Abstract:
An energy filter device for beams which are used in the course of ion beam therapy, wherein at least one passive modulator is provided. The modulator can comprise a scattering film for the beams and a collimator with an opening for controlling the beams, or a magnetic filter and an absorber, or a nonlinear filter and an apparatus for clipping the intensity of individual energy levels of the beams.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to an energy filter device for beams used in low beam therapy for controlling the beam intensity and the dose strength of the beam. 
   Ion beams, that is beams of protons and heavier ions, are used for beam therapy for tumor tissues. Beams of this type have the advantage over photons that they have a considerably better depth dose profile. While, in the case of photon radiation, the dose decreases as the penetration depth increases, it rises slowly in the case of ions and falls away steeply after a sharp maximum, the so-called Bragg peak. The position of this maximum must be distributed accurately over the target volume, in order to concentrate the dose on the tumor and at the same time to reduce the integral dose in the healthy tissue. This also applies to the laser-induced ion beams. Recently, it has been shown that high-intensity, pulsed laser beams can be used to produce relatively tightly focused proton beams from films. However, it has been found that the laser-induced radiation has an energy spectrum which is very poor for cancer therapy, and thus a poor depth dose profile. 
   The energy spectrum can be improved by masking out a narrow energy band, and destroying the rest of the spectrum, by means of a magnet spectrometer. However, in the case of this selection method, the vast majority of the protons that are produced, specifically more than 95% of them, are filtered out and destroyed without being used. 
   SUMMARY OF THE INVENTION 
   The object of the invention is thus to provide an energy filter device by means of which the energy spectrum of laser-induced beams can be utilized better, with fewer protons being filtered out without being used than in the case of conventional magnetic filters. 
   In order to achieve this object, an energy filter device is proposed which is distinguished by including at least one passive modulator to make it possible to change the energy spectrum of the radiation with a simple device design, such that the energy profile is shifted to higher energy levels, that is it is hardened, and such that a good depth dose profile is produced, which is also referred to as the range profile. 
   One preferred exemplary embodiment of the energy filter device is distinguished in that the modulator has a scattering film and a collimator, which is arranged at a distance from it. The use of a passive modulator of this type means that the low-energy component of the radiation is lastingly suppressed in comparison to the primary spectrum, and the range profile becomes considerably flatter. 
   A further preferred exemplary embodiment of the energy filter device is distinguished in that the modulator has a magnetic filter and an absorber. 
   A further exemplary embodiment of the energy filter device is distinguished in that the modulator has a non-linear filter, by means of which the energy profile and the depth dose profile can be set. 
   Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in the following text with reference to the drawings, in which: 
       FIG. 1  shows a schematic illustration of an energy filter device; 
       FIG. 1   a  shows the energy spectrum of an ion/proton beam at the various positions in the energy filter device; 
       FIG. 1   b  shows the depth dose profile of the beam at various points in the energy filter device; 
       FIG. 1   c  shows the scatter of the high-energy and low-energy components of the beam in the energy filter device; 
       FIG. 2  shows an outline sketch of a further exemplary embodiment of an energy filter device; 
       FIG. 2   a  shows the energy spectrum of an ion/proton beam at various positions in the energy filter device; 
       FIG. 2   b  shows the depth dose profile of the beam at various points in the energy filter device; 
       FIG. 3  shows an outline sketch of a further exemplary embodiment of an energy filter device, 
       FIGS. 3   a  and  3   b  show the energy spectrum for the embodiment of  FIG. 3 ; 
       FIG. 3C  is a view of the selective collimator of  FIG. 3 ; and 
       FIG. 4  shows a multi-leaf collimator, seen in the beam direction. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows an ion beam energy filter device, which will be referred to in the following text for short as an energy filter device  1 , and through which an ion beam  3  passes. This ion beam  3  is produced by a high-intensity, pulsed laser beam  5 , which strikes a target, in this case a converter film  7 . Heavy charged particles  9 , protons and relatively heavy ions are deposited on this converter film  7 . The energy spectrum of the ion radiation made up of these particles  9  cannot be used in practice for radiation therapy, nor can the range profile associated with it. 
   A first collimator  11  which has a collimator opening  13  is arranged at a distance from the converter film  7 , with a collimated beam  15  with a broad spectrum and a strong low-energy component emerging through this collimator opening  13 . The outer part of the radiation is masked out by the collimator  11 . 
   The energy spectrum of the beam  15  is shown underneath it, in schematic form, in  FIG. 1   a . Underneath this,  FIG. 1   b  shows a schematic illustration of the depth dose profile of the beam  15 , which has the spectrum illustrated in  FIG. 1   a.    
     FIG. 1   b  shows the beam dose plotted against the penetration depth z (units in millimeters).  FIG. 1   b  clearly shows that the beam dose decreases sharply with the penetration depth. 
   The collimated beam  15  now strikes the energy filter device  1 , which is arranged downstream from the collimator  11 , and passes through it from left to right as shown in  FIG. 1 . 
   The energy filter device  1  has a passive modulator  17  which, in this exemplary embodiment, is in the form of a scattering filter. This scattering filter has a first scattering film  19  which is arranged, for example, at a distance of 50 cm from the collimator in the beam path, and which the collimated beam  15  strikes. The beam is scattered by the Coulomb interaction. In order to keep the energy loss low, relatively thin scattering films with a thickness, for example, of 5 mm are used, composed, for example, of Plexiglas. The Coulomb interaction results in a Gaussian distribution for the scattering angle α in accordance with the following equation: 
             f   ⁡     (   α   )       =         1       2   ⁢           ⁢   π   ⁢           ⁢   σ         ⁢   exp     -       α   2       2   ⁢           ⁢     σ   2                 
where the variance is given by the following relationship:
 
