Abstract:
The invention relates to a collimator ( 1 ) for limiting a bundle of high-energy rays ( 2 ), which is emitted by a substantially point-like radiation source ( 3 ) and directed towards a treatment object ( 20 ) and used in particular for the stereotactic conformation radiotherapy of tumors. According to the invention the collimator ( 1 ) comprises a plurality of diaphragm leaves ( 4, 4 ′) which are arranged opposite each other and which are made of a radiation-absorbing material and which, by means of drive mechanisms, can be moved into the optical path in such a way that the contours and/or exposure period of said optical path can be freely defined, the front edges ( 5, 5 ′) of the diaphragm leaves ( 4, 4 ′) being parallel to the optical path at all times. Avoiding penumbral shadows with this kind of collimator ( 1 ) is made considerably easier if the diaphragm leaves ( 4, 4 ′) consists of a rear partial element ( 6, 6 ′) which can be linearly displaced and a front partial element ( 7, 7 ′) which is hinged to same. Drive means adjust the front partial element ( 7, 7 ′) in accordance with the prevailing position of the rear partial element ( 6, 6 ′) in such a way that the front edges ( 5, 5 ′) are parallel to the optical path at all times.

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns a multiple leaf collimator for limiting a bundle of high-energy rays emitted by a substantially point-like radiation source and directed towards a treatment object and used in particular for the stereotactic conformation radiotherapy of tumors, wherein the collimator contains a plurality of opposing collimator leaves made of a radiation-absorbing material which can be moved by drive mechanisms into the optical path such that the contours of said optical path can be freely defined, wherein the front edges of the collimator leaves are always aligned in parallel to the optical path. 
     The treatment devices used today in oncological radiation therapy are provided with collimators which limit high-energy radiation, in most cases high-energy gamma radiation from a linear accelerator, in such a fashion that the rays assume exactly the same shape as the object to be treated. Since irradiation e.g. of a tumor, is implemented from different directions, a high irradiation intensity on the tumor can be effected with only limited exposure to the surrounding tissue. For absorbing high-energy radiation, the collimator must have a thickness of several centimeters, which produces a half shadow when the passage opening has straight walls in the passage direction. Since the rays diverge from the substantially point-like radiation source, the collimator opening is smaller than the actual shape of the tumor so that the collimated rays diverge to have exactly the size of the tumor upon impingement. When the walls of the collimator opening are straight, part of the radiation will not be shielded by the full material thickness due to the inclined path of the radiation. In consequence thereof, either healthy tissue surrounding the tumor is exposed to considerable radiation or the tumor tissue will receive too little dosage. This causes damage which should be prevented. For this reason, one of average skill in the art has tried to develop different collimators which reduce or prevent these half shadows. 
     One suggestion to prevent half shadows which has been described in the literature, consists in providing the collimator leaves (leaves) of a collimator (multi-leaf collimator) with an irregular trapezoidal shape such that their side surfaces and the side surfaces of the outer limits of the collimator opening have the angle of the optical path. It is, however, more difficult to achieve corresponding alignment of the front edges of the collimator leaves. Many suggestions have been made to solve this problem, none of which is satisfying. 
     In one suggestion made e.g. in EP 0 259 989 B1, EP 0 556 874 B1, EP 0 562 644 B1, U.S. Pat. No. 5,166,531 and DE 33 11 870 C2, the front edges of the collimator leaves have a rounded shape such that the outer rays of the bundle contact these front edges tangentially. Through this solution, the half shadow can be weakened but not prevented. The same is true for a further suggestion made in EP 0 259 989 B1, EP 0 556 874 B1 and EP 0 562 644 B1, wherein the radiation must pass two sequential collimator openings. In DE 195 04 054 A1, such graduation of the front edges of the collimator leaves was further refined by constructing each collimator leaf from a plurality of rods, disposed one on top of the other such that they can be displaced with respect to one another. This collimator is complicated due to the large number of parts and exhibits increased radiation leakage due to the bordering of many collimator leaf elements and the associated unavoidable tolerances. Moreover, no drive mechanism is provided. Adjustments must be made by hand and automatic computer-controlled adjustment of the collimator opening is not possible. 
     DE 33 11 870 C2, U.S. Pat. No. 3,151,245, U.S. Pat. No. 4,987,309, U.S. Pat. No. 5,144,647, EP 0 193 509, EP 0 245 768 B1, the substantially idendical EP 0 387 921, and EP 0 314 214 B1 proposed moving the collimator leaves along curved paths such that the front edges of the collimator leaves are always aligned parallel to the optical path. This requires complicated guidance of the collimator leaves. The arrangement of such complicated guidance means imposes limits on the goal of minimizing the thickness of the collimator leaves. Collimator leaves must be thin to exactly reproduce the shape of the tumor, since rough graduations result in healthy tissue also being irradiated and destroyed or badly damaged. Moreover, if the collimator leaves have the shape of irregular trapezoids and are guided on curved paths, jamming can occur in consequence of this geometrical shape. To prevent same, DE 37 11 245 A1 proposes tapering the collimator leaves towards their front end facing the optical path. Wide opening of a collimator of this type produces gaps which cause increased leakage of rays. Finally, the problems of EP 0 314 213 B1 and U.S. Pat. No. 4,987,309 were believed to be solved by disposing the trapezoidal collimator leaves and the collimator leaves which can be displaced on curved paths such that the bundle of rays must pass through both collimator openings. Although each of the two collimators has a half shadow which is reduced by the other respective collimator, half shadows can only be eliminated with twice the shielding, i.e. almost twice as much material thickness is required. The amount of effort needed for drive and control is also doubled. 
