Abstract:
A multi-leaf collimator for a radiotherapy apparatus comprises at least one array of laterally-spaced elongate leaves, each leaf being driven by an associated motor connected to the leaf via a drive means so as to extend or retract the leaf in its longitudinal direction, the drive means comprising a sub-frame on which at least a subset of the motors are mounted, the sub-frame being mounted at a location spaced from the leaf array in a direction transverse to the lateral and longitudinal directions, and including a plurality of threaded drives disposed longitudinally, each being driven by a motor and being operatively connected to a leaf thereby to drive that leaf.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a Continuation-in-Part of and claims priority of U.S. patent application Ser. No. 12/423,909, filed Apr. 15, 2009, which is a Continuation-in-Part and claims priority of International Application No. PCT/EP2008/003183, filed Apr. 21, 2008, the content of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to multi-leaf collimators. 
     BACKGROUND ART 
     Radiotherapeutic apparatus involves the production of a beam of ionising radiation, usually x-rays or a beam of electrons or other sub-atomic particles. This is directed towards a cancerous region of the patient, and adversely affects the tumour cells causing an alleviation of the patient&#39;s symptoms. Generally, it is preferred to delimit the radiation beam so that the dose is maximised in the tumour cells and minimised in healthy cells of the patient, as this improves the efficiency of treatment and reduces the side effects suffered by a patient, A variety of methods of doing so have evolved. 
     One principal component in delimiting the radiation dose is the so-called “multi-leaf collimator” (MLC). This is a collimator which consists of a large number of elongate thin leaves arranged side to side in an array. Each leaf is moveable longitudinally so that its tip can be extended into or withdrawn from the radiation field. The array of leaf tips can thus be positioned so as to define a variable edge to the collimator. All the leaves can be withdrawn to open the radiation field, or all the leaves can be extended so as to close it down. Alternatively, some leaves can be withdrawn and some extended so as to define any desired shape, within operational limits. A multi-leaf collimator usually consists of two banks of such arrays, each bank projecting into the radiation field from opposite sides of the collimator. 
     The leaves on the MLC leaf bank need to be driven in some way. Typically, this is by a series of lead screws connected to geared electric motors. The leaves are fitted with a small captive nut in which the lead screws fit, and the electric motors are fixed on a mounting plate directly behind the leaves. Rotation of the leadscrew by the motor therefore creates a linear movement of the leaf. The leaf drive motors are inevitably wider than a single leaf thickness, so in order to be able to drive each leaf the motors have to be mounted in a particular pattern as shown in  FIG. 1 . This shows a housing  10  for an array of adjacent MLC leaves  12 . Behind the array, a motor mount  14  is fixed in place to housing  10  via bolts  16  so that it lies behind the leaves  12 . A motor  18  for each leaf  12  is fixed to the motor mount  14 . 
     Each motor  18  is generally tubular and from one end (as shown in  FIG. 1 ) therefore appears circular. The motors are wider than an individual leaf and are therefore arranged in a staggered pattern. In this example, the motors  18  are arranged in four offset rows so that the centre of a motor is aligned with each leaf. As a result of this, the leadscrew nuts therefore have to be fixed to the leaves in one of a variety of positions, meaning that (in this case) four different leaf shapes need to be manufactured. 
     In an alternative system referred to as the “Beam Modulator” and shown generally in  FIG. 2 , leaves are driven by a rack and pinion system. A gear rack  20  is machined into the top or bottom of the leaves  22  and is driven by motors  24  fixed to the side of the leaf bank. The motor gear pinions  26  are mounted to an extension shaft  28  of a suitable length to enable the drive to be carried across to the appropriate leaf to be actuated. 
     In our earlier patent application GB-A-2423909, we describe a modular design similar to the Beam Modulator drive system. The application describes a design where a system of miniature gears and racks are incorporated into a detachable module. The linear motion is transmitted to the leaf via a slotted feature in the rack and engages in a leaf drive coupling fitted to the rear of the leaf. 
     The choice of drive system is influenced by the quantity and thickness of the leaves in the leaf bank. For example, the MLC leaf bank has 40 leaves per side and has an average leaf thickness of 3.6 mm. This thickness and number of leaves allows for a conventional solution of placing the motors directly behind the leaves and actuating them via a leadscrew which passes through the centre of the leaf. 
     The diameter of the leadscrew in this design is limited to 2.5 mm, as this is largest diameter that can pass into the leaf without interfering with neighbouring leaves. Conveniently, it is also a standard ISO thread size. The leadscrew has to drive a leaf weighing around 800 g, and at certain head/gantry angles the full weight of the leaf is suspended by the thread alone. Due to the small engagement area of the thread, the leadscrew therefore experiences high frictional loads and requires regular lubrication to maintain an acceptable service life. The performance of the leadscrew is also adversely affected by a whipping motion that can arise when the leaf nut is close to the motor, in which the long free end of the leadscrew can oscillate as it rotates. In addition, the leadscrew experiences a buckling load when the leaf is pushed to the far end of the leadscrew. There is also a certain degree of noise due to this motion of the leadscrew. 
     The Beam Modulator design employs a thinner leaf in order to increase the resolution of the leafbank. This leaf thickness of only 1.75 mm influences the selection of the drive system. A lead screw system as used on the MLC would not be a viable solution as it would require a 1.5 mm diameter leadscrew; as the leaf travel is longer, the leadscrew would suffer increased whipping and buckling. Leadscrews with a high aspect ratio are also extremely difficult and costly to manufacture and are likely to fracture if they are not adequately supported. In addition, the number of motors required (40 per side) could not be fitted in behind the leaves due to their size. 
