EP0157129A1

EP0157129A1 - Electron accelerator

Info

Publication number: EP0157129A1
Application number: EP85101459A
Authority: EP
Inventors: George Menor; Duc Tran; Volker Stieber; Richard Smith
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1984-02-21
Filing date: 1985-02-11
Publication date: 1985-10-09

Abstract

An electron accelerator which comprises a target (6 or 206) exposed to the electron beam (e-) for producing x-ray deceleration radiation and means (106, 108, 110, 112 or 306, 308, 310, 312) for switching the x-ray radiation between a lower first and a higher second photon energy. A first compensating member (14 or 214) which is arranged centrally in the x-ray radiation at a first distance from the target, flattens the radiation intensity distribution when the x-ray radiation is switched to the first photon energy. In the case the x-ray radiation is switched to the second photon energy a second compensating member (80 or 240) which is arranged in the x-ray radiation in a longer second distance from the target, in combination with the first compensating member flattens the radiation intensity distribution.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electron accelerator. In oarticular, it relates to an electron accelerator for radiation theraoy.

2. Description of the Prior Art

An electron accelerator for radiation therapy is for example described in the essay "Radiotherapy today: The Mevatron 20, a Compact Highoutput Linear Accelerator", by W. Haas, V. Stieber and L. Taumann in Electromedica 3-4/77, pages 101-106. Such an electron accelerator is also specified in the Siemens brochure "Siemens, a total resource comoany for radiation therapy", MG/5020-008 SIQ 785.
In the case of electron accelerators x-ray deceleration radiation is produced due to a deceleration of the electrons in a so-called target. It is known in the art to balance or compensate the dosage in a given soace angle range of the x-rays leaving the target by placing a compensating member (so-called flattening filter) into the portion of the x-ray cone of interest. This compensating member has a conical design and its contour path is adapted to the path of the radiation intensity at the place of use. Special kinds of compensating members are for examole described in the U.S. Patents 4,109,154 ((Taumann), 4,121,109 (Taumann et al.) 4,343,997 (Heinz), 4,286,167 (La Riveria) and 3,917,954 (Boge).
In the case of the first three mentioned U.S. Patents the compensating members are fabricated as one piece from one special material, tungsten for example. In the case of U.S. Patent 4,286,167 the compensating members are combined of two pieces of two different materials, such as iron and tunqsten. However, both oieces are attached to each other such that they form one compensating member only. In case of U.S. Patent 3,917,954 a orimary filter and an external flattening filter are arranged in a certain distance from each other and from an x-ray target. The primary filter is symmetrically aligned about a longitudinal axis. However, the primary filter is of relatively unsophisticated design. So it is the purpose of the external flattening filter to flatten out deficiencies of the primary filter (e.g. column 3, lines 21 to 24 of this patent).
In practice, each electron accelerator produces an x-ray radiation (x-ray beam) with a constant photon energy. However, under certain circumstances it is desirable to have an electron accelerator switched from an x-rav radiation having a first photon energy to an x-ray radiation having a second photon energy. For example, it may he desirable to switch from a 10 MV x-ray radiation to an additional theraneutically useful x-ray radiation with an energy of 20 MV. Switching may be performed by changing the energy of the electron beam. To change the energy of the electron beam the injected e-current and/or the microwave oower of the accelerator may be changed. Also, phase shifting as for example described in U.S. Patents 4,118,653 (Vaquine) and 4,286,192 (Tanabe et al.) may be utilized for energy variation.
In case of switching from one photon energy to another, the compensating member has to be adapted. This may happen by removing the compensating member which is associated with the first photon beam energy and instead inserting another compensating member which is associated with the second photon beam energy.
However, replacing a comoensatinq member by another is problematic. Since the dosage decreases remarkably with the distance from the center beam behind the target, the sides of the compensating member are corresoondingly steep and the tip of the comoensating member must be positioned very precisely with respect to the center beam, as for example indicated in column 1, lines 20-25 of Taumann's U.S. Patent 4,109,154. Precise centering however, if difficult and time consuming, as is for example also discussed in column 2, lines 25 to 39 of Boge's U.S. Patent 3,917,954.

SUMMARY OF THE INVENTION

1. Objects

It is an object of this invention to provide an improved electron accelerator for switching between a first and a second photon energy comprising a compensating member which can be very easily adapted to the energy change in a non-time consuming manner.
It is another object of this invention to provide such an electron accelerator the x-ray dose of which at the first and second photon energy may be monitored with merely one x-ray dose measuring means.