     In addition, collimators have been disclosed (FR A 2519 465 and EP A 0286 858) which are composed of two pairs of shielding blocks offset from each other by 90°. These shielding blocks have front and rear components for preventing half shadows, wherein the latter can be aligned parallel to the optical path. The front blocks have sidewardly projecting bearing pins for pivoting on a holding device which is also connected to the rear blocks and on which drive means act for adjustment. These collimators can, however, only define a rectangular beam and a shape in the form of an object which is to be treated within a living organism, such as a tumor, can not be generated. This requires a shaping multiple leaf collimator of the above mentioned kind. The technical solution cannot easily be transferred to a multiple leaf collimator since a sideward bearing of forward components of this type cannot be effected with leaves of a multiple leaf configuration at those locations at which an adjacent leaf must be disposed for forming the above mentioned shape. A strict requirement for multiple leaf collimators is the absence of any shielding gaps between the leaves forming the shape since the associated leakage would destroy healthy tissue. 
     It is therefore the underlying purpose of the invention to solve the above-mentioned problems and to produce a multiple leaf collimator which eliminates half shadows with as little effort as possible. 
     SUMMARY OF THE INVENTION 
     This object is achieved in accordance with the invention in that the collimator leaves consist of a rear part which can be displaced linearly, and a front part connected thereto, wherein the front part of each collimating leaf is adjusted in correspondence with the respective position of the associated rear part through drive means such that the front edges are always aligned parallel to the optical path and such that the connection between the front part and the rear part does not lead to any significant gaps in the volume of the radiation absorbing material. 
     The present invention omits complicated curved displacement of the collimator leaves to simplify mechanics and reduce leaking radiation, since the linear displacement permits closer tolerances. From the point of view of mechanics and drive technology, adjustment of the front part can be realized in a considerably better and easier fashion compared to the curved displacement of prior art. The complete surface of the full material thickness is used for shielding to completely eliminate half shadows without requiring either the increased effort or additional shielding of the above-mentioned prior art. The suggested technical solution is superior with respect to the previous approaches, in particular when the collimator leaves have a trapezoidal shape to also prevent the half shadow caused by the side surfaces of the collimator leaves. The linear guidance avoids jamming even when the collimator leaves are trapezoidal which permits closer tolerances and reduction of leakage compared to collimator leaves with curved guidance. Only the front parts require a somewhat increased tolerance to prevent interference during adjustment in consequence of the trapezoidal shape. This tolerance is minimal compared to that of a curved guidance system. It is clear that the invention is not limited to trapezoidal collimator leaves in dependence on the size of the collimator and the angle of the optical path. 
     The invention provides that the front part is pivoted on the rear part, such that the volume of the ray-absorbing material is substantially uninterrupted. This should be taken into consideration in the concrete embodiment of pivoting, wherein there are several possibilities which will be explained below. 
     It is possible to provide separate drives for displacing the rear parts of the collimator leaves and for adjustment of the front parts, respectively, wherein these are matched by computer control. The drive means are preferably designed such that forced mechanical coupling into each position of the rear part guarantees the associated alignment of the front part and thereby the front edges, to prevent misalignment of the front edges in consequence of program or drive means error. The reliability is considerably increased, which is particularly important for patients and users. Further advantages of this embodiment consist in that only one drive is required for each collimator leaf which requires correspondingly less computer work to thereby obtain more rapid calculation results and faster adjustment of the collimator to another shape. 
     The front part can be coupled to the rear part in many different ways. The end of the rear part can e.g. have a rounded shape and a front part with a corresponding rounded shape can be disposed thereon. It is also possible to combine segment-shaped front parts with corresponding recesses in the rear parts. However, the front parts are preferably substantially semi-circular bodies which are securely borne in corresponding recesses at the front end of the rear parts, wherein adjustment comprises a pivoting motion about the imaginary axis of rotation at the center of the circular shape. There are different possibilities of secure mounting without considerably interrupting the volume of the radiation-absorbing material, e.g. dove-tailed guidance means, guidance in grooves, retaining pins guided in slots, etc. The height of the rear part preferably substantially corresponds to the diameter of the semi-circular body, wherein the front ends of the rear part are displaced to the rear such that any required inclined position of the front edges of the collimator leaves is possible. This embodiment has the advantage that the pivoted front part also has the same height as the associated rear part for all possible positions. This is advantageous for the concrete design for bearing and guiding the collimator leaves. 
