Patent Number: 
Section: description

FIG. 1 has been described already, and therefore no further description will be given herein. FIGS. 2 and 3 illustrate the first embodiment of the present invention. According to this embodiment, a first collimator 100 is suspended above a second collimator 102. The first collimator 100 is constructed as a standard multi-leaf collimator comprising a plurality of leaves 104 alongside each other and which are slidable longitudinally relative to one another. FIG. 2 shows the view along the length of the leaves. Thus, the leaves appear head-on. The second collimator 102 is below the first and is a solid mass except for slits 106. These slits 106 are positioned immediately below the centre lines of leaves 104 of the first collimator 100, although this position may be moveable as described later. Referring to FIG. 3, the combination of the first and second collimators 100, 102 means that the view along the x-ray beam shown in FIG. 3 is essentially that of the slits 106 delimited in length by the leaves 104 of the first collimator 100. FIG. 4 shows the effect of this collimation arrangement. An area 108 is to be irradiated, shown shaded. The first irradiation, as delivered by the collimator positions shown in FIG. 3, will produce a series of stripes 110 across the area 108. These correspond to the exposed areas of the slits 106. It is then necessary either to move the patient beneath the collimator by a distance equal to the projected width of the slits 106, or to move the first and second collimators over the patient by an equal distance, or to move the second collimator 102 relative to the first collimator 100 by that distance. All three arrangements will then allow a further irradiation to be made in which the stripes 110xe2x80x2 are effectively adjacent the stripes 110 of the first irradiation. The leaf positions of the first collimator 100 can be adjusted between each irradiation as required. The projected stripe will almost always include a penumbra and reference herein to stripe width, adjacent stripes etc should be interpreted taking this into account. For example, it is known to adjust the spacing of neighbouring stripes so as to match the intensity profile and achieve a near uniform delivered intensity. After several such irradiations have been completed, the total number required being determined by the relative widths of the leaves 104 of the first collimator as opposed to the slits 106 of the second collimator, the complete area 108 will have been irradiated. The net effect of the multiple irradiations will be as shown in FIG. 5, in which it can be seen that the excess irradiated area 112 is significantly smaller than the area 16 of FIG. 1. However, this pseudo-micro-multi-leaf collimator effect has been achieved using only a standard size multi-leaf collimator in combination with a simple (and hence inexpensive) secondary collimator. It was mentioned above that either the patient, both collimators, or only the second collimator could be moved between irradiations. It is preferred to move both collimators together, for reasons which will be explained. If the patient alone is moved, this will normally be in a transverse direction relative to the leaves 104 and slits 106. However, there will inevitably be some divergence of the beam during its passage between the collimator and the patient, and the result of this will be that areas within the patient which are distant from the source will be irradiated twice by the divergent portions of adjacent beams, whilst areas on the patient closest to the source are likely to be under-irradiated. As the motion is very small, typically 2-3 mm, this effect is negligible with this device, unlike other applications of radiotherapy where the motion is 20-100 mm and this xe2x80x98matchlinexe2x80x99 is an undesirable effect. Another possibility is to move the patient in a continuous motion simultaneously moving the leaves to correspond. This motion will be very slow, typically 1 mm in 30 seconds and therefore imperceptible to the patient. This technique averages any inaccuracies in matching the fields over a larger area, thus reducing the maximum variation. FIG. 6 illustrates the potential difficulty in moving the second collimator 102 relative to the first 100. This arises from the fact that the irradiation source is inevitably non-point like. In FIG. 6, collimator leaves 104A and 104C of the first collimator 100 are extended but leaf 104B lying immediately between is withdrawn. Thus, the intention is to project a stripe 106 only at the position corresponding to leaf 104B (not visible). The second collimator 102 has corresponding slits 106A, 106B and 106C. Slits 106A and 106C-are not intended to be irradiated since they are beneath extended leaves 104A and 104C. Slit 106B should be irradiated since it is beneath withdrawn leaf 104B. In the position shown, the second collimator 102 is at the limit of its travel relative to the first collimator 100, with the left hand (as shown) edge of the slits 106A being directly beneath the left hand edge of the leaves 104. As shown, the x-ray beam 114 passes between leaves 104A and 104C of the first collimator 100 and correctly passes through slit 106B of the second collimator. However, the necessary engineering tolerances and the slight transparency of all materials to x-rays means that there is a small leakage path on one side along the edge of leaf 104C and through slit 106C. This gives rise to a xe2x80x9cghostxe2x80x9d irradiation in areas which were not intended to be irradiated. Whilst it may be possible to minimise this ghosting in particular arrangements, this means that this method of relative movement is not preferred. FIG. 7 illustrates the preferred method of achieving relative motion. The first collimator 100 and the second collimator 102 are both supported on a frame 118 in an essentially fixed arrangement. In this arrangement, the slits 106 of the second collimator 102 are preferably directly beneath the leaves 104 of the first collimator 100. It is possible to offset the slits 106 slightly without affecting performance, but an excessive offset would of course introduce the difficulties with respect to FIG. 6. The entire support 118 is in turn supported on a set of bearings 120 on one side and a threaded drive screw 122 on the other side. The drive screw 122 is held in a rotatable nut 124. Thus, rotation of the nut 124 will cause the support 118 to slide within the bearing 120, adjusting the positions of both collimators simultaneously. As shown in FIG. 7, the bearing 120 and drive screw 122 