     The drive system for Beam Modulator therefore incorporates a rack and pinion system, with the motors disposed on either side, top and bottom of the leaf bank. The motors are fixed to the side of the leaf bank, and pinions are driven from the motors on extension shafts requiring 10 different lengths, in addition a staggered bearing block is incorporated in which the extension shafts runs. 8 such bearing blocks are required for the leaf bank. 
     Because the motors are dispersed along the 4 sides of the leaf bank, the bank has to be removed for motor servicing. Removal of the leaf bank is a lengthy process, and problems can occur with radiation performance if the leaf bank is not replaced in the same position. 
     The rack is machined into the top or bottom of the tungsten leaf; the bearing surface that would be positioned at the top of the leaf therefore has to be offset in order to make way for the rack. This has the undesired effect of reducing the shielding effect of the leaf, as some 8 mm is lost off the top/bottom of the leaf for the rack and bearing surface. 
     In order for smooth operation of the rack a certain amount of clearance has to be maintained between the rack and pinion. Each of the 80 motors therefore has to be checked when assembling the leaf bank. This clearance can vary leaf to leaf, depending on manufacturing tolerances, and can lead to unwanted backlash once the pinion and motor gearbox begin to wear. Such backlash will affect the positional accuracy of the leaves. 
     GB-A-2423909 describes a removable module which alleviates many of the service issues problems experienced with the beam modulator. However, as it incorporates a rack and pinion system it will suffer from backlash in the same way. The MLC Rack and Pinion System was originally designed around a 160 leaf MLC, but limitations in available space in the treatment head above and below the leafbank as well as restrictions on the overall head diameter create problems for fitting this type of Actuator. The gear racks in the actuator are positioned to match the leaf pitch; during operation the racks extend into the radiation beam, which may have effects on beam performance—particularly if there is an error in the pitching. The Actuator module also contains a high part count, including many precision cut gears and racks making this expensive to produce. 
     Thus, the leaf thickness/pitch and motor size affects the method in which the actuation is carried to the leaf, and once a suitable method is derived (of the 2 practical drive solutions, leadscrew and rack and pinion) the design can have inherent problems with wear, noise, production and assembly costs, backlash and servicing issues. 
     SUMMARY OF THE INVENTION 
     The present invention therefore seeks to provide a compact MLC actuator, that addresses many of the problems associated with a conventional leadscrew system, with the potential to drive a greater number of leaves without relying on a complex drive design and a high part count (relative to the number of leaves). This has the benefit of reducing production costs and assembly times. The drive mechanism should ideally not reduce the shielding effect of the tungsten leaves or interfere with the radiation beam. A modular design would also improve servicing issues by allowing the complete removal of the drive system from the leafbank. 
     The MLC actuator of the present invention is designed for use on a 160 leaf MLC, but can of course be applied to MLCs with a lesser or greater number of leaves. The drive will ideally be capable of moving the leaves faster than previous MLCs to offer better dynamic treatment therapies, and will be useable for MLCs with smaller width and/or pitch of the leaves of, say, 1.5 mm as compared to the 10 mm diameter of the drive motors even within a limited overall head height. 
     The width above the leaves (i.e. on the source side) is generally smaller than that below the leaves, due to the tapered design of the leafbank. Therefore, any design should ideally encompass this difference in leaf width and available space without complicating the design and increasing the required numbers of component parts. 
     The present invention therefore provides a multi-leaf collimator for a radiotherapy apparatus, comprising at least one array of laterally-spaced elongate leaves, each leaf being driven by an associated motor connected to the leaf via a drive means so as to extend or retract the leaf in its longitudinal direction, the drive means comprising a sub-frame on which at least a subset of the motors are mounted, the sub-frame being mounted at a location spaced from the leaf array in a direction transverse to the lateral and longitudinal directions, and including a plurality of threaded drives disposed longitudinally, each being driven by a motor and being operatively connected to a leaf thereby to drive that leaf. 
     The threaded drives will typically be leadscrews, but other arrangements such as a ballscrew can be used. 
     Mounting the drive motors in this way allows them to be distributed more space-efficiently, and allows the drive system to be modular, without requiring rack and pinion gears. 
     To take advantage of the ability to distribute the motors in a more space-efficient manner, we therefore prefer that a plurality of the motors mounted on the subframe are mounted at a first longitudinal end, and the remainder of the motors mounted on the subframe are mounted at a second, opposing, longitudinal end. Those leadscrews not at an edge of the array are preferably neighboured on either lateral side by one leadscrew driven by a motor mounted at the same longitudinal end and a second leadscrew driven by a motor mounted at the opposite longitudinal end. This results in the motors being arranged in pairs with a gap between which provides space for mounting the motors. The pairs of motors can be arranged one above the other to allow the necessary clearances, meaning that the leadscrews will be mounted in the subframe at one of two spacings from the leaf, with laterally neighbouring leadscrews being mounted at alternating spacings. The leadscrews can be mounted within a bore in the subframe. 
     Still greater space efficiency can be achieved by including a lower subframe, mounted at a location spaced from the leaf array in an opposite direction to that of the upper array and on which the remainder of the motors are mounted. This can be designed in a generally similar manner to that of the (upper) subframe, except as regards the leaf pitch which will need to be adjusted as a result of the varying inclination of the leaves. We prefer that half of the leaves are driven from the subframe and half are driven from the lower subframe. Adjacent leaves in the array can be driven alternately from the subframe and from the lower subframe. 