     The cross-sections of the collimator leaves preferably have asymmetrical trapezoidal shapes such that their side surfaces extend approximately parallel to the optical path, wherein the inner surfaces of the lateral sides bordering the outer collimator leaves extend at an inclination such that they join with the outer collimator leaves without leaving gaps. In this fashion, a half shadow is prevented since all limitations of the collimator opening correspond approximately to the optical path. As was mentioned above, such an embodiment of the collimator in accordance with the invention is very advantageous. The front parts preferably have sufficient lateral play to guarantee adjustment, despite the trapezoidal shape. 
     One embodiment provides that the collimator leaves can be displaced beyond the central line of the possible collimator opening to permit reproduction of tumors having any irregular contour, e.g. including U-shaped contours which require that the collimator leaves cross the central line of the collimator opening. This embodiment also facilitates modulation of the intensity of the rays through temporary covering of certain regions. A further advantage is that the collimator leaves can be closed asymmetrically, e.g. like a zipper. This considerably reduces leakage of rays in the closing region compared to that associated with closing all collimator leaves in the center. Clearly, to achieve such displacement beyond the central line, the length and distance of displacement of the collimator leaves must be dimensioned correspondingly. 
     In order to position the collimator leaves (like in the above-mentioned EP 0 245 768 B1 and in the largely identical EP 0 387 921 A1) a drive mechanism may be provided which adjusts several collimator leaves, one after the other. It is, however, preferred to provide each collimator leaf with one single controllable drive to permit quick computer-controlled shape changes. This is particularly important for dynamic irradiation of a tumor, wherein irradiation is enabled from different sides with relatively frequent changes. Even if the object of irradiation has an irregular shape and rapid change of the contour is required, maximum protection of the surrounding tissue is thereby ensured. The individual drives are also suitable if collimator leaves must be temporarily moved into the collimator opening during irradiation to weaken the intensity of radiation in certain regions. To increase the safety and reduce the number of drives, these individual drives preferably exhibit the forced coupling mentioned above. 
     Control of the collimator during operation thereof is preferably effected by a computer which adjusts the contour and position of the collimator opening to the object of irradiation in the respective direction of radiation, wherein the computer receives the data from a device for detecting the shape of the object of radiation and a control means examines the result of the adjustment. The collimator leaves can thereby be advantageously disposed in a displaceable collimator block which is provided for positioning the collimator opening relative to the object of irradiation and to the radiation source. The collimator block can be divided along a central line which permits separate displacement of these halves. Moving apart of the two halves additionally increases the collimator opening. The collimator block can also be mounted to a gantry which permits a relative motion between the collimator and the patient such that the patient can be irradiated from all sides through adjustment of the collimator opening to the shape of the object of irradiation. In this fashion, the collimator can be used to encircle a tumor to be irradiated, wherein this motion must not necessarily be circular but can also extend through three dimensions. A radiation method of this type is known, but is facilitated in combination with the invention since the inventive collimator provides improved construction and drive and the computing effort is considerably reduced. This method offers, in particular, high safety with regard to malfunction due to the forced coupling between the two drives. 
     The forced coupling of the drive of the rear parts of the collimator leaves and the actuator for the front parts can be realized by a transmission. To increase the space for the transmissions, the transmissions, optionally also the drives, can be disposed alternately at the top of one collimator leaf and at the bottom of the neighboring collimator leaf. This is particularly important if the collimator leaves must be very narrow, as is required for an exact reproduction of the shape of the tumor. A first embodiment of the drives provides that the actuator for the front parts is designed to align the front parts with respect to the radiation source when individual collimator leaves are adjusted, when all leaves are adjusted, or when some of the collimator leaves are adjusted. This permits movement of the overall collimator block or, through adjustment of the collimator block halves, to move them apart and thereby increase the collimator opening. This permits treatment of larger irradiation objects with a relatively small collimator without having to do without the inventive alignment of the front edges of the collimator leaves. 
     As drive, the rear part can have an associated collimator toothed rack into which a driving toothed wheel engages, wherein the collimator toothed rack associated with the rear part can also be embodied as teeth in a longitudinal edge of the rear part of the collimator leaves. 
     To provide good guidance of the collimator leaves, a rear section proximate the region of the gearing in the longitudinal edge of the rear part can be disposed in the collimator block at a displaced height such that a guiding element connected to the side of the collimator block above the gearing can engage in a guiding groove of the rear part or in the rear section. This produces secure guidance directly in the vicinity of the engagement region of the toothed wheel to provide exact play-free displacement of the collimator leaves. Clearly, additional guidance means can also be provided, e.g. a guidance for the edge of the rear part opposite to the gearing. In a particularly advantageous fashion, a guiding element is provided which is guided in a groove of the longitudinal edge of the rear part for securely retaining the collimator leaf in its position even if the neighboring collimator leaf assumes a substantially different position and is therefore no longer directly adjacent. 
     The driving means for adjusting the front part may be a front edge toothed rack which is hinged to the front part outside of its axis of rotation and into which a toothed gear engages to produce an adjustment path which differs from that of the rear part. The differing adjustment path permits corresponding adjustment of the front edge. Although this can be effected e.g. with separate drives, a forced mechanical coupling is preferred. 
