Abstract:
This invention includes an iso-energetic intensity modulating filter and method for therapeutic charged-particle beams, preferably electron beams, in which attenuating members are arranged to completely attenuate portions of the beam while permitting other portions of the beam to pass through the space between attenuating members. The attenuating members block the charged particles without producing any significant bremsstrahlung contamination in the filtered beam. The attenuating members can be arranged in a large number of configurations to obtain the desired modulated beam intensity profile. This invention also includes an improved iso-energetic, intensity-modulated charged-particle beam produced by the filtering device and method of this invention. A charged-particle beam therapeutic device and treatment method which has the iso-energetic intensity modulating filter is also included in this invention.

Description:
This application claims the benefit of priority based upon U.S. Provisional Application No. 60/037,390, filed on Feb. 7, 1997, the entire disclosure of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of this invention pertains to a device and method for isoenergetic intensity modulation of a beam of charged particles, and more particularly to a device and method for iso-energetic intensity modulation of a therapeutic electron beam. 
     2. Description of Related Art 
     High energy electron beams (MeV range) have been widely used in the field of radiotherapy. The principal applications for this type of radiotherapy are for skin cancer, chest well irradiation for breast cancer, administering boost doses to nodes, and the treatment of head and neck cancers (Faiz M. Khan, The Physics of Radiation Therapy, William &amp; Wilkins). 
     Modern therapy linear accelerators provide electron beams with satisfactory field flatness and symmetry of the intensity profile. However, many clinical situations indicate that an electron beam with a tilted, concave, or asymmetric intensity profile is useful for the treatment of a curved or oblique skin surface. 
     Photon (gamma or x-ray) beams with a tilted beam intensity profile have become common practice and is achieved by placing a wedge shaped absorber (a commercially available standard accessory) in the photon beam path. While the same principle has been attempted in order to produce a tilted electron beam profile, the technique used for photon beams causes the electron beam energy to be seriously degraded throughout the modified electron field. Thus, heretofore there has been no commercially available electron wedge filter. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a device and method to modulate the intensity of a charged particle beam without substantially changing the energy of the charged-particle beam. 
     Another object of this invention is to provide a charged-particle therapeutic device which provides an intensity modulated charged-particle beam of substantially the same energy as the source beam. 
     Another object of the present invention is for a device and method to provide a desirable tilted (wedge, asymmetric) or concave (symmetric), or other shape of electron beam profile without substantially altering the original beam energy. 
     Another object of the present invention is to provide an improved isoenergetic intensity modulated charged-particle beam. 
     The principles of the present invention can be used to provide a single direct electron beam with adequate dose uniformity for total skin treatment. 
     When using the electron beam for total skin lesions, the patient is conventionally arranged at an extended distance from the radiation source (3 to 4 meters). The electron field at such extended distance will not provide adequate dose uniformity over a patient. The conventional treatment technique used to overcome this problem utilizes either a dual-field or triple-field configuration. The present invention provides a flattening filter to produce a large uniform electron beam profile without degrading the beam energy. Hence, the total skin electron beam treatment can be simplified by using a single direct beam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the invention will become more apparent and more readily appreciated from the following detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, of which: 
     FIG. 1(a) is an illustration of an example of a charged-particle beam therapeutic device designed to produce iso-energetic intensity modulated beams according to this invention. 
     FIG. 1(b)-1(c) is a perspective view showing the parallel bar (also known as para-bar) electron wedge filter of the present invention used in conjunction with an electron beam collimator cone. 
     FIG. 2(a) is a cross-sectional view of the para-bar electron wedge of the present invention take through the line 2--2 in FIG. 1(b), and 
     FIG. 2(b) shows the resulting electron beams and intensity distribution. 
     FIGS. 3(a) and 3(b) are perspective views of a concentric ring filter of the present invention. FIG. 3(a) further shows an incoming convex electron beam intensity profile and the flattened intensity profile of the electron beam output by the concentric ring filter, while FIG. 3(b) shows an incoming flat electron beam intensity profile and resulting concave electron beam intensity profile output by the concentric ring filter of the present invention. 
     FIG. 3(c) is a cross sectional view of the concentric ring flattening filter taken through the lines 3(c)--3(c) in FIGS. 3(a) and 3(b), and shows the progressively increased spacing between rings as the radial distance from the center of the flattening filter increases. 
     FIG. 4 is a graph of electron beam intensity profiles for a 9 MeV electron beam produced parallel bar wedge filter in accordance with the present invention. The following setup for this is as follows: 10 cm×10 cm cone, 100 cm Source -to- Surface Distance (SSD), and measurements were taken for preprogrammed scanning depths of 0.00 cm, 1.25 cm, 2.50 cm, 3.24 cm, 3.75 cm, and 5.00 cm. Notice that all profiles are parallel to each other, indicating the homogeneity of energy across the field. 
     FIG. 5 is a graph which plots a superposition of percentage depth dose curves obtained with open beam and wedge filtered beam (including central axis and off-axis depth dose curves, off-axis depth dose curves were taken at (-1 cm, -1 cm) and at (1 cm, 1 cm)). This plot shows that the electron beam energy remains virtually unchanged before and after wedge filtration. 