     The leaves are preferably mounted in a machined guide thereby to allow longitudinal motion. The subframe(s) can be mounted on the guide. 
     In this way, drive can be supplied to the leaves from an elongate edge thereof. This drive can be transmitted to the substantially radio-opaque leaves via a drive coupling attached to the rear of each leaf. This can be located outside the radiation beam and can therefore be of a lightweight non-radio-opaque material. 
     The drive means can further include a threaded member on the leadscrew. This is preferably restrained from rotation around the leadscrew by the remainder of the operative connection between it and the leaf. One way of doing so is for the threaded member to urge a laterally extending lug, thereby to connect to the drive coupling. The lug can engage with a recess on the drive coupling, and can include laterally-spaced flanges positioned to lie adjacent the drive coupling to prevent rotation of the lug around the threaded member. The lug ideally has a reasonable length in a direction parallel to the threaded member, to prevent rotation around axes transverse to the threaded member. A length that is at least 50% of its length transverse to the threaded member will typically suffice. 
     The lug can alternatively be held in a machined slot in the subframe; that slot can be machined with non-parallel sides to assist in guiding the lug in the light of the offset nature of the load that it needs to carry. 
     If desired, a collimator can be provided with 160 leaves, for future expansion, but operated as an 80 leaf collimator for compatibility purposes, by grouping adjacent leaves (such as in pairs), each leaf of a group being identically oriented and driven in unison by the same drive means. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; 
         FIG. 1  shows a view along the leaf direction of a known MLC drive arrangement; 
         FIG. 2  shows a perspective view of a known beam modulator; 
         FIG. 3  shows a single leaf according to the present invention; 
         FIG. 4  shows a view of the leaf drive according to the present invention, along the direction of a leaf; 
         FIG. 5  shows a bank of leaf drives according to the present invention; 
         FIG. 6  shows the retention and removal of a single drive motor of the bank; 
         FIGS. 7 to 10  illustrate different profiles for the lug and the associated guide slot; and 
         FIG. 11  illustrates an alternative embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The inherent limitation on the minimum length of the rack and pinion-type system is the number of motors mounted on the side of the module. For example, assuming that each module is designed to drive 40 leaves, that each motor is 10 mm in diameter and (therefore) spaced 14 mm apart in a double row, then the length of the module will have to be 14×(40/2), i.e. 280 mm, plus the distance over which the leaves are expected to travel. If we take a rough figure of 70 mm for this distance, this makes an overall length for the system of 350 mm. The minimum overall height will be the motor diameter plus the height of the rack, i.e. about 32 mm. A rack and pinion module when mounted on the leafbank will therefore increase the treatment head diameter significantly. 
     The MLC actuator described herein features a lead screw that runs parallel to the leaf, which means that the length of the drive modules are shorter overall, as the leadscrew only needs to be a slightly longer than the required leaf travel. The overall length of actuator including motors can therefore be about 200 mm, with a height of about 24 mm. 
     This however faces the difficulty noted above, i.e. that the leadscrew needs a minimum diameter in order to be economic to produce and sufficiently rigid in operation. For MLC arrays in which the individual leaf thickness falls close to or below this diameter, this raises difficulties in accommodating both the leadscrews and the motors that drive them. 
     The MLC actuator described herein incorporates a leadscrew drive assembly which actuates the leaf indirectly via a lug which projects out from the drive assembly and engages with a drive coupling for the leaf. The leadscrews and lugs run in machined guide slots in a bearing block which both houses the lugs (etc) and provides mounting for the drive assemblies. 
     It still remains, of course, that the leadscrews may be wider than the leaves, and it will usually be the case that the motors are wider. Accordingly, each leaf will (generally) only be a fraction of the width of its associated drive mechanism. An alternative way of viewing this is that laterally arrayed drive mechanisms will only be able to drive a fraction of the leaves. Therefore, a number of such arrays can drive all of the leaves, if the drive from each array can be transmitted to the leaves satisfactorily. A specific pattern of drive mechanisms is therefore needed in order to mount the leadscrews drives into a compact removable module. 
     We have chosen to divide the drive to the leaves in a number of ways so as to distribute the drive mechanism arrays. First, leaves can be driven from their upper edge or their lower edge. This is defined by the convention that MLC arrays are usually described as having a top that is closest to the radiation source and a bottom that is closest to the patient. Such a convention is necessary since the MLC array is mounted in a radiation head that rotates around the patient, and therefore in use the array may take up any orientation. Thus, an upper subframe can carry half of the drive mechanisms and drive every other leaf, and a lower subframe can carry the other half to drive the remaining leaves. Next, each subframe can carry two rows of leadscrews, one above the other. The lugs associated with each leadscrew can be of a corresponding length. This spaces the motors and allows them to drive laterally adjacent leadscrews. Finally, the leadscrews do of course have two ends and can be driven from either. Accordingly, half the leadscrews in each subframe can be driven from the front (which we define as the end most distant from the beam) and half from the rear (defined correspondingly). These three binary divisions allow 2 3  combinations, i.e. each situationally identical drive means drives one in eight leaves. This division can be as follows: 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Leaf 
                 Subframe 
                 Row 
                 Bank 
               