     One embodiment of simple construction and reliable function provides that the collimator toothed rack and the front edge toothed rack are disposed on a longitudinal edge of the rear part and have different subdivisions to produce the differing adjustment paths, wherein a toothed wheel engages both toothed racks, wherein the subdivision difference lies within the tolerance limits of the gearing. With respect to a transmission disposed below a collimator, the subdivisioning of the front edge toothed rack is larger than that of the collimator toothed rack. With respect to a transmission disposed above a collimator, the subdivisioning of the front edge toothed rack must be smaller than that of the collimator toothed rack. This embodiment has, of course, only exemplary character and further possibilities are feasible, e.g. spindle drives with different pitches. 
     The driving toothed wheels or further toothed wheels which engage both toothed racks can be disposed in the collimator block. Alternatively, the further toothed wheels can be borne by the base frame. One toothed wheel can take over both functions or two separate toothed wheels can be provided. 
     In a further development, the driving toothed wheel engages in a driving toothed rack and is disposed in a displaceable collimator block or in two displaceable collimator block halves, and one further toothed wheel, which engages the collimator toothed rack and the front edge toothed rack, is disposed on a base frame. The collimator block as a whole or the collimator block halves—one for each part of the collimator leaves—can be displaced on the base frame. The base frame can thereby be disposed between the collimator block and the gantry or the base frame can be the gantry itself. The driving toothed rack can be a separate toothed rack or a continuation of the collimator toothed rack relative to a shorter front edge toothed rack. This embodiment has the advantage that even when displacing the collimator block or the collimator halves, any setting of the front edges of the collimator leaves with respect to the radiation source, once adjusted, is maintained so that the front edges are always aligned parallel to the radiation even if the collimator block or the collimator block halves are adjusted. This is effected by the further toothed wheel which engages both toothed racks thereby ensuring the relative orientation and positioning of both toothed racks throughout the entire travel region to considerably increase the possible adjustment region and the collimator opening. 
     Of course, there are many further possible types of control and forced mechanical couplings. The drive for the rear parts can e.g. be connected to a link drive for the adjustment of the front parts. These link drives can have different designs. A connecting link guide can be directly connected to the bearing of the driving toothed wheel and a slider of the connecting link guide can cooperate with the front part. The connecting link guides can also be directly connected to a base frame and displaceable collimator block halves—one for each part of the collimator leaves—are directly connected to the bearings of the driving toothed wheels. A concrete embodiment of a connecting link guide provides that a slider is mounted to a cable control which is guided to the front part and is mounted with one end above, and with the other end below the imaginary axis of rotation of the front part. A further possible embodiment consists in that the slider is mounted at a rear end of a two-armed lever wherein the axis of rotation of the lever is disposed on the rear part and its front end engages in the rear region of the front part to effect adjustment. 
     Preferably, a guidance is provided on at least one, preferably both longitudinal edges of each rear part which can be designed e.g. such that a groove is formed on the longitudinal edge in which a guiding element of the collimator block slides. Further possible guidance means are feasible to ensure that a collimator leaf is securely guided even when the neighboring collimator leaf is displaced to such an extent that the collimator leaf is exposed. 
     The collimator leaves can serve as compensating means for generating different radiation intensities by temporarily introducing them into the collimator opening during irradiation. This reduces the need for separate compensating means without excluding use thereof along with the inventive collimator. 
    
    
     The problems on which the invention is based, and embodiments of the invention are described below with reference to schematic drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the basic construction of a radiation device in which the inventive collimator can be used; 
     FIG. 2 shows a collimator opening of a multi-leaf collimator; 
     FIG. 3 a  shows the principle of half shadow production with collimators according to prior art; 
     FIG. 3 b  shows the principle of half shadow prevention by the inventive collimator; 
     FIG. 3 c  shows partial prevention of half shadows through a trapezoidal embodiment of the collimator leaves; 
     FIG. 4 shows the principle of an inventive embodiment; 
     FIG. 5 a  shows tooth subdivisions in a first configuration; 
     FIG. 5 b  shows tooth subdivisions in a second configuration; 
     FIG. 6 shows coupling of a front part of a collimator leaf; 
     FIG. 7 shows an arrangement of collimator leaves; 
     FIG. 8 shows mounting of collimator leaves; 
     FIG. 9 a  shows a first positioning of a second embodiment; 
     FIG. 9 b  shows a second positioning of the second embodiment; 
     FIG. 10 a  shows a first positioning of a third embodiment; 
     FIG. 10 b  shows a second positioning of the third embodiment; 
     FIG. 11 shows a fourth embodiment; and 
     FIG. 12 shows a further design of the second embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows the basic design of a radiation device in which the collimator  1  in accordance with the invention can be used. The collimator  1  is disposed on a collimator block  19  which is mounted to a gantry  41 . The gantry  41  contains the radiation source  3 . The radiation can be produced e.g. by a linear accelerator  43 . The gantry  41  can be rotated about a horizontal axis of rotation  44 , wherein the rays  2  are directed towards a radiation object  20 , e.g. a tumor. The radiation object  20  is located in the isocenter of the rays  2 , and the radiation source  3  as well as the collimator  1  circle around the patient  46  through rotation of the gantry  41 . The treatment table  42  can also simultaneously rotate about a rotational axis  45  to further change irradiation of rays  2  onto the treatment object  20  within the patient  46 . Of course, further adjustments are feasible. The intent is that the object to be treated  20  experiences maximum radiation dose by changing the different radiation directions while, however, protecting surrounding tissue to the greatest extent possible by only exposing it to the rays  2  for short periods of time. Furthermore, certain regions of the body must often be completely avoided such as e.g. the spinal cord or organs, wherein the irradiation directions must be chosen accordingly. The rays  2  are formed by the collimator opening  18  such that they impinge on the radiation object  20  in correspondence with its shape to protect the surrounding tissue. The profile of the tumor is detected e.g. through computer tomography recordings. This data is processed to generate a collimator opening  18  corresponding to this shape and can optionally irradiate different portions of the irradiation object  20  with different intensities. The shape and intensity are calculated and adjusted for each irradiation direction. 