   
     
       
         
           
             σ 
             2 
           
           ≈ 
           
             1 
             
               E 
               2 
             
           
         
       
     
   
   Particles  9  which pass through the first collimator  11  and strike the first scattering film  19  are thus scattered by it, with the scattering becoming greater the lower the energy level of the particles  9 . 
     FIG. 1   c  shows various distributions for the scattering angle α, with the first curve  23  showing the scattering for the high-energy component of the particles  9  in the scattered beam  21 , and the second curve  25  showing the scattering of the low-energy component of the particles. 
   The energy filter device  1  has a second collimator  27  which is arranged at a distance from the first scattering film  19  and has a collimator opening  29  with a diameter of, for example, 5 mm, through which the majority of the high-energy component of the particles  9  passes, corresponding to the first curve  23 , and only a small proportion of the low-energy particles. This results in a beam  31  which is hardened by the collimator, that is to say a beam which has an energy spectrum that is considerably better for radiation therapy than the broadly scattered ion/proton beam which leaves the converter film  7 . 
   Thus, if the second collimator  27  is used to mask out the outer part of the scattered beam  21  after passing through a distance downstream from the first scattering film  19 , this results in a transmission filter with a 1/E 2  characteristic. This is because the collimator which, for example, has a square or round collimator opening clips the beam in two directions. 
   Underneath the hardened beam  31 ,  FIG. 1   a  shows the energy spectrum for this beam  31 . Underneath this,  FIG. 1   b  shows a schematic illustration of the depth dose profile of the hardened beam  31  with the energy spectrum illustrated in  FIG. 1   a.    
     FIG. 1   b  clearly shows that the depth dose profile of the hardened beam  31  is not the same as that of the collimated beam  15 : the beam dose does not fall away so sharply with the penetration depth, and the depth dose profile is thus considerably flatter than that of the collimated beam  15 . 
   The energy filter device  1  that is illustrated in  FIG. 1  accordingly has a first hardening stage which comprises the first scattering film  19  and the second collimator  27 . However,  FIG. 1  also shows that, in the case of the exemplary embodiment of the energy filter device  1  illustrated here, the passive modulator  17  has a second hardening stage, which comprises a second scattering film  33  (which is arranged at a distance from the second collimator  27 ) and a third collimator  35 , which is once again arranged at a distance from the second scattering film  33  and has a collimator opening  37 . 
   The explanation given with regard to the first hardening stage applies in a corresponding manner on passing through the second hardening stage: the hardened beam  31  is scattered by the Coulomb interaction as it passes through the second scattering film  33 . A thin scatterer is also used in this case, in order to keep the energy loss low, so that this once again results in the scattering angle α having a Gaussian distribution, as has been described above. 
   A scattered beam  39  is thus once again produced downstream from the second scattering film  33 , and within this beam, the particles with a high energy level have a lower scattering angle than low-energy particles, as has been explained with reference to  FIG. 1   c.    
   The outer part of the scattered beam  39  is once again masked out, in the same way as the scattered beam  21 , by the third collimator  35 , which is arranged downstream from the second scattering film. Considerably more high-energy particles thus emerge through the collimator opening  37  than low-energy particles, thus resulting in a hardened beam  41 . The energy spectrum of this beam is shown underneath this beam in  FIG. 1   a , in this case having a significant maximum, for example, at 200 MeV, and the depth dose profile of this beam with the spectrum as illustrated in  FIG. 1   a  is shown in  FIG. 1   b.    