               
                   
                   
               
             
             
               
                   
                  1* 
                 Lower 
                 bottom 
                 front 
               
               
                   
                  2 
                 Upper 
                 top 
                 front 
               
               
                   
                  3 
                 Lower 
                 top 
                 front 
               
               
                   
                  4 
                 Upper 
                 bottom 
                 front 
               
               
                   
                  5 
                 Lower 
                 bottom 
                 rear 
               
               
                   
                  6 
                 Upper 
                 top 
                 rear 
               
               
                   
                  7 
                 Lower 
                 top 
                 rear 
               
               
                   
                  8 
                 Upper 
                 bottom 
                 rear 
               
               
                   
                  9* 
                 Lower 
                 bottom 
                 front 
               
               
                   
                 10 
                 Upper 
                 top 
                 front 
               
               
                   
                 11 
                 Lower 
                 top 
                 front 
               
               
                   
                 12 
                 Upper 
                 bottom 
                 front 
               
               
                   
                 13 
                 Lower 
                 bottom 
                 rear 
               
               
                   
                 14 
                 Upper 
                 top 
                 rear 
               
               
                   
                 15 
                 Lower 
                 top 
                 rear 
               
               
                   
                 16 
                 Upper 
                 bottom 
                 rear 
               
               
                   
                 17* 
                 Lower 
                 bottom 
                 front 
               
               
                   
                 18 
                 Upper 
                 top 
                 front 
               
               
                   
                 19 
                 Lower 
                 top 
                 front 
               
               
                   
                 20 
                 Upper 
                 bottom 
                 front 
               
               
                   
                 21 
                 Lower 
                 bottom 
                 rear 
               
               
                   
                 22 
                 Upper 
                 top 
                 rear 
               
               
                   
                 23 
                 Lower 
                 top 
                 rear 
               
               
                   
                 24 
                 Upper 
                 bottom 
                 rear 
               
               
                   
                 25* 
                 Lower 
                 bottom 
                 front 
               
               
                   
                 26 
                 Upper 
                 top 
                 front 
               
               
                   
                 27 
                 Lower 
                 top 
                 front 
               
               
                   
                 28 
                 Upper 
                 bottom 
                 front 
               
               
                   
                 29 
                 Lower 
                 bottom 
                 rear 
               
               
                   
                 30 
                 Upper 
                 top 
                 rear 
               
               
                   
                 31 
                 Lower 
                 top 
                 rear 
               
               
                   
                 32 
                 Upper 
                 bottom 
                 rear 
               
               
                   
                   
               
             
          
         
       