are tilted slightly so as to cause the support 118 to move along a circumferential path, centred on the radiation source. This means that beam divergence effects mentioned above are minimised. FIG. 7 also shown the second collimator 106 to be received within an aperture 126. This allows the second collimator 102 to be removed as necessary and replaced with alternative collimators. A suitable indexing means will of course need to be provided to locate the respective second collimator 102 in the correct position, but this should not present difficulties. The advantage of providing interchangeable second collimators 102 is that alternatives could be provided in which the width ratios as between the first and second collimators differ, or in which the spacial extent of the slits 106 on the second collimator differs. This means that the accuracy of the collimation can be selected according to the dose to be applied and the time available, and that small-area high precision second collimators can be provided for precision work together with alternative large-area lower precision devices for standard work. FIG. 8 shows an alternative arrangement in which the potential difficulty illustrated in FIG. 6 is eliminated. As shown, the slits of the second collimator are placed at a 20 mm pitch with a 5 mm width. The first collimator has leaves A, B, C and D of a 10 mm width. According to this arrangement, the second collimator is moved relative to the first collimator such that the slit straddles leaves A and B. This is shown as position 1. A first irradiation than takes place in which the width of the radiation stripe is controlled by both leaves acting together. Normally, this would involve them being placed at the same point. The second collimator is then moved by 5 mm to position 2 in which it lies completely beneath leaf B. A second irradiation takes place, delimited by leaf B only. The second collimator then moves by a further 5 mm to position 3 and a third irradiation takes place, delimited this time by leaves B and C. Finally, the second collimator moves by a still further 5 mm to position 4 and a final fourth irradiation take place delimited by leaf C only. In this manner, each slit is alternately delimited by a single leaf and a pair of leaves. It will be apparent that during the fourth irradiation, leaf A covers the next adjacent slit and therefore delimits the adjacent radiation stripe. At no time during this process are the edges of any leaves and slits aligned during an irradiation. Therefore, the potential difficulty illustrated in FIG. 6 is avoided completely, the only cost being further irradiations. In this example, a doubling of the resolution is achieved through a mere quadrupling of the number of irradiations. FIGS. 9a to 9c illustrate a further alternative in which the same doubling of resolution is achieved without any xe2x80x9cexcessxe2x80x9d irradiation but though only three irradiations. In this further alternative, the leaves A, B, C, D, E and F of the first collimator are at a 10 mm pitch and the slits X, Y and Z in the second collimator are 5 mm wide and at a 15 mm pitch. FIG. 9a illustrates the first irradiation. Slit X is delimited by leaves A and B, positioned over the join between them. Slit Y is therefore directly over leaf C, as its pitch is equivalent to {fraction (11/2)} leaves. Sit Y is therefore delimited by leaf C alone. Slit Z lies over leaves D and E. For the second irradiation, the second collimator is moved by 5 mm so as to irradiate the adjacent stripe. Slit X therefore lies over and is delimited by leaf B only. Slit Y is delimited by leaves C and D, and slit Z is delimited by leaf E. Finally, prior to a third irradiation, the second collimator is moved by a further 5 mm. Slit X will then lie over and be delimited by leaves B and C. Slit Y is delimited by leaf D and slit Z is delimited by leaves E and F. Thus, by simultaneously delimiting by single leaves and adjacent leaf pair, greater efficiency is achieved and the number of irradiations required is reduced. This will in turn reduce the treatment time necessary. FIG. 10 illustrates schematically a preferred linear arrangement of the structure of the invention along the path of the radiation beam. A beam of radiation 200 is emitted by a source 202. It is conventional for there to be a light reflecting mirror 204 in the path of the beam 200, angled at approximately 45xc2x0 to the radiation beam axis, in order to reflect a light beam 206 from a light source 208 onto the path of the radiation beam 200. Of course, the mirror 204 can be at other angles, and the location of the light source 206 adjusted accordingly. Thus, the therapist can position the patient beneath the apparatus with the radiation beam 200 off (ie shielded) and the light beam 206 on. This will then cast a light on the patient illuminating the area to be treated. This enables the therapist to confirm that the patient is correctly located. During irradiation, the mirror 204 is effectively transparent to X-rays and the like and therefore has substantially no effect on the treatment. As shown in FIG. 10, the first collimator 210 is below the mirror 204 and therefore modulates the light beam for the purpose of positioning the patient. This modulation will be to the lower resolution of the first collimator as compared to the apparatus as a whole, but for positioning purposes this will not matter. The second collimator 212 is above the mirror 204 and therefore does not affect the light beam 206. As a result, positioning can be carried out using a full light source as opposed to a series of narrow stripes. Locating the second collimator 212 above the mirror 204 also means that the beam size to be collimated is substantially narrower. The second collimator can therefore be correspondingly smaller, reducing its weight and associated material cost. It will also be within the interior of the apparatus, rendering it easier to displace by small distances for successive irradiations. It is preferred that the second collimator can be removed from the radiation beam 200, eg by sliding to one side, in order to allow conventional use of the apparatus of use with other collimators. It will be appreciated that many variations can be made to the above-described embodiments without departing from the scope of the present invention. Such variations are of course intended to fall within the scope of this Application. This device can of course be used in place of existing multi-leaf collimators, in similar treatment patterns. This could include (for example) intensity modulated radiotherapy methods.