2. Summary

According to this invention an electron accelerator is provided, which comorises:

a) an electron beam;
b) a target means exposed to the electron beam for producing x-ray deceleration radiation;
c) means for switching the x-ray deceleration radiation between a lower first photon energy and a higher second photon energy;
d) a first compensating member for flattening the radiation intensity distribution when the x-ray radiation is switched to the first photon energy;
e) a second compensating member;
f) means for arranging the first compensating member centrally in the x-ray radiation in a first distance from the target means; and
g) means for arranging the second compensating member centrally in the x-ray radiation in addition to the first compensating member in a second, with respect to the first distance longer distance from the target means, when the x-ray radiation is switched to the second photon energy such that the first compensating member in combination with the second compensating member flattens the radiation intensity distribution at the second photon energy.

Also according to this invention an electron accelerator is provided which comorises:

a) an electron beam;
b) a target means exoosed to the electron beam for producing x-ray deceleration radiation;
c) means for switching the x-ray deceleration radiation between a lower first ohoton energy and a higher second photon energy;
d) a first compensating member for flattening the radiation intensity distribution when the x-ray radiation is switched to the first photon energy;
e) a second compensating member;
f) an x-ray dose measuring means;
g) means for arranging the first compensating member centrally in the x-ray radiation between the target means and the x-ray dose measuring means in a distance from the latter one, such tnat a free space is formed between the first compensating member and the x-ray dose measuring means; and
h) means for arranging the second compensating member centrally in the x-ray radiation in addition to the first compensating member in the free space between the first compensating member and the x-ray dose measuring means, when the x-ray radiation is switched to the second photon energy, such that tne first compensating member in combination with the second compensating member flattens the radiation intensity distribution at the second ohoton energy.

According to the invention a first compensating member is positioned very orecisely with respect to the center of the x-ray radiation and kept in this precise position independent from the photon energy of the x-ray radiation. Only in the case, the x-ray deceleration radiation is switched to the higher second photon energy the second compensating member is also arranqed in the x-ray radiation in addition to the first compensating member. Positioning of a second compensating member in the radiation, however, is not critical, since the second compensating member is located in greater distance from the target means than the first compensating member. Errors in positioning will be projected with a relatively small projection coefficient into the isocenter (patient plane). Therefore, when making use of a second compensating member in addition to a first one an easy and non-time consuming manner has been found to adapt a compensating member to an energy change.
In case the second compensating member is arranged in the free space between the first compensating member and the x-ray dose measuring means, both the lower x-ray dose, when the x-ray radiation is switched to the lower first photon energy and the higher x-ray dose, when the x-ray radiation is switched to the higher second photon energy, can be monitored by one x-ray dose measuring means.
The second compensating member may be arranged in the x-ray radiation by hand. However, in a preferred embodiment, the second compensating member arranging means may also comprise drive means for driving the second compensating member from a first position outside the x-ray radiation into a second position inside the x-ray radiation.
In another preferred embodiment of the invention, the drive means may also be connected with the switching means for driving the compensating member from the first position to the second position when the x-ray deceleration energy is switched from the first photon energy to the second photon energy.
In still another preferred embodiment of the invention the second compensating member is arranged in the free space between the first compensating member and the x-ray dose measuring means with its tip aligned away from the target means. Due to this the influence of errors in oositioning is further decreased.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

In the drawings:

Fig. 1 is a partial cross-section of a beam-defininq system of an electron accelerator according to this invention, the x-ray radiation of which has been switched to a lower first photon energy;
Fig. 2 is a partial cross-section of a beam defining system of an electron accelerator according to this invention, the x-ray radiation of which has been switched to a higher second photon energy;
Fig. 3 is a schematic block diagram of the invention;
Fig. 4 is an embodiment for the first compensating member in a partially cutted side elevation;
Fig. 5 is the first compensating member in a top view;
Fig. 6 is an embodiment for the second compensating member in a cross-section;
Fig. 7 is the second compensating member in a top view;
Fig. 8 is a partial cross-section of a beam-defining system of an electron accelerator according to this invention wherein the second conpensating member is arranged in a free space between the first compensating member and an x-ray dose measuring means and wherein the x-ray radiation of the electron accelerator has been switched to a lower first photon energy;
Fiq. 9 is a partial cross-section of a beam defining system of the electron accelerator according to Fig. 8 wherein the x-ray radiation has been switched to a higher second photon energy; and
Fig. 10 is a schematic block diagram of the electron accelerator according to Figs. 8 and 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. 1 affords a view of the relative positions of the exit window 2 of an electron accelerator vacuum envelope 4, of a target 6 for generating x-rays when hit by high energy electrons, a collimator 8, an x-ray shielding jaws system 10, an absorption member 12 for electron absorption, and a conical compensating member 14 (flattening filter), in an x-ray beam defining system of an electron accelerator. The target 6 is arranged on a target slide 16 in the radiation direction directly behind the exit window 2 of the vacuum envelope 4. The collimator 8, which is for example made of tungsten, comprises a conical x-ray passageway 18, which may be stepped according to U.S. Patent 4,343,997. The x-ray shielding jaws system 10 comprises a pair of inner x-ray shielding jaws 20 and a pair of outer x-ray shielding jaws 22. The collimator 8 is subdivided in two collimator parts 24 and 26. The collimator part 24 is mounted by means of a slide 28 on a support 30 in a steel housing 32. The collimator part 26 is mounted on a suoport 34 of steel housing 32.
The slide 28 comprises a first window 36 and a second window 38. In the first window 36 of slide 28 an x-ray dose chamber 40 is arranged for the purpose of monitoring the issued x-ray radiation. In the second window 38 of slide 28 an electron dose chamber (not shown) may be positioned. By moving the slide 28 in the direction of arrow 42 the collimator part 24 with x-ray passageway 18 together with slide window 36 and inserted x-ray dose chamber 40 are removed from electron exit window 2 of the vacuum envelope 4. Instead, window 38 is moved in a position beneath exit window 2 of the vacuum envelope 4. Thus, the system may be switched from x-ray mode to electron mode.
As illustrated in Fig. 1 electrons e^- of high energy are generated at exit window 2 of vacuum envelope 4 after acceleration within a waveguide and beam bending within the vacuum envelope 4. The x-ray radiation is produced by collision of accelerated electrons with the target 6. The x-ray radiation is in the shape of a cone 44. Its intensity maximum coincides with the direction of the impinging electron beam, that means in the direction of center ray 46 of radiation cone 44.
The conical compensating member 14 installed in the x-ray passageway 18 of collimator 8 comprises a base 48 and a tip 50. It is precisely adapted with regard to its absorption value and its shape to the intensity characteristic of the x-ray radiation issuing from the target 6, when the x-ray radiation is switched to a first photon energy, for example 10 MV. Thus, the compensating member 14 as illustrated in Fig. 1 corresponds with the first compensating member according to this invention, which is arranged centrally (e.g., on slide 28) in the cone of the x-ray radiation with base 48 in a first distance ( < 15 cm, e.g. approximately 9 cm) from the target 6 within the x-ray passageway 18 of tne collimator 8.
Further, according to this invention the system of Fig. 1 also comprises a mounting device 52 for a second compensating member, which will be described later in more detail in connection with Figs. 2, 3, 6 and 7. The mounting device 52 which is located in the interior of an accessory holder 54 contains a support 56, which is part of the accessory holder 54 and a first slot 58 along the left inner edge of the support 56 and a second slot 60 along the right inner edge of the support 56. Both slots 58, 60 provide guide rails for a mounting tray for the second compensating member.
For arrangement in the cone of the x-ray radiation, the second compensating member may be attached to the mounting device 52 by hand, e.g. through window 62 of the accessory holder 54. However, as will be specified later in more detail with respect to Fig. 3, the second comoensating member may also be attached to the mounting device 52 by means of a slide. The slide may be driven by a motor such that the second compensating member will be moved into the cone of the x-ray radiation, when the x-ray radiation is switched from the first to the second photon energy.
In the system of Fig. 1 the accessory holder 54 also bears an electron applicator 64, as for examole described in U.S. Patent 4,140,129 (Heinz et al.) or in U.S. Patent 4,484,078 (Tayag et al.). The element 66 is a light field mirror for the x-ray field. The associated light source is generally designated by 68. The portion 70 is a plate disc in front of the light source 68. The x-ray shielding jaws system 10 is arranged in a steel housing 72. The steel housing 72 is attached to the steel housing 32 by means of a bearing 74 for rotation. The isocenter (patient plane) is generally designated with 76.
