Patent Number: 051446472
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1, there is shown a linear electron accelerator in which a radiation exposure field limiting apparatus according to the present invention is incorporated. The linear electron accelerator shown is substantially similar in construction to the conventional electron accelerator shown in FIG. 9, and description of common construction is omitted herein to avoid redundancy. The linear electron accelerator of FIG. 1 is different from the conventional linear accelerator in that it includes a pair of radiation shielding blocks 20 of the multi-leaf type in place of the radiation shielding blocks 13a and 13b. FIG. 2 is a similar view to FIG. 10 but shows an apparatus for generating X-rays and a radiation exposure field limiting apparatus for defining an exposure field to which the present invention is applied, and FIG. 3 is a similar view to FIG. 14 but shows one of the radiation shielding blocks 20 shown in FIGS. 1 and 2. Referring to FIGS. 2 and 3, each of the radiation shielding blocks 20 is composed of a plurality of radiation shielding members or leaves including a center leaf 22a for defining an exposure field portion on a center axis of an exposure field to be formed and a suitable plural number of, three in the arrangement shown in FIG. 3, pairs of leaves 23a, 24a and 25a disposed in a symmetrical relationship on the opposite sides of the center leaf 22a. Though not shown, corresponding leaves of the other radiation shielding block 20 are disposed in an opposing relationship to the leaves 22a to 25a of the one radiation shielding block 20 in such a manner as in the radiation shielding blocks shown in FIG. 13. The leaves 22a to 25a of the radiation shielding blocks 20 are connected to be driven independently of each other by respective suitable driving mechanism not shown so that portions W1, W2, . . . of the exposure field may be defined independently of each other. The leaves 22a to 25a, however, may be driven to move such that they may be moved subordinately by adjacent ones or opposing ones thereof. The leaves 22a to 25a of the radiation shielding blocks 20 are constructed so as to have a common imaginary radiation source at a location denoted at 21 on a radiation center axis 7. In particular, each of the leaves 22a to 25a has an outer face at which it contacts with an inner face of an outer adjacent leaf and which makes part of a face of a circular cone having the apex at the imaginary radiation source 21, and a generating line of such face of the circular cone is denoted by 26l, 27l or 28l. Thus, for example, reference character 26l denotes a generating line of a face of a circular cone which is provided by an outer face of the leaf 22a contacting with an inner face of the outer adjacent leaf 23a and has the apex at the imaginary radiation source 21. Meanwhile, reference characters 25x, 26x and 27x denote X-rays which come to the outer faces of the leaves 22a, 23a and 24a from an actual radiation source 11. Referring now to FIG. 4, there is shown a modification to the radiation shielding block shown in FIG. 3. The radiation shielding block shown is modified such that the individual leaves 22a to 25a do not have a common imaginary radiation source but have different imaginary radiation sources on the center axis 7. In particular, a generating line 26l of a face of a circular cone of an outer face of the center leaf 22a has an imaginary radiation source at the apex 21a of the circular cone, and generating lines 27l and 28l of faces of circular cones of outer faces of the leaves 23a and 24a have imaginary radiation sources at the apexes 21b and 21c, respectively. Referring now to FIG. 5, there is shown another modification to the radiation shielding block shown in FIG. 3. The radiation shielding block shown is modified such that the individual leaves 22a to 25a have a common imaginary radiation source 21 on an axis 29 which passes the actual radiation source 11 and extends in parallel to the axis 6 of rotation of the rotatable frame 2. Such axis 29 will be hereinafter referred to as an imaginary radiation source axis. Referring now to FIG. 6, there is shown a modification to the modified radiation shielding block shown in FIG. 5. The radiation shielding block shown is modified such that the leaves 22a to 24a have different imaginary radiation sources 21a to 21c on the imaginary radiation source axis 29, that is, the generating lines 26l to 28l of faces of circular cones of outer faces of the leaves 22a to 24a intersect the imaginary radiation source axis 29 at different positions. Referring now to FIG. 7, there is shown a modification to the radiation shielding block shown in FIG. 6. The radiation shielding block shown is modified such that an outer face of the center leaf 22a which contacts with an inner face of the outer adjacent leaf 23a does not make part of a face of a circular cone but makes a plane which includes a line 22l parallel to the radiation center axis 7 and extends perpendicularly to the imaginary radiation source axis 29, that is, a flat plane. Consequently, the center leaf 22a has a rectangular cross section as seen in FIG. 7. The other leaves 23a and 24a have outer faces which make part of faces of circular cones having the apexes at imaginary radiation sources 21b and 21c, respectively, on the imaginary radiation source axis 29. FIG. 8 is a view similar to FIGS. 17 and 18 but shows a manner in which a difference in visually observable exposure field portions is provided by a difference in mutual positions of adjacent leaves of any of such radiation shielding blocks shown in FIGS. 3 to 