that they cast and thus improve the definition of the beam that is allowed through. This does therefore mean that opposing collimator elements cannot be extended so that they meet and close the field completely. Usually, an aperture will be collimated by a pair of opposed block collimators operating in one direction (say, the y direction) and a pair of opposed multi-leaf collimator (“MLC”) banks operating in the transverse direction (say, the x direction), both directions being transverse to that of the beam (the z direction). Our earlier application WO2008/141667 discloses a design for the block collimator leaves which can co-operate with the MLC leaves so as to minimise the necessary weight of the block collimator elements; the reader is specifically directed to that disclosure (which is incorporated herein by reference) for a fuller understanding of the present invention. 
         [0007]    To produce very small radiation fields, the resolution of a standard MLC and block collimator is usually too coarse, and the rounded tips of both are unsuited to forming very narrow beams. Therefore, one of a micro-MLC or “stereotactic cones” are often used. A micro-MLC is akin to a standard MLC but much smaller. This is however very difficult to engineer, as the leaves are very thin and therefore less rigid. Stereotactic cones offer a range of preset shapes and sizes, such as 5 mm, 10 mm, 15 mm and 20 mm diameter circles, and are provided as “add-on” collimators that can be attached to the radiation head as and when needed. These are usually fitted to the exterior of the radiation head, in the path of the beam, so as to place them as close as possible to the patient for best accuracy. As an alternative, US2014/0048727 discloses stereotactic cones that are integrated into the beam generation system adjacent the primary collimator; this will be more convenient for the operators but at the expense of lesser accuracy in delivery. 
       SUMMARY OF THE INVENTION 
       [0008]    We would like to provide stereotactic cones that can be placed into the beam easily and securely, but can nevertheless be in the correct position to achieve the necessary levels of accuracy, i.e. close to the patient. 
         [0009]    To that end, the present invention proposes a radiotherapy apparatus comprising a source of radiation adapted to emit a beam of radiation, and a collimator structure for limiting the lateral extent of the beam, the collimator structure comprising a primary collimator, a collimator comprising a plurality of adjacent elongate leaves, each being controllably extendable into the beam in a first direction transverse to the radiation beam, and at least one block collimator, controllably extendable into the beam in a second direction transverse to the radiation beam and transverse to the first direction, wherein the block collimator has at least one aperture aligned to permit the beam to pass through. 
         [0010]    Thus, when a stereotactic cone is needed, the block collimator can be extended sufficiently to place the aperture within the radiation beam, the aperture then acting as a stereotactic cone. The aperture is thus preferably cone-shaped. When a stereotactic cone is not needed, the block collimator can be used as normal and the MLC leaves extended as necessary in order to cover the aperture. As the block collimator can be located beneath the MLC/first collimator, it can be substantially closer to the patient than that of US2014/0048727. Indeed, it is possible for the block collimator to be the last beam-shaping element of the radiation head, and therefore very close to the patient indeed. Thus, the block collimator may be adjacent (in the beam direction) to an external cover of the source of radiation. 
         [0011]    The aperture can be a through-hole, which may be empty or filled with a radiotransparent material. There can be several apertures in the block collimator, in order to provide several stereotactic cones. These may be of varying dimensions. Where there are at least three apertures, the minimum total spacing on each side of an aperture is preferably 2d, such that 2d≧(w−a) where w is the width of the leaves of the first collimator and a is the diameter of the aperture. If the spacings on either side of the aperture is unequal, then 2d will be the sum of the two spacings and d will be the average of them. Ideally, no two of these apertures are the same size. 
         [0012]    The block collimator can have a central region of a first thickness flanked by side regions of a second and lesser thickness, with the aperture being located in the central region. It may have a front edge portion of the first thickness. Alternatively, it may comprise a spine portion extending in the second direction and in which the aperture is located, and a frontal portion extending transversely to the spine portion in either direction to define a T shape. Thus, the frontal portion can extend in the first direction. The spine region can thus be of a first thickness, flanked by side regions of a second and lesser thickness. 