     FIG. 1   b  clearly shows that the effective dose of the beam increases as the penetration depth increases and falls away after reaching a maximum, for example at a penetration depth of about 100 mm. 
   The energy filter device  1  shown in  FIG. 1  has two hardening stages. However, it is also feasible to use more than two such stages. 
   The thickness of the scattering films  19 ,  23  and the size of the collimator openings  29 ,  37  as well as the distance between the scattering films and the collimators  27 ,  35  are variable and can be matched to the energy spectrum of the ion/proton beam that enters the energy filter device. 
   The energy filter device  1  can be matched to the collimated beam  15 , which is also referred to as the primary beam, by means of the thickness of the scattering films  19 ,  33  and the size of the collimator openings  29 ,  37 . 
     FIG. 2  shows a second exemplary embodiment of an energy filter device  1 ′, through which an ion beam  3  passes. In this case as well, it is assumed that a laser beam  5  which strikes a target, in this case a converter film  7 , is used to produce a widely scattered ion beam with a broad spectrum, by forcing particles  9  (protons and heavy ions) out of the converter film  7 . 
   A first collimator  11  with a collimator opening  13  is located at a distance from the converter film  7 , through which collimator opening  13  a portion of the broadly scattered proton beam passes, thus resulting in a collimated beam  15 . Underneath this beam,  FIG. 2   a  shows the energy spectrum of this beam, while  FIG. 2   b  shows the depth dose profile of this beam. 
   The production of the particle beam and its first collimation corresponds to the characteristics illustrated in  FIG. 1 . 
   The energy filter device  1 ′ has a passive modulator  17 ′, which comprises a magnetic filter  33  and an absorber  45 . 
   The magnetic filter  33  has a homogeneous magnetic field and is formed, for example, by an electromagnetic dipole. The collimated beam  15  passes through the magnetic field, so that this results in a spectrally spread ion/proton beam  47 .  FIG. 2  shows the spread beam  47  in the form of a fan downwards, with high-energy particles having been deflected to a lesser extent by the magnetic field than particles with a lower energy level, and whose velocity is thus lower. 
   The absorber  45 , which is wedge-shaped, is introduced into the profile of the spread beam  47  in such a way that the thicker part of the wedge is arranged at the top, and the thinner part at the bottom. The high-energy ions/protons thus pass through the thicker part of the absorber, and the low-energy ions/protons pass through its thinner part. The absorber is used for energy homogenization, and for increasing the transmission associated with this. 
   The angle of the walls  49   a ,  49   b  (which are arranged in the form of a wedge with respect to one another) of the absorber  45  can be matched to the desired energy loss, which is desired when the spread beam  47  passes through the absorber  45 . It is also possible for the walls  49   a  and  49   b , which in this case run in a straight line, that is to say lie on one plane, to be designed to be curved in a concave or convex shape, in order to influence the energy absorption in a specific manner. 
   By appropriate dimensioning of the absorber wedge, that is to say by the absorber  45  having a suitable thickness profile, it is thus possible to compensate for the different energy levels of the particles in the spread beam  47  by means of the different energy loss within the absorber, and thus to obtain a greatly improved range profile. 
   The absorber  45  is followed by a second collimator  51  with a collimator opening  53 , which is used to filter out a desired energy band from the spread beam  47  which has passed through the absorber  45 . 
   The beam which emerges through the collimator opening  53  is made parallel by means of a magnet  55  by forming a second magnetic field, with the opposite polarity to the first magnetic field. 
   After passing through the second magnetic field, this results in a beam  57  with a narrower energy range. 