     FIG. 2 shows a collimator opening  18  of a multi-leaf collimator. The invention concerns such a collimator  1  having the above-mentioned improvements which are also shown and explained in the subsequent figures. In accordance with the invention, the collimator leaves  4  and  4 ′ can produce a collimator opening  18  which corresponds to the shape of the object to be treated  20  without producing a half shadow  47 . This will be further explained below. FIG. 2 shows the principle of a collimator  1  designed as a multi-leaf collimator which reproduces the shape of a tumor in a collimator opening  18  using collimator leaves  4 ,  4 ′. Advantageous embodiments of the invention thereby provide that the collimator leaves  4  and  4 ′ can be pushed beyond the central line  17  of the maximum possible collimator opening  18 . This is required e.g. if the radiation object  20  has a U- or similar shape which can be reproduced only if the collimator leaves  4  and  4 ′ extend beyond the central line  17 . Moreover, the collimator leaves  4  and  4  can be closed, as shown at the left and right sides. The collimator leaves on these sides do not abut at the central line  17  but are staggered to reduce leakage radiation in this region. 
     FIG. 3 a  shows the principle of half shadow  47  production in some collimators of prior art. The collimator leaves  4  and  4  therein have straight front edges  5  and  5 ′. If rays  2  from a substantially point-like radiation source  3  pass through the collimator opening  18 , part of these rays  2  must pass through the entire material thickness and another part of the rays does not contact the material. In the intermediate region, the rays penetrate only part of the material of the collimator leaves  4 ,  4 ′ and are partly absorbed to produce half shadows  47 . The further the collimator leaves  4  and  4 ′ are moved apart by the adjustment  48 , the larger this half shadow  47 . Due to this half shadow  47 , the surroundings of this radiation object  20  are also irradiated with attenuated intensities  2 , in addition to the irradiation object  20 . This causes unnecessary damage to the surrounding tissue of the patient  46 . With rounded front edges  5 ,  5 ′ or with collimator leaves  4 ,  4 ′ which are disposed on top of one another in steps, such half shadows can be reduced, however, not eliminated. 
     FIG. 3 b  shows the principle of avoiding half shadows  47  in the inventive collimator  1 . The inventive collimator leaves  4  and  4 ′ are designed such that their front edges  5  and  5 ′ are always oriented in parallel to the rays  2 , despite their linear displacement to thereby ensure that a ray  2  either completely passes through the collimator opening  18  and hits the radiation object  20  or is absorbed by the entire material thickness of the collimator leaves  4  and  4 ′. The front edges  5  and  5 ′ are aligned according to the adjustment  48  of the collimator leaves  4  and  4 ′ thereby ensuring that a half shadow  47  is prevented for all widths of the collimator opening  18 . 
     FIG. 3 c  shows partial prevention of a half shadow by designing the collimator leaves  4  and  4 ′ in an asymmetrical trapezoidal shape  13 . The viewing direction of the collimator  1  is rotated through 90° with respect to the representations of FIGS. 3 a  and  3   b  and directed onto the front edges  5 ,  5 ′. The design shown prevents the side surfaces  14  of the collimator leaves  4 ,  4 ′ and the lateral borders  16  from producing half shadows  47 . The collimator leaves  4  and  4 ′ thereby have an asymmetrical trapezoidal shape  13  such that the two side surfaces  14  of each collimator leaf  4 ,  4 ′ extend parallel to the rays  2 . The inner surfaces  15  of the lateral borders  16  also have a corresponding alignment and are adjacent to the side surfaces  14  of the outer collimator leaves  4  and  4 ′, without leaving gaps. 
