Patent Number: 039379710
Section: description

A focused shield blank 10 is preferably cast or molded from molten lead to present a generally rectangular body having a planar top side 12, a rounded bottom side 14 opposite the top side 12 and longitudinal and transverse edges 16 and 18, respectively, generally perpendicular to the planar top side 10. The shield blanks 10 may be uniformly cast having overall standard dimensions that are compatible with a radiation therapy machine (not shown) in connection with which the blanks 10 are to be ultimately used. If desired, the edges 16 and 18 may be subsequently trimmed for purposes of reducing weight if the full area of the blank 10 is not required. Normally the blanks 10 are at least 2 inches thick. The method of making a focused shield 20 from the shield blank 10 in accordance with the present invention includes the use of a shield blank holding fixture 22 especially constructed for placement on a conventional cutting table 24 of a band saw 26. The fixture 22 may be molded or fabricated in any suitable manner out of fiber glass, plastic, or the like and is comprised of a flat bottomed receptacle 28 having an upwardly facing concavity 30 presenting a rounded, shield blank supporting surface 32 provided with a radius of curvature complementary to the curvature of the bottom side 14 of the shield blank 10. An opening 34 is provided at the bottom center of the concavity 30 to provide clearance for a saw band 36. Reference to FIG. 4 of the drawing will clearly illustrate the complementary nature of the respective radii of curvature of the shield blank 10 and the blank-supporting surface 32 of the fixture 22. Prior to the placement of the shield blank 10 in the concavity 30, a desired radiation field outline 38 corresponding to a predetermined area of a patient that is to be exposed to a field of radiation is drawn on a piece of paper or other sheet material 40. Initially the area to be exposed to a radiation field is outlined on an X-ray radiograph (not shown) of the patient by a radiotherapist and the outline as sketched is then reduced to the appropriate size on the paper 40 using a pantograph (not shown) resulting in the outline 38. It is, of course, to be understood that the degree of reduction of the outline is dependent upon the relative location of the completed focused shield 20 in a radiation therapy machine with respect to the source of the cobalt rays and relative location of the patient. The procedure employed in determing the extent of outline reduction and the relative distances involved between the patient and shield 20, as well as the distance from the shield 20 to the source of the radiation rays, is well known to those experienced in the field of rendering radiation treatments and will not be detailed herein. Although it is not essential, it is desirable to cut the paper 40 to a size corresponding to that of the transverse dimensions of a radiation beam, which has a rectangular cross section at the point the latter strikes the top side 12 of the shield 20. In any event, the desired radiation field outline 38 is to be selectively placed on the paper 40 at a predetermined point relative to a center marking 42 representing the center of a radiation beam when the focused shield 20 is operably inserted in a radiation therapy machine. Once the radiation field outline 38 has been properly located on the paper 40 the latter is then placed on the top side 12 of the shield blank 10 such that the center marking 42 is superimposed over a predetermined center 44 of the shield blank 10 as shown in FIG. 2. Normally the paper 40 is pasted or otherwise secured to the top side 12 to insure that there is no inadvertent displacement of the outline 38 once the paper 40 has been positioned on the shield blank 10. After the paper 38 has been secured to the top side 12, the shield blank 10 is positioned on the fixture 22 with the convexly rounded bottom side 14 of the shield blank 10 resting on the rounded, concave surface 32 of the bowl-shaped receptacle 28. Thus, it will be seen that as the shield blank 10 is normally manipulated, the actuated saw band 36 cuts an aperture 46 therein having a transverse configuration corresponding to that of the predetermined area of a patient to be exposed to a field of radiation as represented by the outline 38. Further, not only will the aperture 46 have the correct configuration, but the aperture 46 will also have beveled sidewalls 48 which are angularly disposed to be in parallelism with the angularity of the radiation rays passing through the aperture 46 adjacent the sidewalls 48 when the shield 20 is properly associated with a radiation therapy machine. By way of further description it will be seen in referring to FIG. 4 that were the blank 10 positioned at the exact center of the concavity 30 any aperture sidewall located at this point would be perpendicular or normal to the top side 12 and be in alignment with the center of a radiation beam during use of the shield 20. Additionally, it will be further observed that when the blank 10 is shifted in any direction away from center, the angularity of the corresponding cut is increased in accordance with the increased angularity of the radiation rays as the outer perimeter of a radiation beam is approached with the sidewalls 48 diverging as the rounded bottom side 14 is approached. Once the focused shield 20 has had the aperture 46 cut therein the focused shield is then securely attached to a mounting plate 50 by means of screws 52 for insertion into a radiation machine. The relative disposition of the focused shield 20, after it is inserted in a radiation machine, is depicted in FIG. 6 wherein the numeral 54 generally identifies a radiation beam as it would appear while emanating from a radiation source 56 of a radiation therapy machine. It will be seen in referring to FIG. 6 that the shield 20 stops the radiation rays of any portion of the radiation beam 24 that is not in direct linear alignment with the aperture 46 while those rays, identified by the numeral 58 in alignment with the aperture 20 pass therethrough in an unimpeded manner. The unique construction of the focused shield 20, typified by the beveled aperture sidewalls 48, permits those rays 58 proximal thereto to pass through the aperture 46 in substantial parallelism with their respective adjacent sidewall 48. As is further apparent in viewing FIG. 6, the radiation rays will be precisely focused on the desired area of the patient for full exposure within the boundary of the outline 38. There will be no unintended "dead" spots beneath the shield 20 because the relative angularity of all portions of the sidewalls 48 is the same as that of the radiation rays passing thereby. The aperture 46 is, in effect, disposed within the shield 20 in axial alignment with the radiation rays 58 passing therethrough and no sections of the upper edge 60 of the aperture 46 block off any of the radiation rays 58. Likewise, none of the radiation rays 58 entering the aperture 46 strike any portion of the sidewalls 48 because of the parallelism of the same with the radiation rays 58. Manifestly, it is to be understood that all radiation rays 58 entering the aperture 46 at the top side 10 will exit in an unimpeded manner at the bottom side 14. The angularity of the sidewalls 48 relative to the top side 12 is determined by the radius of curvature of the surface 32 and the rounded bottom wall 14. However, the required angularity of the sidewalls 48 and therefore the radius of the blank bottom side 14 and surface 32, needs to be calculated, keeping in mind the angle of divergence of the rays within the radiation beam 54. While it is not necessary, the edges 16 and 18 of the blank 10 may also be trimmed to present beveled edges 16a and 18a as shown in FIGS. 5 and 6 to reduce the size of the finished focused shield 20 for purposes of weight reduction. However, in no event should the blank 10 be reduced to a size less than that of the paper 40 or the transverse width of the radiation beam 54 at the point it strikes the top side 12. It is to be further understood that the blank 10 is to be kept in full contact with the surface 32 during the cutting of the aperture 46 to insure the proper angularity of the sidewalls 48. Once the blank 20 has been secured to the mounting plate 50 the focused shield 20 may be repeatedly used to treat a patient simply by inserting the assembled plate and shield into a radiation thereapy machine with the assurance that the exact same spot is being treated each time, that all rays in alignment with the aperture 46 enter the same, and that the rays exit the aperture in the exact same pattern they enter the aperture.