     FIG. 6(a) is a plan view and 
     FIG. 6(b) is a perspective view of an electron wedge filter in accordance with the present invention and used to produce the wedge profile shown in FIGS. 4 and 5. 
     FIG. 7(a) is a plan view and 
     FIG. 7(b) is a perspective view of a flattening filter used for single direct field total skin electron beam treatment. This filter flattens the electron field in one direction (patient&#39;s head to foot). When used at standard distance, this filter can be used to produce a concave symmetric profile. 
     FIG. 8 is a plan view of a star-shaped wedge filter that produces a concave intensity profile similar to that shown in FIG. 3(b). 
     FIG. 9 is a plan view showing a tapered parallel bar configuration for a wedge filter in accordance with another embodiment of the present invention. 
     FIG. 10 is an illustration of an embodiment of the invention in which the width and the separation of the parallel bars can be automatically changed by a computer controlled system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Due to the Coulomb multiple-scattering effect in an air medium, Gausian spread in air of the electron beam broadens with increasing distance from the linear accelerator source. Two parallel electron beams with limited separation will join together as one composite distribution at a certain distance from the source. Based on this physical principle, the intensity distribution of a broad electron beam can be modulated by dividing the broad beam into a plurality of narrow slit electron beams with different spatial separation. Since an intensity modulated electron beam results from the plurality of narrow electron beams which have the same energy characteristics (energy spectrum), the modulated beam will have the same energy as the original open beam. Although an electron beam is used in the preferred embodiments, this invention is not in limited to only electron beams. One skilled in the art would recognized that the teachings of this invention can be applied to charged particle beams, in general, since they exhibit Coulomb scattering off of the air molecules. However, the specification will refer to electron beams throughout since electron beams are preferred according to this invention. Unlike other electron beam modulation methods, this intensity modulation method is isoenergetic. 
     FIG. 1(a) illustrates an example of a charged-particle beam therapeutic device 2a according to this invention. The charged-particle beam therapeutic device 2a has a charged-particle beam source 4a, an iso-energetic intensity modulating filter 6a, and a patient positioning region 8a. 
     A first preferred embodiment of this invention is for an iso-energetic electron beam wedge filter. 
     As shown in FIG. 1(b), the electron wedge filter of the present invention, indicated generally at 10, consists of a number of spaced high density metal bars 12 disposed in parallel side-by-side arrangement. Preferably the bars are metal bars. Each bar has a thickness and density sufficient to block the electron beam without producing any significant bremsstrahlung contamination. Preferably, the bars have a thickness of about 1.5 cm and are made from lead, tungsten, cerrobend, copper, steel, or iron. Most preferably, lead or tungsten is used. In any event, the same preferred metals are used for all embodiments of the present invention and preferably have a minimum density of 10 grams/cc. 
     As shown in FIG. 2(a), the width and spacing between the bars 12 are varied to form a gradient of separation and openings. This configuration converts the broad beam into a plurality of narrow slit electron beams which emanate through the openings or spaces 14 between the parallel metal bars while the rest of the broad electron beam impinging upon the bars is blocked. 
     When in use, the wedge 10 is placed on top of an electron applicator at an adequate distance from the patient surface. The minimum distance between the wedge filter and patient required to produce a smooth profile depends on the spatial separation between each of the wedge filter bars and the electron beam energy. As the energy of the electron beam increases, a greater distance between the wedge and the surface of the patient is required to obtain a smooth intensity profile. Preferably, the minimum distance ranges from 15 cm to 40 cm for electron energies ranging from 6 MeV to 20 MeV. The electron wedge is preferably placed in the same slot in which photon wedges or other treatment modules such as total skin electron therapy (TSE) modules are usually placed. By doing so, the height of a conventional electron cone will provide the adequate distance of about 40 cm between the electron wedge and patient surface for all practical energy ranges. On the patient surface, these narrow electron beams merge to form a smooth tilted intensity profile, as seen in FIG. 2(b). The slope of the profile is controlled by the spacing between the bars and the width of the bars. A concave profile can also be produced if the spatial gradient is symmetric along the midline, for example, as could be achieved with the embodiment shown in FIGS. 7(a) and 7(b). 
     Electron wedges are advantageous in treating superficial tumors along a curved surface, as well as in electron field matchings. 
     A second application of this invention relates to the construction of a flattening filter to form a uniform beam profile at an extended distance (e.g., about 3-4 meters from the electron source) with a single direct field for total skin electron therapy (TSE). The current treatment technique for TSE is either a dual-field or triple-field configuration, meaning that two or three fields are used with different angles to form a flattened field at the aforementioned extended distance of about 3-4 meters. 
     The electron flattening filter of the present invention applies the same principles as the electron wedge filter of the present invention, except that the gradient of the separation between the narrow beams is symmetrical along the central axis of the beam. The purpose of this specific design is to create a concave electron fluence which will compensate the convex Gausian distribution at the extended treatment distance in order to form a flattened profile. Again, the process is iso-energetic. 