     
     The precise pattern of the leadscrews, lugs, and guiding slots in the bearing block is derived from the angle and pitch of the leaf and the required space for the drive motor. Such a pattern can also allow the drive motor axis to match the leaf centre line, ensuring an efficient transfer of linear motion. 
     By mounting the drive motors on the front and rear surfaces of the drive modules (upper and lower subframes) the area required to mount the drive motors can be dispersed over 2 faces. This also has the advantage of only requiring 2 sizes of drive mechanism, thereby maintaining a low parts count. Thus, the drive system is split into 2 modules; 2 per side, upper and lower. Each of these modules contains 40 motor/leadscrew drives, allowing for 80 leaves in total. Each module has 20 motors mounted on the front face and 20 on the rear face. The method for mounting of the motor/leadscrew drives is designed specifically to fit the pattern of machined slots in the modules. 
     This leadscrew design incorporates a precision machined leadscrew with an Acme thread form. The leadscrew nut is injection moulded in a low friction plastic material, which allows the assembly to run quietly without lubrication. The leadscrew nut fits into the lug, and can be easily replaced by removing the motor assembly. 
     The machined guide slots for the lugs can also be formed with non-parallel sides, and the lugs profiled correspondingly. Thus, viewed along the guide slot, the profile can be akin to that of a key for a cylinder lock. This provides non-vertical surfaces which act as bearings, removing from the leadscrew nut the side and moment loads which will occur in moving the mass of the tungsten leaf. On previous designs, these loads adversely affected the life of the nut. The leadscrew is also supported in this way, reducing both whipping and buckling tendencies. The guide slot profile may also feature a “V” or fir tree shape in the leg of the slot, which will increase the bearing surface area of the key and reduce friction. 
     A lower portion of the lugs are exposed below the drive module. These sections engage into the top or bottom of a drive coupling for the leaf via a mating cut-out in the drive coupling. 
     Referring to  FIG. 3 , this shows a single leaf and its associated drive. The tungsten attenuation portion  100  is relatively thin in a lateral direction in order to allow good resolution, is long in its longitudinal direction to allow a wide range of movement, and is deep in the beam direction to allow good attenuation of the beam. A front edge  102  of the attenuation portion  100  is curved in a generally known manner so as to provide a sharper penumbra. A rear edge of the attenuation portion  100  is vertical, and is joined to a drive coupling  104 . 
     The drive coupling  104  has one edge, in this case the upper edge, which is co-linear with the corresponding edge of the attenuation portion  100  except for a recess  106  into which a lug  108  fits snugly. The opposing edge of the drive portion  104  is rebated back from the corresponding edge of the attenuation portion  100  in order to reduce the overall weight of the device and to avoid interference with the drive mechanism on the other side. It will be apparent that the relative orientations of the attenuation and drive portions can be reversed to allow the leaf to be driven from the top edge (as shown) or from the bottom edge. 
     The lug  108  fits snugly in the recess  106  of the drive coupling  104  but is not fixed in place. The lug  108  is however attached to a pair of cylinders  110 ,  112  through which a leadscrew  114  passes, and between which a leadscrew nut  116  is fixed. Thus, as the leadscrew  114  is rotated, the nut  116  is forced in one direction or another and takes with it the cylinders  110 ,  112 , the lug  108 , the drive coupling  104  and the attenuation portion  100 . The cylinders offer rigidity to the structure retaining the leadscrew nut  116 , and also offer lateral support to the leadscrew  144  to inhibit both whipping and buckling. 
     Finally, at one end of the leadscrew  114 , a motor  118  is provided in order to drive the leadscrew. 
     Thus, by simple reversal of the orientations of the drive coupling  104  and/or the motor  118 /leadscrew  114 , two of the above divisions can be achieved. The remaining third division is achieved by substitution of a longer lug  108 . Accordingly, the spatial distribution of the various drive motors is achieved with an exceptionally low parts count. 