         [0013]    Typically, there will be two block collimators, i.e. a first block collimator and a second block collimator, each extending into the beam from opposing sides. In this case, we prefer there to be apertures on both block collimators. Preferably, the first block collimator contains apertures that are all of different sizes to each of the apertures on the second block collimator, to avoid duplication and provide the maximum range of sizes. To allow the greatest number of apertures in the space available, and thus offer the widest choice of sizes, the smallest aperture of one block collimator can be sized between the sizes of the smallest and the second-smallest apertures of the other block collimator. Likewise, the largest aperture of one block collimator can be sized between the sizes of the largest and the second-largest apertures of the other block collimator. We prefer that where a block collimator has more than one aperture, the smallest is located nearest the front edge of the block (i.e. the edge that lies in the radiation beam). 
         [0014]    The apparatus preferably includes a control apparatus adapted, when in a first mode, to extend leaves of the first collimator in order to cover the aperture. In a second mode, corresponding to the use of stereotactic cones, it will extend the block collimator so as to place the aperture or one of them in the radiation beam, and cover any other apertures with leaves of the MLC. It may also cover any remaining areas or locally thinner areas of the block collimator with leaves of the MLC. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; 
           [0016]      FIG. 1  shows a known arrangement of a block collimator and a multi-leaf collimator; 
           [0017]      FIG. 2  shows a block collimator and a multi-leaf collimator for a radiotherapy head, according to the present invention, viewed along the beam axis; 
           [0018]      FIG. 3  shows the block collimator and a multi-leaf collimator of  FIG. 2 , viewed transverse to the beam axis; 
           [0019]      FIG. 4  shows the collimators of  FIG. 3 , with the stereotactic cone out of use; 
           [0020]      FIG. 5  shows a block collimator and a multi-leaf collimator being a second embodiment of the present invention, with the stereotactic cones being out of use; 
           [0021]      FIG. 6  shows the embodiment of  FIG. 5 , with a selected stereotactic cone in use; and 
           [0022]      FIG. 7  illustrates the spacing of apertures relative to the leaf size. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0023]    Referring to  FIG. 1 , a view along the beam axis of a conventional collimation arrangement is shown. The purpose of the collimators is to allow the transmission of a beam which has a desired cross-section and to provide as complete shielding as possible across the remainder of the beam field (i.e. the maximum extent of the beam). To allow shaping of the beam, a multi-leaf collimator (MLC)  10  is provided which comprises a series of individual leaves  12  of a radiopaque material such as tungsten, arranged side-by-side relative to each other, in two opposing arrays  10   a,    10   b.  Thus, the lower array  10   a  extends into the beam field in the x direction from one side of the field, and the upper array  10   b  extends into the beam field in the x direction from the opposing side of the field. The leaves  12  can each be moved independently of the others so as to define a chosen shape  14  between the tips of the opposing leaf banks  10   a,    10   b.  Each leaf is thin in its transverse (y) direction to provide good resolution, is deep in the (z) direction to provide adequate absorption, and long in its longitudinal (x) direction to allow it to extend across the field to a desired position. Generally, the longitudinal length of the leaf will be greater than its depth, and both will be much greater than its transverse thickness. 
         [0024]    To allow for the fact that directly opposing MLC leaves  12  from opposing banks  10   a,    10   b  cannot fully approach each other (as described above) to close off parts of the field, and that there is usually some degree of leakage through the gap between adjacent leaves, a pair of block collimators  16 ,  18  are also provided. These consist of a pair of solid blocks of radiopaque material such as tungsten, which extend inwards in the y direction from the two opposing sides of the field . They have a front edge that spans the entire width of the field and which is straight (in the x-direction), and which can each be advanced and/or withdrawn independently across the field in a direction transverse to the front edge. Thus, the block collimators provide additional shielding in locations spaced from the field shape along the y direction, limiting inter-leaf leakage between the tips of opposing leaves and between adjacent leaves. 