7. Subsequently, operation of the linear electron accelerator will be described. Referring back to FIG. 1, X-rays 8 are generated from the radiation source 11 and restricted in exposure field thereof by the radiation shielding blocks 12a and 12b and then by the radiation shielding blocks 20 so that it is used for the radiation therapy of a patient 3. FIG. 2 illustrates a manner of generation of X-rays and definition of an exposure field similarly as FIG. 10, but the radiation shielding blocks 13a and 13b of FIG. 10 are replaced here by the multi-leaf radiation shielding blocks 20. The leaves of the multi-leaf radiation shielding blocks 20 operate in one of such manners as seen in FIGS. 3 to 7 wherein faces of circular cones defined by them do not have the apex at the radiation source 11 but have the apex or apexes at the imaginary radiation source 21 or sources 21a to 21c. In particular, referring to FIG. 3, faces of the leaves 22a to 25a which contact with each other make part of faces of circular cones, and they have a common apex at the imaginary radiation source 21 spaced from the actual radiation source 11. If the actual radiation source 11 is located at the position of the imaginary radiation source, then X-rays from the same directly pass through gaps between adjacent ones of the leaves and make leakage X-rays outside the intended exposure field, which will make trouble to intended radiation therapy. Actually, however, since the actual radiation source 11 is located at a different position from the imaginary radiation source 21 in the present arrangement, leakage X-rays outside the intended exposure field are prevented by the radiation shielding blocks 20. In particular, taking an X-ray 26x as an example, the X-ray 26x which comes to a gap (represented by a generating line 26l) between the leaves 22a and 23a cannot pass through the gap linearly. Now, if it is assumed tha the leaf gap is 0.1 mm and a lower end of the leaf 22a remote from the actual radiation source 11 is spaced by 50 cm from the actual radiation source 11 while the leaf 22a has a thickness equal to 70 mm in the direction of the radiation center axis, then X-rays which may pass directly through the leaf gap can be prevented if the imaginary radiation source 21 is located at a position spaced by 60 mm on the radiation center axis from the actual radiation source 11. In actual designing, such dimensions may be set to values having some tolerance, but they will be determined taking a magnitude in displacement between visually observable exposure field portions arising from relative positions of adjacent leaves into consideration. In the case of the dimensions given above, the difference in visually observable exposure field portions with respect to the leaf 22a, that is, the difference between the dimensions Le and Lf shown in FIG. 8, is 0.2 mm or so on the iso-center plane. The dimension of the leaf gap specified above is a value which is almost applicable to a linear electron accelerator wherein the distance between the radiation source 11 and the iso-center 9 is 1 m in most cases and which is operating actually in the technical field. Further, from an ordinary case in the field of medical treatment wherein an error of 1 mm in visual observation of an X-ray exposure field by visible light is permitted, the error of 0.2 mm in visual observation is a sufficiently small value. Accordingly, the imaginary radiation source 21 can be spaced by a greater distance from the actual radiation source 11. Since such error in visual observation is greater at an outer side leaf, the position of the imaginary radiation source should be determined with a maximum value of such errors in visual observation. An investigation must also be made for leakage X-rays which may pass in a scattered condition through any leaf gap in the system in which no X-rays pass directly through such leaf gap. A concept of leakage which is normally called streaming can be applied to such leakage. Such leakage does not amount to a significant value because the leaf gap is 0.1 mm or so (a further smaller value can be achieved actually) and very small but amounts to such a value which is resulted from attenuation of X-rays while they pass through a leaf as a radiation shielding material. An ordinary system is designed such that X-rays may be attenuated normally to one hundredth or less when they pass through a leaf (of each of the radiation shielding blocks 12a, 12b, 13a and 13b). Where the multi-leaf radiation shielding blocks are designed in such a manner as described above, the difference between visually observable exposure field portions arising from a difference between relative positions of adjacent leaves which has been called in question hereinabove with reference to FIG. 14 can be restricted to a value sufficiently lower than an allowable value, and direct passage of X-rays through a leaf gap which has been called in question hereinabove with reference to FIG. 15 can be achieved without employing such projection 39 as seen in FIG. 15. Accordingly, the multi-leaf radiation shielding blocks are simple in structure, easy to produce and economical, and restrict leakage of X-rays outside an exposure field to an allowable level as different from insufficient attenuation of directly passing X-rays by the projections 39. Besides, shading off or a penumbla around an edge portion of an exposure field of X-rays which is a common problem to the arrangements shown in FIGS. 14 and 15 is minimized or moderated significantly because the faces of the leaves extend along radiation planes of X-rays, which enables medical treatment with a low penumbla Accordingly, there are