       FIG. 4  shows one leaf bank from one end. The side-by side (i.e. laterally arrayed) leaves  100  are supported at their top and bottom edges in a leaf guide (not visible). Counting the leaves from the left hand side of  FIG. 4 , the odd-numbered leaves are driven from their lower edge and the even-numbered leaves are driven from their upper edge. Thus, an upper subframe  120  carries leadscrews, lugs, motors etc for the even-numbered leaves and a lower subframe  122  carries leadscrews, lugs, motors etc for the odd-numbered leaves. Apart from dimensional issues relating to the divergent nature of the leaves  100 , the two subframes are functionally and structurally identical. 
     Within each subframe, for example the upper subframe  120 , the first two leaves that are controlled (i.e. leaves 2 and 4) are connected via lugs  108  of varying lengths to a leadscrew running in a guide machined in the otherwise solid block that forms the subframe. These two guides are placed at differing heights so as to separate the motors  118 . 
     The next leaf (i.e. leaf 6) is then connected to a leadscrew at the same upper level as leaf 2. To provide sufficient space, the motor for leaf 6 is located at the other end of the subframe  120  and drives its associated leadscrew from its other end. The pattern then continues, so that the next leaf that is driven in a manner identical to leaf 2 is leaf 10. 
       FIG. 5  shows one subframe, with the leaf bank and leaf guide removed. An array of motors  118  can be seen at one end, distant from the beam, and an opposing array of motors  124  can be seen at the other end, closest to the beam. The lugs  108  can be seen projecting from the guide slots  126 ; when this sub-assembly is replaced under (or over) the leaf array then these lugs will project into the recesses  106  of the drive portions  104  of the leaves  100 . In this way, the drive mechanism can be easily removed for service, repair or replacement. 
       FIG. 6  shows how the motors  118  are retained on the subframe  122 . Each motor has a pair of flanges projecting outwardly in two opposed directions around a part (but not all) of the circumference of the motor  118 . Fortuitously, there will be a pair of guide slots  126   a  and  126   b  either side of the motor  118  which contain a leadscrew that is driven from the other end of the subframe  122 . Thus, the ends (at least) of these slots  126   a  and  126   b  will be empty, and thus a mushroom-head screw  128   a  and  128   b  respectively can be screwed into the end of these slots  126   a  and  126   b  by providing a suitable tapping in the ends of the slots. In this way, by rotating the motor  118  so that the flanges are located under the mushroom-headed screws, then tightening the screws, the motor  118  will be retained securely. To remove the motor  118 , both screws can be loosened, and the motor rotated in the direction of arrow  130  to move the flanges clear of the screw heads and allow the motor to be withdrawn in the direction of arrow  132 . 
     In this arrangement, each screw will retain two motors, one on either side. This still permits individual motors to be removed, since the motors either side will still be retained by one screw, on their other side. This is generally preferable to providing each motor with a single flange and a single retaining screw; whilst this could be done, and would mean that each screw only held one motor, it would weaken the retention of the motors generally. 
     There could of course be further layers of leadscrews and motors beyond the two illustrated. Although this will incur a cost in terms of a greater complexity, it will permit a still greater ratio of motor spacing to leaf thickness to be achieved. 
       FIGS. 7 to 10  show alternative profiles for the lug and  108  and the guide slot  126  in which it slides.  FIG. 7  shows the simplest option, a parallel-sided guide slot  126  formed in the subframe  122 , with an enlarged root  134 . The leadscrew  114  sits in the enlarged root  134  and is surrounded by the leadscrew nut  116 . The lug  108  extends from the leadscrew nut  116 , along the guide slot  126  and out of the subframe  122 , to engage with the drive portion  104  of the leaf  100 . This arrangement is obviously easiest to manufacture. However, it then requires the lug  108  to support the leaf  100  despite the fact that the centre of mass of the leaf  100  is offset from the line along which the lug  108  is driven. This will create a rotational moment on the lug  108  which will seek to rotate the lug  108  within the plane of the guide slot  126 . This will create an uneven wear pattern on the lug  108 , the leadscrew nut  116 , and the leadscrew  114  and may be detrimental to the long-term performance of the drive mechanism. 
       FIG. 8  therefore shows an adjustment to this design to alleviate this. The lug  108  is no longer parallel-sided, but includes a step  136  to one side part way along its length. The thickness of the lug  108  remains the same through the step; that is, the outward bulge  138  on one side is matched by a corresponding recess  140  on the other side. Matching formations are provided in the guide slot  126 , to accommodate the outward bulge and to project into the recess. 
