Iso-energetic intensity modulator for therapeutic electron beams, electron beam wedge and flattening filters

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.

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 & 
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.

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's width, the 
flattening filter can be constructed to flatten the dose distribution 
along the patient'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.