         [0025]      FIG. 2  shows a development of this arrangement, based on the concept of our earlier disclosure WO2008/141667 but incorporating the present invention as well. As disclosed in WO2008/141667, the block collimator  20  is reduced in weight by having a spine  22  and a front edge  24  that are of the usual thickness, flanked on either side of the spine  22  and behind the front edge  24  by lateral regions  26 ,  28  that are substantially thinner in the (z) direction of the beam.  FIG. 2  shows one block collimator  20  but there will usually be a second block collimator extending from the opposite side of the field. As shown in  FIG. 2  and WO2008/141667, the MLC leaves  12  in the region covered by the block collimator  20  are extended so as to cover the thinner lateral regions  26 ,  28 . The inevitable gap between opposing leaves  12  is therefore covered by the thicker spine region  22 , whereas the lateral regions  26 ,  28  only need to block leakage from between the MLC leaves  12 . A triangular transition region  30  is provided at the junction of the spine  22  and the front edge  24  which is of substantially the same thickness of the spine  22 , in order to allow time for the MLC leaves  12  to extend or retract in the x direction to the spine position, while the block collimator  20  is itself extending or retracting across the field in the y direction. The minimum shape and dimensions of the transition region  30  are dictated by the various movement speeds of the collimator elements as explained in WO2008/141667. 
         [0026]    The specific shapes of the spine  22  and the transition region  30  can be varied as desired and (in particular) it is not necessary for the spine  22  to be at the centreline of the block collimator, although that may often be the most convenient location. Equally, it is not necessary for the block collimator of the present invention to be of the type described in WO2008/141667 (although we do prefer this), and the invention can be applied to simple plain block collimators such as that shown in  FIG. 1  or other designs of block collimator. 
         [0027]    According to the present invention, a radiotransparent aperture  32  is provided in the block collimator. Normally, this would be avoided at all costs, as the purpose of the block collimator is to provide a complete block to radiation and an aperture goes against this principle. However, it means that when desired, the block collimator can be extended so as to locate the aperture  32  at a desired position in the beam field, usually the centre. This implies that the aperture should ideally be at the centreline of the block collimator. Where the block collimator has a spine  22 , the aperture  32  is preferably located on the spine  22  in order to provide adequate attenuation in the regions immediately around the aperture  32 . The spine may be thicker in this region, if desired. The MLC leaves  12  can then be extended to cover the lateral regions  26 ,  28  as before. 
         [0028]    The aperture  32  can thus act as a stereotactic cone, limiting the radiation beam to just a pencil beam of the desired diameter (dictated by the size of the aperture). The aperture  32  is therefore preferably frusto-conical in shape, matching the divergence angle of the beam at that location so as to provide a beam with the minimum penumbra. The aperture  32  can be hollow, or it can be filled with a radiotransparent material such as perspex or glass. 
         [0029]    Where the apparatus is being used in a conventional manner, i.e. collimated by the block collimators and the MLC rather than in a stereotactic manner, the MLC leaves  12  can simply be extended across the aperture  32  in order to cover it and substantially prevent transmission through the aperture  32 , as shown in  FIG. 2  where individual leaves  36 ,  38  together cover the aperture  32 . The spine  22  may need to be made slightly wider than would otherwise be the case, or offset slightly, to provide an adequate width to cover the gap  34  between opposing MLC leaves  12 . 
         [0030]      FIGS. 3 and 4  shows a cross-section through the relevant parts of the radiotherapy head in the x-z plane of  FIG. 2 . A conventional pointlike radiation source  40  is formed by an x-ray target (not illustrated in  FIGS. 3 and 4 ) onto which is directed a beam of electrons produced by a linear accelerator(not illustrated in  FIGS. 3 and 4 ). This is shaped by a conical primary collimator  49  which allows a conical x-ray beam  42  to be produced. This is then filtered and measured in the conventional manner. The primary collimator  49  (shown only in  FIG. 3 ) is in the form of a block of a material which is substantially opaque to radiation, such as tungsten, with an aperture (usually frustoconical) machined into it so as to define the extent of the beam of x-rays after they are created at the X-ray target (not shown). 