   The energy spectrum of the spread beam  47  is illustrated underneath it in  FIG. 2   a , and its depth dose profile is illustrated in  FIG. 2   b . This clearly shows that the energy spectrum has a discrete high point, and thus that the depth dose profile has a relatively sharp maximum, a Bragg peak. 
   The second collimator  51  governs the upper and the lower energy limit of the beam  47   a  that is influenced by the absorber  45 . The beams, which now effectively have a single energy state, are made parallel by the magnetic field of the magnet  55  of the passive modulator  17 ′. The parallel beam  57  which leaves the further magnet filter  45  is focused by a suitable lens, in this case, by way of example, a quadrupole pair  58 . This results in a beam with a higher energy density. 
   The advantage of the wedge-shaped absorber  45  is its high transmission. The disadvantage is that the entire spectrum of the spread beam  47  is shifted towards lower energy levels. An energy filter device  1 ′ of the type described here can be used to brake the intensity component of the radiation which is above a desired energy level to a desired energy level without any significant loss of intensity. If magnetic energy selection were to be carried out without a wedge-shaped absorber, thus by means of a collimator, the intensity component would be lost. A filter such as this would therefore have a poor transmission curve. 
     FIG. 3  shows a third exemplary embodiment of an energy filter device  1 ″, through which an ion beam  3  passes. 
   In the case of the exemplary embodiment illustrated here, the ion beam is once again produced by means of a laser  5  which strikes a target, in this case a converter film  7 , so that charged particles  9 , that is to say protons of heavy ions, are forced out of this converter film  7 . The converter film  7  is followed by a first collimator  11 , which has a collimator opening  13  through which a collimated beam  15  passes, whose energy spectrum is illustrated in  FIG. 3   a , and whose depth dose profile is illustrated in  FIG. 3   b.    
   This beam is supplied to the energy filter device  1 ″, which comprises a passive modulator  17 ″. In the exemplary embodiment illustrated here, the modulator  17 ″ has a non-linear filter, which has a first magnetic filter  59  and an apparatus  61  for clipping the intensity of the individual energy levels in the beam. 
   The magnetic filter  59  has a preferably homogeneous magnetic field, which spectrally spreads the collimated beam  15  passing through the magnetic filter  59 . As is shown in  FIG. 3 , the collimated beam  15  is spread into a fan downwards, with high-energy particles, whose velocity is thus higher, being deflected less than low-energy particles in the spread beam  63 , as in the case of the beam  47  shown in  FIG. 2 . The charged particles  9  are thus separated on the basis of their energy and velocity in the first homogeneous magnetic field of the magnetic filter  59 . 
   The magnetic filter  59  is followed by a second collimator  65  with a collimator opening  67 , which defines the upper and lower energy limits for the spread beam  63 , thus resulting in a beam  69  being produced. 
   This beam  69  is made parallel by a magnet  71 , with a second magnetic field, in the opposite direction to the first magnetic field, being used here, and the beam being deflected in the opposite direction to the direction produced in the first magnetic filter  59 . 
   The parallel beam  73  which emerges from the magnet  71  thus has a narrower energy range than the spread beam  63 . This parallel beam  73  is passed through the apparatus  61  for clipping the intensity of the individual energy levels, specifically through a selective collimator, so that the beam components which have been spatially separated on the basis of their energy are clipped to different extents, and a filtered beam  77  is produced. 