     In FIG. 3 c  the two outer collimator leaves  4  and  4 ′ are shown in cross section, since they are closed. The other collimator leaves  4 ,  4 ′ are opened to a greater or lesser degree to thereby form the collimator opening  18 . A corresponding design of the collimator leaves  4 ,  4 ′ was disclosed in prior art, but had the functional problems discussed above. Only the inventive design of the collimator leaves  4  and  4 ′ permits guaranteed trouble-free function despite the asymmetrical trapezoidal shapes  13  without having to accept large tolerances or introduce a further set of collimator leaves, displaced by 90°, in the optical path  2 . In this fashion, both the functions shown in FIG. 3 b  and FIG. 3 c  can be provided by the same set of collimator leaves  4  and  4 ′. This is a considerable advantage over prior art. 
     FIG. 4 shows the principle of an inventive embodiment of the collimator  1 . The collimator leaves  4  and  4 ′ comprise rear parts  6  and  6 ′ and front parts  7  and  7 ′. The latter are formed as semi-circular bodies  8  and  8 ′ and are disposed in corresponding recesses  9  and  9 ′ of the rear parts  6  and  6 ′ of the collimator leaves  4  and  4 ′. Such mounting can e.g. be effected when the front parts  7  and  7 ′ have a groove  56  about their semi-circular shape into which the rear parts  6  and  6 ′ engage with corresponding graduation in the region of the corresponding recesses  9  and  9 ′ such that full material thickness is maintained. Retaining pins  49  are provided within corresponding slots  50  for securely mounting the front parts  7  and  7 ′. The length of the slots  50  defines the adjustment region. When the collimator leaves  4  and  4 ′ are displaced in accordance with the arrows  48 , the front parts  7  and  7 ′ are simultaneously turned about an imaginary axis of rotation  36  such that the front edges  5  and  5 ′ are always aligned parallel to the rays  2 . This means that the front edges  5  and  5 ′ are perpendicular in the region of the central line  17  of the possible collimator opening  18  and, when displaced from this central line  17 , are oriented in the one or the other direction such that they point towards the radiation source  3 . To guarantee these adjustments, the front ends  12  of the rear parts  6  and  6 ′ must be set back correspondingly such that the front edges  5  and  5 ′ are located in the region of these front ends  12  only when maximum adjustment has been reached. The height  12  of the rear parts  6  and  6 ′ is preferably as large as the diameter  11  of the semi-circular bodies  8  and  8 ′, thereby ensuring constant material thickness. This embodiment has the further advantage that the front parts  7  and  7 ′ always have the same height as the rear parts  6  and  6 ′. 
     FIG. 4 also shows a transmission for the collimator leaves  4  and  4 ′ which ensures that the front edges  5  and  5 ′ are correctly aligned for each position of the collimator leaves  4  and  4 ′. This can be effected through forced mechanical coupling defined by a driving toothed wheel  23  which engages a collimator toothed rack  21  as well as a front edge toothed rack  22 , wherein these toothed racks  21  and  22  have different subdivisions  52 ,  53  or  54  (see FIGS. 5 a  and  b ) to achieve the different required adjusting motions. It must thereby be guaranteed that the different subdivisions  52 , 53  or  54  lie within the tolerance limits of the gearing of the driving toothed wheel  23  to prevent jamming. The arrow  51  shows the direction of rotation of the driving toothed wheels  23  and the arrows  48  show the adjustment of the collimator leaves  4  and  4 ′ caused by this driving direction. The collimator leaf  4  of FIG. 4 assumes the maximum opening position  64  and the collimator leaf  4  assumes the maximum over-travel  63 . The latter represents maximum traverse of the central line  17 . This over-travel permits the collimator opening  18  to reproduce a tumor  20  of any shape, up to the size of the maximum collimator opening  18 . 
     The arrangement of the drives in FIG. 4 at the lower end of the collimator leaves  4  and  4  is merely an example. It is also possible to dispose these drives  23 , 21 , 22  in the upper region or alternately at the bottom of a collimator leaf  4  or  4 ′ and at the top of the neighboring collimator leaf  4  or  4 ′ to thereby obtain more space for the drives. In the embodiment of FIG. 4, the teeth of the collimator toothed rack  21  are milled into a longitudinal edge  37  (see FIG. 7) of a rear part  6  or  6 ′. The front edge toothed rack  22  is disposed within a central groove  66  of this milled collimator toothed rack  21  (FIG. 8) and is pivotally connected to the front part  7 , 7 ′ via a pivot  57  to effect adjustment. Since the collimator toothed rack  21  and the front edge toothed rack  22  have different divisions  52 , 53 , 54 , the driving toothed wheel  23  provides different advances. The advance difference can be defined by the subdivision differences. 