         [0031]      FIG. 3  shows the apparatus in use in a stereotactic cone mode. Thus, the block collimator  20  is positioned so that the aperture  32  is at the centre of the beam field and the block collimator covers substantially all of the beam  42 . The MLC leaves  12  are advanced into the beam  42  to cover the thinner lateral regions  26 ,  28 ; if the block collimator is a conventional block collimator as illustrated in  FIG. 1  then this may be considered unnecessary, although doing so will assist in reducing leakage. The MLC leaves  12  and the block collimators  20  provide the accurate shaping of the beam for radiotherapy. Although the MLC leaves  12  are shown in  FIGS. 3 and 4  as being closer to the radiation source than the block collimator  20 , those skilled in the art will appreciate that these positions may be reversed, so that the block collimator  20  is closer to the source  40  than the MLC leaves  12 . 
         [0032]      FIG. 4  shows the apparatus in a normal mode of operation, in which the conical beam formed by the primary collimator (not shown) is shaped by the MLC  10  and the block collimators  20 . The illustrated block collimator  20  may be at any position called for by the currently-required beam shape, and the relevant MLC leaves  12  are advanced so as to block the beam  42  over the aperture  32 . The opposing leaf  12  will be advanced so as to cover the relevant lateral region  26 . The gap between the opposing leaves is covered by a section of the spine region  22  of the block collimator  20  that is adjacent the aperture  32 . 
         [0033]      FIGS. 3 and 4  also show a part of the housing  44  of the radiotherapy head containing the collimators (etc.). This has an opaque outer section  46  which holds a glass or perspex window  48  through which the collimated beam is emitted. The window may be opaque (so long as it is radiotransparent) but is preferably transparent as radiotherapy heads often include visible light sources that are in optically equivalent positions to the x-ray source, to provide a visual check on the operation of the collimators. As can be seen in  FIGS. 3 and 4 , the block collimator  20  is the last beam-shaping element of the radiation head, located immediately prior to the window  48  which defines the exterior shape of the head. Thus, the aperture  32  will define as accurate as possible a shape for the stereotactic beam without requiring additional beam-shaping devices to be affixed to the outside of the radiation aperture. 
         [0034]      FIGS. 5 and 6  show a second embodiment of the present invention and illustrate the block collimators and the MLC leaves along the direction of the beam, i.e. the z direction. These collimators are fitted within a radiation head that is otherwise conventional. The radiation head produces an x-ray beam with a rectangular maximum field  50 . This is collimated in the y direction, as before, by a pair of block collimators being a first block collimator  52  which extends upwardly (in  FIGS. 5 and 6 ) from the lower part of the field  50  and a second block collimator  54  which extends downwardly from the upper part of the field  50 , together delimiting the beam in the y direction. In the x direction, a pair of MLC leaf banks  56 ,  58  each comprise a plurality of leaves  60  arranged side-by-side and extendable into the field  50  in the x-direction to define a chosen profile. The two collimator sets thus define a desired beam shape  62 , with the shape defined by the MLC leaves and the block collimators providing additional backup on either side of the shape. 
         [0035]    As before, the block collimators each comprise thinner lateral regions  66 ,  68  on either side of a central spine region  64  which leads from the rear of the collimator to a front edge  68  of substantially the same thickness as the spine  64 . Thus, as before, the MLC leaves  60  are advanced so as to cover the thinner lateral regions  66 ,  68 , with the gap between opposing leaves  60  being covered by the spine region  64 . 
         [0036]    In this embodiment, each block collimator carries several apertures within its spine region  64 , all being of different sizes. Thus, the first block collimator  52  has three apertures  70   a,    70   b,    70   c  and the second block collimator  54  has three apertures  70   d,    70   e,    70   f.  On each collimator, the apertures  70  are arranged linearly along the respective spine  64 , along the centrelines of the blocks  52 ,  54 . The spines  64  are offset slightly from the centre of the blocks  52 ,  54  so that there is adequate space to one side of the apertures  70  in order to cover the gaps between opposing leaves  60 . As illustrated, the spines  64  are straight-sided but this is not necessarily the case and the spines  64  may be narrowed (on one side or the other) in regions away from the apertures  70 . 