     The arrangement of the attenuator for the TSE filter can be either parallel bars (see FIGS. 7(a) and 7(b)), concentric rings (see FIG. 3(a)), or bars forming a star pattern (see FIG. 8). 
     The application of this invention can be expanded to the general electron beam flattening filter, intensity modulation for treating other irregularly shaped surfaces instead of using an electron beam bolus. 
     Electron Wedge Application 
     The results shown in FIGS. 4 and 5 were obtained by using the wedge filter shown in FIGS. 6(a) and 6(b). The density and thickness of the metal bars is sufficient to block an electron beam without producing any significant bremsstrahlung contamination, for example, 1.5 cm thickness of cerrobend can be used. Correction of beam divergence can be neglected, especially when higher density metal bars are used (since the thickness can be reduced). The constructed electron wedge is then mounted on a frame which can be inserted into the slot where typical photon wedges or other treatment modules such as TSE modules are usually placed. The electron wedge should be placed above the electron cone, as shown in FIG. 1(b), to produce a smooth tilted profile on the patient surface. The design shown in FIGS. 7(a) and 7(b) can be used at a standard distance to produce a concave intensity profile from a relatively flat intensity profile broad electron beam. 
     It is contemplated that a multileaf collimator (MLC), which is a computer control system designed and used to form an irregular field shape for photon beam radiation therapy as opposed to the conventional cerrobend block, can be used to provide an electron wedge filter in accordance with the principles of the present invention by using selective leaf openings to form a parallel bar arrangement with designed opening variations. The limitation is that the current MLC can only provide a few discrete opening variation arrangements due to the fact that the location of each leaf is fixed and the width of the leaf is not thin enough to serve this purpose in general. 
     Heretofore, multileaf collimators (MLC) for electron beam intensity modulation with parallel bar arrangement have not been used. Hence, the application of an existing or a modified MLC to form the parallel bar arrangement is also contemplated by this invention to achieve iso-energetic intensity modulation for electron beams. This means that the width and the separation of parallel bars can be automatically changed by a computer controlled system. FIG. 10 illustrates an example of this embodiment of the invention. 
     Flattening Filter For Total Skin Electron Therapy Application 
     Since dose uniformity is usually adequate along a patient&#39;s width, the flattening filter can be constructed to flatten the dose distribution along the patient&#39;s height only. This leads to the wedge configuration shown in FIGS. 7(a) and 7(b). The wedge in FIGS. 7(a) and 7(b) can be mounted to the TSE module frame and the insert the frame to the accessory slot. The spacing between bars should be designed based on the treatment distance (3, 4 or 5 meters). 
     A tapered parallel bar configuration shown in FIG. 9 is another design of the present invention which will also serve as an electron beam wedge filter. The difference between FIGS. 6(a), 6(b) and FIG. 9 is the direction of spatial separation gradient. In other words for FIGS. 6(a), 6(b), the intensity gradient is in a direction perpendicular to the bars, while in FIG. 9 the gradient is parallel to the bars. 
     A filter with a concentric ring configuration can be built with different sizes, and can be used in the head of an accelerator below the scattering foil to flatten the beam at normal distance (100 cm) as shown in FIG. 3(a), or be used to modify a flattened beam profile into a concave profile at normal distance (100 cm), as shown in FIG. 3(b). The concentric ring configuration can also be used to produce a flattened beam at an extended distance (3-4 m) for total skin treatment. 
     In an alternate embodiment, it is contemplated that an alternate concentric ring configuration can be used to convert a flat electron beam intensity profile into a convex intensity profile. In this embodiment, the spacing between the rings is wider at the center portions of the filter than at the peripheral regions (compare this to FIG. 3(a)). 
     A star shaped configuration can produce an effect similar to that for the concentric ring configuration. In other words, it can be use to convert an incoming convex dome shaped intensity beam into a flattened beam, or convert an incoming flattened beam into a concave beam. FIG. 8 demonstrates this star shaped design. 
     Conclusion 
     While the use of electron beams is a widely used modality for radiation cancer treatment, the present invention provides a general technique which can be used to modulate the intensity of a static electron field without changing the electron beam energy. Typical intensity modulations are used to produce tilted wedge beam profiles and flattened beam profiles. Although the electron beam can be filtered through a wedge shape absorber to produce a tilted intensity profile (which is the technique used in x-ray or gamma ray therapy), the serious beam energy degradation by the absorber makes this design undesirable for electron beams. The present invention utilizes the Coulomb multiple-scattering principle to produce a tilted or flattened intensity profile by merging multiple narrow slit electron beams. Using this method, the energy of the electron beam remains substantially unchanged, hence the modulation is substantially iso-energetic. 
     The present invention is advantageous in that it can be used to produce (1) a static tilted wedge, concave, or other shape electron beam profile without degrading the beam energy across the whole field, (2) a static flattened electron beam at extended treatment distance without degrading the beam energy across the whole field. In addition, the internal scattering/flattening electron foil/filter can be designed based on the proposed concentric ring configuration to improve the electron beam energy spectrum. Although only the preferred embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications of the exemplary embodiments are possible in without materially parting from the novel teachings and advantages of this invention.