     The different tooth subdivisions  52 , 53 , 54  are shown in FIGS. 5 a  and  5   b . FIG. 5 a  shows the subdivisions  52 , 54  of collimator toothed rack  21  and front edge toothed rack  22  when they are disposed above the collimator leaves  4  and  4 ′. In this case, the subdivision  52  of the collimator toothed rack  21  is larger than the subdivision  54  of the front edge toothed rack  22  which produces a larger advance of the collimator toothed rack  21  compared to that of the front edge toothed rack  22 . If the rear part  6 ,  6 ′ is moved in the direction of the double arrow  48 , its displacement is somewhat larger than that of an upwardly disposed front edge toothed rack  22  to turn the front part  7 , 7 ′ such that the front edge  5 ,  5 ′ extends parallel to the rays  2 . This alignment is ensured in all positions, even when the central line  17  is crossed. In FIG. 5 b , the subdivision  52  of the collimator toothed rack  21  is smaller than the subdivision  53  of the front edge toothed rack  22  when it is disposed below the collimator leaves  4  and  4 ′. The function is the same as described above with the difference that, in this arrangement, the advance of the front edge toothed rack  22  must be larger than that of the collimator toothed rack  21  for corresponding alignment of the front edges  5  and  5 ′. 
     Of course, other arrangements are also possible. The toothed racks  21  and  22  can also be disposed on rear extensions of the collimator leaves  4  or  4  and it is also possible to provide a separate driving toothed wheel  23  to guarantee allocation of the advances for the two toothed racks  21  and  22  via a no-load toothed wheel  24 . 
     FIG. 6 shows coupling of a front part  7  or  7 ′ to a rear part  6  or  6 ′ of a collimator leaf  4  or  4 ′ and shows how the front edge toothed rack  22  is guided in a groove  66  which was milled in the center of the collimator toothed rack  21 . Both gearings are therefore at the same height and a single toothed wheel  23  or  24  can engage both gearings. Since the collimator toothed rack  21  is directly milled in a longitudinal edge  37  of a rear part  6  or  6 ′, this adjustment will be transferred directly onto this rear part  6  or  6 ′. To adjust the front parts  7  and  7 ′, the front edge toothed rack  22  is pivotally mounted  57  to the front part  7  or  7 ′ for transmitting the adjustment motion. 
     FIG. 7 shows displaced arrangement of the rear parts  6  or  6 ′ of the collimator leaves  4  or  4 ′. This displaced arrangement serves to accommodate guidance means  38  via grooves  26  and  39 . Such grooves  26 ,  39  can be milled into either the side surfaces  14  or into the longitudinal edges  37 . 
     FIG. 8 shows the arrangement of such guidance means  38  as well as disposition of a driving toothed wheel  23 , a toothed wheel  24 , the collimator toothed rack  21 , and the front edge toothed rack  22 . A first guidance  38  is defined by a groove  39  milled into the longitudinal edge  37  of the rear part  6  or  6 ′ in which a guiding element  40  of the collimator block  19  runs. A further guidance  38  has a guiding groove  26  located in the side surface  14  of a rear part  6  or  6 ′. A guiding element  25  of the collimator block  19  also engages in this guiding groove  26 . The guiding groove  26  is disposed at the end of the rear part  6  or  6 ′ where the collimator toothed rack  21  is located. The collimator toothed rack  21  is milled into a longitudinal edge  37  of the rear part  6  or  6 ′. The central region of this collimator toothed rack  21  is provided with a groove  66  in which the front edge toothed rack  22  is disposed such that a toothed wheel  24  or  23  engages in said gearing and also in the gearing of the collimator toothed rack  21 . Different advances are achieved due to the different subdivisions, as described above. 
     FIGS. 9 a  and  9   b  show a second embodiment of the invention which differs from the first embodiment in that a driving toothed rack  55  and a driving toothed gear  23  are located in the front region of the rear part  6  or  6 ′ and the collimator toothed rack  21  and the front edge toothed rack  22  are disposed in the rear region. A non-loaded toothed wheel  24  engages both toothed racks  21  and  22  to transmit the differing advance to the front edge toothed rack  22 . In the present embodiment, the driving toothed wheel  23  is disposed in a collimator block  19  or in a collimator block half which can be displaced with respect to a base frame  58 . The further toothed wheel  24  is connected to the base frame  58  via a bearing  59 . In this arrangement, the front edges  5  and  5 ′, once correctly adjusted, remain aligned and parallel to the rays  2  even when the entire collimator block  19  is displaced with respect to the radiation source  3  or if two collimator block halves are moved apart to enlarge the collimator opening. This is shown in FIGS. 9 a  and  9   b . The collimator block  19  of FIG. 9 a  is in a first position with respect to the center line  17  and, in FIG. 9 b , in a second position displaced in the direction of the arrow  60 . This displacement produced a change in the angle α 2  of the front part  7  or  7 ′ via the mechanics shown, such that the front edges  5  or  5 ′ also extend parallel to the rays  2  in the new position. The figure shows that the front edge  5  or  5 ′ in FIG. 9 b  has a larger distance from the center line  17  than in FIG. 9 a , and the angle α 1  was increased to α 2  through displacement. 
     The driving toothed rack  55  in the front region can thereby be identical to the gearing of the collimator toothed rack  21  or have a different subdivision or tooth size. In any event, the front edge toothed rack  22  must not have any teeth in this region and lies in the groove  66  at a depth which permits the driving toothed wheel  23  to run in the driving toothed rack  55  and the front edge toothed rack  22  to be freely displaced in this region. 