     By providing a non-flat surface to the lug  108  and a corresponding shape to the guide slot  126 , rotation of the lug  108  in the guide slot  126  is inhibited. Support for the lug  108  against rotation is provided by the interaction of the bulge  138  and the recess  140  with the corresponding formations in the guide slot  126 . Some lubrication may be useful in these areas, and a coating of graphite is suitable. 
     The arrangement shown in  FIG. 8  is a simple and straightforward one which illustrates the concept. In practice, the bulges and recesses could be located elsewhere along the height of the lug  108 /guide slot  126 , and/or they could be duplicated so that multiple such formations are present. Where several such formations are provided, they could be oriented in the same direction, or in different orientations such as alternate directions or a mix of directions. 
       FIG. 9  shows a further variation. In this arrangement, the lug  108  has a pair of adjacent bulges  142 ,  144  on one side, duplicated on the other side. Corresponding recesses are formed in the guide slot  126 . This arrangement has the advantage of being symmetrical as compared to that of  FIG. 8 , and also avoids any narrowing of the lug  108  that might cause it to be weakened. 
       FIG. 10  shows a further alternative. A pattern of recesses  146  are formed in the sides of the lug  108 , in this case four on each side in two groups of two each. Corresponding bulges are provided on the internal surfaces of the guide slot  126 . 
     The shapes described above can be formed at the necessary scale by processes such as wire discharge machining. 
       FIG. 11  illustrates an alternative embodiment which may be simpler to manufacture in that the potentially complex shapes illustrated in  FIGS. 7 to 10  are avoided. 
     In the above embodiments, the leafbank comprises a set of leaves that run in a leafguide, driven via separately attached drive couplings in the form of ‘tails’ that can be made of a lighter and cheaper material. A separate drive module uses guided ‘keys’ running in accurately machined slots, which fit into slots in the drive couplings. This allows the drive module to be removed and replaced very quickly. In the alternative embodiment, the keys are made with slots that fit over the edges of the slots in the drive couplings. It is therefore no longer necessary to constrain the keys against movement in their roll axis (around the axis of the leadscrew). This allows the drive couplings to be fitted with a looser tolerance, reducing manufacturing time and cost. This also allows the key drive profile to be greatly simplified. 
     With the key restrained in roll, it is possible to use the leadscrew to constrain the key in pitch and yaw, eliminating the need for the sliding contact and complicated machining of the drive module. The key can be simplified in material and form, reducing cost further. 
     Thus, referring to  FIG. 11 , a plurality of leaves  200  are provided in the usual side-by-side relationship.  FIG. 11  shows a single leaf for clarity purposes, but this will be supplemented by many other leaves on either side—typically making up a bank of 20, 40 or 80 leaves in total on each bank. The leaves  200  are supported in a guide  202  which supports the upper edge  204  and the lower edge  206  of the leaves in slots  208  formed in the guide. The guide  202  can be fixed to one side of the radiotherapy beam so that the leaves  200  are extendable into the beam by sliding in the guide slots  208 , thereby limiting the lateral extent of beam on that side to a desired shape. Alternatively, the guide  202  can be mounted on a moveable support, its position thereby being adjustable in rotation around the beam and/or longitudinally relative to the leaves so as to enable a wider range of adjustment of the leaf positions. A similar bank of leaves is usually provided on the opposite side of the beam in order to collimate the other lateral extent of the beam. 
     The leaf  200  is illustrated in  FIG. 11  in a partially advanced position, shown in solid lines, and a withdrawn position shown in dotted lines. The withdrawn position illustrated is one that lies beyond the normal fully retracted position, in which the leaf has been fully retracted and then withdrawn further so that is no longer supported by the guide slots  208 . Such a position would only be reached during assembly, maintenance, or disassembly, but allows us to illustrate the construction of the leaf. 