     FIG. 3C  also shows a view of the selective collimator seen in the beam direction. This clearly shows the specially formed collimator opening  79 , which is illustrated in the beam direction and has a broader free opening at the top than at the bottom. The collimator opening  79  effectively tapers in the form of a wedge downwards, in which case the profile of the opening can be influenced particularly well, especially by means of a multi-leaf collimator  85 , in order to produce the desired opening contour. 
   A multi-leaf collimator  85  of the type mentioned here is illustrated in  FIG. 4 , with this figure showing a view of the collimator seen in the beam direction. The multi-leaf collimator  85  which is illustrated here is preferably in the form of a multi-leaf collimator and has two groups  87   a  and  87   b  (which are located alongside one another) of thin laminates  89   a  and  89   b , which are arranged in pairs and can be adjusted by a motor. 
     FIG. 4  shows that the uppermost laminates of the right and the left group  87   a  and  87   b  have been moved towards one another, and that mutually facing ends are touching. Underneath the touching laminates, there is a group of laminates whose mutually facing ends are arranged at a distance from one another, with this distance decreasing from top to bottom. Finally, the lowermost laminates of the two groups  87   a  and  87   b  once again touch in the region of their mutually facing ends. 
   The thickness of the laminates, of which two mutually opposite laminates  89   a  and  89   b  are picked out here, is 2-4 mm. 
   The laminate depth measured at right angles to the image plane shown in  FIG. 4 , that is to say in the beam direction, is 5-10 cm, depending on the metal that is used for production of the laminates. 
   In the energy filter device  1 ″ shown in  FIG. 3 , the beam  73  leaving the magnet  71  passes through the multi-leaf collimator  85 . In the illustration shown in  FIG. 4 , the beam enters the image plane chosen here at right angles. 
   The distance between the upper laminates is greater than that between those located underneath. The width of the collimator opening  91  transversely with respect to the beam direction and transversely with respect to the deflection direction of the magnet  71  is greater for the high-energy component of the beam, which is deflected only slightly in the dipoles of the magnet  71 , and becomes increasingly less for the low-energy component of the beam  73 , which is deflected more strongly. The moving laminates  89   a  and  89   b  can be used to change the shape of the collimator opening  91 , which in this case tapers from top to bottom, in order to predetermine the energy spectrum of the filtered beam  77  that is produced downstream from this special collimator. The spectrum of the beam  77  can be adapted individually by means of a multi-leaf collimator of the type mentioned here. It is thus possible to produce an energy spectrum which results in a depth dose profile with a high plateau (spread-out Bragg peak). This is shown underneath the beam  77  in  FIGS. 3   a  and  3   b.    
   The production of a beam such as this is medically highly advantageous. 
   The filtered beam  77  may be passed through a suitable lens, for example through a quadrupole pair  81 , in order to obtain a filtered and focused beam  83 . 
     FIG. 3   a  shows the energy spectrum of the filtered beam  77 , and  FIG. 3   b  shows its depth dose profile. 
   Various exemplary embodiments of energy filter devices are shown in  FIGS. 1 to 3 . These have different passive modulators. In the exemplary embodiments described here, one particular type of modulator has been used in each case. However, it is possible to combine the various modulators illustrated in  FIGS. 1 to 3 , and their elements, with one another in order to optimally harden the ion beam  3 . 
   The energy filter device explained with reference to the figures is accordingly used for beams which have a broad spectrum with a relatively high low-energy component, such as those which result when ions are produced by pulsed laser beams. It is clearly evident that the modulators  17 ,  17 ′ and  17 ″ described in  FIGS. 1 to 3  act as energy modulators and are used with the aim of hardening the energy spectrum of the beam, in order to produce a depth dose profile which can be used to good effect for ion beam therapy. 
   Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.