significant effects in the field of medical treatment that medical treatment planning is facilitated and that accuracy in medical treatment is improved. Since in the arrangement shown in FIG. 3 the difference between the dimensions Le and Lf shown in FIG. 8 increases toward the opposite outer sides from the center leaf 22a as described above, the arrangement shown in FIG. 4 is constituted otherwise such that the generating lines 26l, 27l and 28l for the leaves 22a, 23a and 24a pass the different imaginary radiation sources 21a, 21b and 21c, respectively, so that the difference between adjacent visually observed exposure field portions shown in FIG. 18 (difference between Le and Lf) may be restricted to a sufficiently small value within an allowance and such differences may be substantially similar values for the individual leaves in order to assure medical treatment with X-rays having penumblas which are equal at any position of the affected portion 42. It is to be noted that different leaves may have a common imaginary radiation source depending upon the difference between the dimensions Le and Lf. On the other hand, the arrangement shown in FIG. 5 is constructed such that the common imaginary radiation source 21 is positioned on the imaginary radiation source axis 29. The radiation shielding blocks 13a and 13b shown in FIG. 11 make opening and closing movement so as to normally make generating lines of a circular cone having the apex at the actual radiation source 11 in order to define an exposure field. In the case of the arrangements shown in FIGS. 3 and 4, the apexes of similar generating lines assume different positions from the radiation source 11 in the condition shown in FIG. 11, and consequently, an exposure field will have some penumblas at the opposite boundaries in the W direction. Thus, if the generating lines have the imaginary radiation source 21 on the imaginary radiation source axis 29 as shown in FIG. 5, then the imaginary radiation source 21 and the actual radiation source 11 are overlapped with each other in FIG. 11. Consequently, there is an effect that penumblas of an exposure field in the W direction can be prevented. The arrangement shown in FIG. 6 is somewhat similar in construction to the arrangement shown in FIG. 6 but is modified such that the imaginary radiation sources 21a, 21b and 21c are disposed at different positions on the imaginary radiation source axis 29 in order to attain an effect that differences between the distances Le and Lf of all of the leaves are made substantially equal to each other similarly as in the modification of the arrangement of FIG. 4 to the arrangement of FIG. 3. Also in this instance, different leaves may have a common imaginary radiation source depending upon a degree of the difference between the dimensions Le and Lf. The arrangement shown in FIG. 7 is a most practical arrangement and includes the center leaf 22a which has a rectangular cross section. Since the center leaf 22a has the opposite faces which are located at the innermost positions among other faces of the leaves of the radiation shielding block, even where it has such a rectangular cross section, the difference between the dimensions Le and Lf shown in FIG. 18 is sufficiently small. In this instance, if the leaf faces are formed not as faces of circular cones (curved faces) but as flat faces in order to facilitate working of the leaves, then a significant effect is provided that production of the leaves is easy. Further, if the difference between the dimensions Le and Lf of a visually observable irradiation field portion is smaller than an allowable value, not only the center leaf 22a but also an adjacent pair or pairs of leaves on the opposite sides of the center leaf 22a may have a rectangular cross section or sections. Such modification as is made in the arrangement shown in FIG. 7 can be applied to any of the radiation shielding blocks shown in FIGS. 3 to 7. It is to be noted that while in the embodiment described above a radiation exposure field limiting apparatus of the present invention is incorporated in a linear electron accelerator for the X-ray medial treatment, it can be incorporated similarly in any other radiation generating equipment which generates X-rays such as a cobalt 60 equipment, a betatron, a microtron and a synchrotron and also in an X-ray generating equipment which generates X-rays of energy lower than 1 MeV. Further, a radiation exposure field limiting apparatus of the present invention can be applied to any other radiations than X-rays such as an electron beam, gamma-rays, a neutron beam, a proton beam and a corpuscular beam, and a radiation shielding block may be made of a material which is effective for radiations to be used, such as a heavy metal for X-rays, a light metal for an electron beam and paraffin or an acrylic resin material for a neutron beam in order to attain intended similar effects. Further, similar effects can be attained also where, in distribution of radiations for any other application than for the medical treatment, that is, in non-destructive inspection with radiations, an exposure field is defined by means of multi-leaf radiation shielding members in order to prevent possible fading of an image of a portion for the inspection arising from scattered light. It is to be noted that, while the multi-leaf radiation shielding blocks 20 shown in FIG. 2 replace the radiation shielding blocks 13a and 13b of the system shown in FIG. 10, they may otherwise replace the radiation shielding blocks 12a and 12b shown in FIG. 10. Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit and scope of the invention as set forth herein.