     Each leaf  210  is of a substantially radio-opaque material such as tungsten, and the drive couplings  212  can be of a lighter and less expensive material such as steel or aluminium. This allows the tungsten forward portion  210  to be projected into the beam, driven by a rearward drive coupling that never enters the beam and does not therefore need to be of a radiopaque material. The overall weight and cost of the unit is thereby minimised. 
     The drive coupling  212  of each leaf  200  includes a rectangular cut-out section  214 , visible more clearly in the dotted outline version of the leaf  200  shown in the withdrawn position. This receives a corresponding drive lug  216  that is threaded onto a leadscrew  218 . The leadscrew  218  is, in turn, mounted in a subframe  220  and provided with a drive motor (not shown) in a pattern similar to that described above. 
     In this embodiment, the leadscrew  218  is supported by the subframe  220  at either end. The drive lug  216  has an extent in the longitudinal direction (i.e. parallel to the leadscrew  218  and the leaf  200 ) of (for example) 10 mm or more, generally at least 50% of its extent transverse to the leadscrew  218 . It is therefore constrained against rotation about axes transverse to the leadscrew  218 . 
     The drive lug  216  extends transversely away from the leadscrew  218  toward the cut-out  214  of the leaf tail  212 . The lug  216  ends with an interface region that keys with the cut-out  214 ; in this example it comprises a solid rectangular section  222  that matches the rectangular cut-out  214  and (when assembled) fits into the cut-out  214 . On either longitudinal side of the rectangular section  222 , there are laterally-spaced flanges  224  that fit snugly either side of the leaf tail  212  and prevent the lug  216  from rotating around the leadscrew  218 . 
     Thus, the drive lug  216  is prevented from movement in all axes other that longitudinal translation along the leadscrew  218  as the leadscrew  218  rotates. This movement of the drive lug  216  will then cause a corresponding movement of the leaf  200 . 
     Through the use of the above-described embodiments, it is possible to produce a reliable 160-leaf multi-leaf collimator, that is a collimator with 80 leaves on each side of the beam. Current commercially-available large-aperture MLCs have a total of 80 leaves, i.e. 40 leaves per side as illustrated in  FIG. 4 , but the increased space efficiency achieved by the present invention allows this to be doubled by appropriate thinning of the leaves. This means that instead of a projected width at the isocentre of 1 cm, each such leaf will have a resolution of 5 mm—with an attendant improvement in resolution and accuracy of delivery. 
     An improvement of the resolution to 160 leaves instead of 80 will also require improvements in the treatment planning systems and software, and the associated control systems and software in order to take advantage of the additional degrees of freedom offered by doubling the number of leaves. In the longer term, this does not present a particular difficulty, but in the short term clinics may wish to replace hardware and other systems incrementally. Accordingly, there may be advantages in an MLC that retains the ability to operate in a 160-leaf mode but which is fully compatible with 80-leaf control systems. 
     This is indeed possible through the present invention. If the same leaves are inserted into the same leaf guide, but oriented so that they are organised in identical pairs, then these leaf pairs can be driven together, in unison, by providing suitable upper and lower subframes  120  as illustrated in  FIG. 3  et seq. Adjacent leaf pairs will have co-located recesses  106  in their associated drive couplings, into both of which the same lug  108  can project. Some care may need to be taken in designing the appropriate width for the lug  108  to ensure that an adequate drive is transmitted to both leaves. 
     Thus, the device will operate as an 80-leaf collimator and can be controlled and driven in the same way. However, as and when the clinic is able to upgrade other aspects of their radiotherapy equipment, the upper and lower subframes can be replaced with items adapted for 160-leaf operation and the leaves removed and re-inserted in the pattern appropriate to independent operation of each leaf. 
     Another use of the described collimator drive is for a variable-pitch collimator. Such a collimator includes leaves having a plurality of different thicknesses, such as a group of narrow leaves in the central region flanked on either side by relatively thicker leaves. Thus, a fine resolution is available in the central area of the aperture where it is usually needed, but the full aperture of the MLC is available when needed. Such collimators are limited by (inter alia) difficulty in driving the various leaves accurately and the present invention can assist with this. 
     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. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.