     FIGS. 10 a  and  10   b  show a third embodiment wherein the adjusting motion of the front parts  7  and  7 ′ is effected through a linkage. In this embodiment as well, the rear parts  6  or  6 ′ are provided with a driving toothed rack  55  for adjusting the rear part  6  or  6 ′ via a driving toothed wheel  23 . A connecting link guide  27  is joined by a rigid connection  61  to the driving toothed wheel  23  for producing adjustment of the front part  7  or  7 ′. A slider  28  runs in this connecting link guide  27  which is mounted to a cable drive  29 . One end  30  of this cable drive  29  is mounted above the imaginary axis of rotation  36  at the front part  7  or  7 ′ and the other end  31  below said imaginary axis of rotation  36 . 
     FIGS. 10 a  and  10   b  show the possible adjustment range. FIG. 10 a  shows the position of the maximum over-travel  63  and FIG. 10 b  shows the maximum opening  64 . The adjustment displacement  48  is transferred via the driving toothed wheel  23  to the rear part  6  or  6 ′ and the slider  28  is displaced by the connecting link guide  27  in the direction of the arrow  65 . 
     FIG. 11 shows a fourth embodiment which differs from the third embodiment in that the slider  28  is located at the end of a two-armed lever  32 . The lever  32  pivots on the rear part  6  or  6 ′ via a rotation axle  33 . The front end  35  of the two-armed lever  32  pivots on the front part  7  or  7 ′, i.e. in the rear region, removed from the imaginary axis of rotation  36 . 
     In this embodiment, the two-armed lever  32  is pivoted by the connecting link guide  27  thereby effecting the adjustment leading to the corresponding alignment of the front edges  5  or  5 ′ of the collimator leaves  4  or  4 ′. A certain recess must be provided in the rear parts  6  or  6 ′ for accommodating the two-armed lever  32 . FIG. 11 shows the maximum over-travel  63  on one side and the maximum opening  64  on the other side. 
     FIG. 12 shows a further embodiment which is a variation of FIGS. 10 a  and  b . It differs in that the connecting link guides  27  are connected to the base frame  58  and the driving toothed wheels  23  are connected to the collimator block  19  or collimator block halves. In this fashion, the collimator opening  18  can also be enlarged. The connecting link guides  27  must have a length which corresponds to the entire adjustment distance, i.e. the adjustment distance of the collimator leaves  4 , 4 ′ and the adjustment distance of the collimator block halves. The embodiment of FIG. 11 can be modified accordingly. 
     The embodiments shown are, of course, only exemplary. Further embodiments are feasible in particular with respect to the forced coupling, and also with respect to the drives and design of the two parts of the collimator leaves. 
     List of reference numerals 
       1  collimator 
       2  rays 
       3  radiation source 
       4 , 4 ′ collimator leaves 
       5 , 5 ′ front edges of the collimator leaves 
       6 , 6 ′ rear part of the collimator leaves 
       7 , 7 ′ front part of the collimator leaves 
       8 , 8 ′ semi-circular body 
       9 , 9 ′ corresponding recesses 
       10  height of the rear part 
       11  diameter of the semi-circular body 
       12  front ends of the rear part 
       13  asymmetrical trapezoidal shape 
       14  side surfaces of the collimator leaves 
       15  inner surfaces of the lateral borders 
       16  lateral borders of the possible collimator opening 
       17  central line of the possible collimator opening 
       18  collimator opening 
       19  collimator block 
       20  radiation object (tumor) 
       21  collimator toothed rack 
       22  front edge toothed rack 
       23  driving toothed wheel 
       24  toothed wheel 
       25  guiding element 
       26  guiding groove 
       27  connecting link guide 
       28  slider 
       29  cable control 
       30  end of the cable control 
       31  other end of the cable control 
       32  two-armed lever 
       33  rotation axle of the two-armed lever 
       34  rear end of the two-armed lever 
       35  front end of the two-armed lever 
       36  imaginary axis of rotation 
       37  longitudinal edge of the rear part 
       38  guidance 
       39  groove 
       40  guiding element 
       41  gantry 
       42  treatment table 
       43  linear accelerator 
       44  horizontal axis of rotation of gantry 
       45  axis of rotation of treatment table 
       46  patient 
       47  half shadow 
       48  arrow: adjusting motion of the collimator leaves 
       49  retaining pin 
       50  slot 
       51  arrow: direction of rotation of the driving toothed wheel 
       52  subdivision of the collimator toothed rack 
       53  subdivision of a front edge toothed rack disposed below a collimator 
       54  subdivision of a front edge toothed rack disposed above a collimator 
       55  driving toothed rack 
       56  groove for guiding the front part in a rear part 
       57  pivoting of the front edge toothed rack to the front part 
       58  base frame 
       59  pivoting of the further toothed wheel 
       60  arrow: displacement of the collimator block 
       61  fixed connection: connecting link guide—driving toothed wheel 
       62  deflecting rollers 
       63  maximum over-travel of a collimator leaf 
       64  maximum opening of a collimator leaf 
       65  arrow: adjustment of the slider  28   
       66  groove