         [0037]    Each aperture  70  is of a different size, in order to offer a choice of sizes for the stereotactic cone. To allow the minimum possible leakage of radiation through the collimator set, the apertures are placed in size order. Thus, the smallest apertures  70  are placed closest to the front edge  68 , and the largest apertures  70  are placed furthest from the front edge  68 . As the regions of the block collimator  52 ,  54  closest to the front edge  68  are the parts most commonly in the beam field  50 , this minimises the average area of aperture that is in the beam field  50  (and hence covered by an MLC leaf or leaves only) at any particular time. 
         [0038]    Likewise, the apertures are distributed between the block collimators  52 ,  54 , so that the smallest aperture  70   a  is placed on (say) the first block collimator  52 , and the second-smallest aperture  70   d  is placed on the other block collimator  54 . The third-smallest aperture  70   b  is then adjacent the smallest aperture  70   a  on the first block collimator  52 , and so on with the apertures alternating from one block collimator to the other in increasing order of size, allowing the leakage to be minimised. 
         [0039]    As the apertures are located along a straight line substantially parallel with the y direction, this means that each successive aperture can be moved (when in use) to a consistent point within the radiation field. This means that the aperture size can be varied effectively independently and without requiring a corresponding adjustment in the head location. In addition, placing that straight line along the centre of the radiation field means that the apertures can be placed at the centre of the field, aligned with the peak in the radiation beam and creating the minimum penumbra. However, other arrangements may allow for a greater number of apertures, if this is felt necessary. 
         [0040]      FIG. 6  shows the apparatus in the stereotactic mode. In this instance, the cone size needed is that of the aperture  70   e.  The first block collimator  52  is therefore withdrawn from the centre of the field and the second block collimator (carrying the selected aperture  70   e ) is extended to place the aperture  70   e  at the centre of the radiation field. The MLC leaves  60  are then extended so that their tips lie under the region of the spine  64  adjacent the apertures  70 , with the exception of (in this case) two leaves  64   a,    64   b  which are withdrawn as necessary to reveal the selected aperture  70   e.    
         [0041]    In this example, there are three apertures on each block collimator. To ensure that any selected aperture can be revealed exclusively of the others, the spacing between the central aperture  70   e  and the apertures  70   d,    70   f  on either side should be sufficient to allow the two outer apertures  70   d,    70   f  to be blocked. This calls for the leaves immediately adjacent to the leaf or leaves that are withdrawn to reveal the central aperture  70   c,  to be capable of completely covering the two outer apertures  70   d,    70   f.  In other words, the spacing between apertures should be sufficient that the leaf or leaves that need to be withdrawn in order to expose the aperture in question should not also expose an adjacent aperture.  FIG. 7  illustrates this constraint in practice, in a case where the leaves are wide relative to the aperture sizes, and implies that the aperture spacing is at least d, such that 2d≧(w−a) where w is the width of the leaves  60  of the MLC and a is the diameter of the aperture  70   e  in question. This is derivable from the inequality w≦2d+a which is the constraint that ensures that the particular leaf  60  that is withdrawn in order to reveal the middle aperture  70   e  does not also expose any part of the adjacent apertures  70   d,    70   f.  If the spacings on either side of the aperture are unequal, then d can be measured as the average of them, i.e.  2   d  will be the sum of the two spacings on either side of the aperture. Where the leaf width is less than the size of the aperture (i.e. w&lt;a), other constraints will apply. In general, the exact constraint that applies will depend on the geometry of the system in question, including such factors as the precise intended location of the aperture in use relative to the leaf edges. 
         [0042]    Some form of control apparatus will usually be needed in order to send appropriate control instructions to the collimators and to the beam production apparatus. At its simplest, this will, in a first mode, act to extend leaves of the first collimator as required at any one time in order to cover the aperture. In a second mode, corresponding to the use of stereotactic cones, it will extend the block collimator so as to place the aperture or one of them in the radiation beam, and cover any other apertures with leaves of the MLC. It may also cover any remaining areas or locally thinner areas of the block collimator with leaves of the MLC. Usually, such a control apparatus will be a suitably-programmed computer. 
         [0043]    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.