As already mentioned above, Fig. 1 illustrates the beam defining system of an electron accelerator, when the x-ray deceleration radiation is switched to a first photon energy, e.g., 10 MV. In this case, the second compensating member is not arranged in the cone 44 of the x-ray radiation. Thus, when the x-ray radiation is switched to the lower first photon energy, merely the first compensating member 14 flattens the radiation intensity distribution. Before switching to the higher second photon energy, e.g., 20 MV, the second compensating member has to be arranged centrally in the cone of the x-ray radiation in addition to the first compensating member. This situation is illustrated in Fig. 2. In the case of Fig. 2 a second compensating member 80, which is flatter than the first compensating member 14 and which also has a larger basic diameter has been arranged by means of mounting device 52 centrally in the cone 44 of the x-ray radiation in addition to the first compensating member 14. The second compensating member comprises a base 82 and a tip 84. It is mounted on a tray 86. The left and right edges of the tray 86 slide in slots 58 and 60 of the mounting device 52. When arranged in the x-ray cone 44 the base 82 of the second compensating member 80 is at a distance of approximately 40 cm ( ) 35 cm) from the target 6. Thus, the ratio between the first distance of the base of the first compensating member 14 and the second distance of the base of the second compensating member 80 is approximately 1:4. After having arranged the second compensating member 80 as shown in Fig. 2, the x-ray deceleration radiation can now be switched from the lower first photon energy to the higher second photon energy. After having switched the x-ray deceleration radiation to the higher second photon energy the first compensating member 14 in combination with the second compensating member 80 now flattens the radiation intensity distribution.
Fig. 3 shows in a schematic block diagram a linear accelerator waveguide 90 comprising an electron gun 92, a suitable radio frequency (RF) source 94, a radio frequency coupling element 96, a radio frequency input window 98 and an electron exit window 100. The power supply for the electron gun 92 is generally designated by 102. The power supply 102 is connected with a power supply adjusting means 104. The radio frequency source 94 comprises a radio frequency power adjusting means 106, which is connected with the output of a radio frequency power control circuit 108. The radio frequency power control circuit 108 comprises a first input switch 110 and a second input switch 112. Both input switches 110, 112 are controlled such that when one switch is closed the other one is open. When closing the first input switch 110 the electron beam of the accelerator will be switched to a first electron energy by means of the radio frequency power adjusting means 106 via radio frequency power control circuit 108. Due to this the x-ray deceleration radiation will be switched to a lower first photon energy, e.g., 10 MV. When closing the second input switch 112 the electron beam will be switched to a second electron energy and due to this the x-ray deceleration radiation will be switched to a higher second photon energy, e.g. 20 MV.
As discussed above, the second compensating member 80 may be arranged centrally in the cone of the x-ray radiation in addition to the first compensating member 14 by hand. However, it may also be arranged by means of a motor drive 114 as illustrated for example in Fig. 3. The motor drive 114 in Fig. 3 comprises a motor 116, a rack wheel 118 and a rack 120. The rack 120 is mounted on the bottom of the mounting tray 86 for the second compensating member 80. The edges of the mounting tray 86 are slidably arranged in slots 58, 60 of the mounting device 52. The motor 116 may be connected by control line 122 with the second input switch 112 of the radio frequency power control circuit 108.
The motor 116 is rotatable between a first position and a second position according to double arrow 123. In the first motor position the mounting tray 86 and the second compensating member 80 are in a position I (as indicated in Fig. 3 by solid lines). In the second motor position the mounting tray 86 and the second compensating member 80 are in a position II (as depicted in Fig. 3 by dotted lines). In position I the second compensating member 80 is arranged outside the cone 44 of the x-ray radiation. However, in position II the second compensating member 80 is positioned centrally in the cone of the x-ray radiation.
As long as input switch 110 of the radio frequency power control circuit 108 is closed (as indicated in Fig. 3), the control signal on line 122 keeps motor 116 in the first motor position. Thus, also the second compensating member 80 is in position I. As desired, the second compensating member 80 is arranged outside of the cone 44 of the x-ray radiation, when the x-ray radiation is switched to the lower first photon energy, i.e., 10 MV. Therefore, the first compensating member 14 alone flattens the radiation intensity distribution.
In the case input switch 112 is closed a control signal is generated on control line 122 which rotates thr motor clockwise into its second motor position. Due to this the mounting tray 86 and the second compensating member 80 are moved into the position II. Thus, when switching from the lower first photon energy, i.e. 10 MV, to the higher second photon energy, e.g. 20 MV, the second compensating member 80 is driven into a position, where it is arranged centrally in the cone 44 of the x-ray radiation. Now, at the higher photon energy the second comoensating member 80 in combination with the first compensating member 14 flattens the radiation intensity distribution.
As indicated in Fig. 3 by dotted lines a delay member 124 may be inserted between the radio frequency power adjusting circuit 108 and the second input switch 112. The delay member 124 delays switching of the x-ray radiation to the higher photon energy until the motor 116 has completed its rotation into the second position. This preventive measure guarantees that the second compensating member 80 is already in its position II before the x-ray radiation is switched to the higher second photon energy.
In the embodiment of Fig. 3 switching the x-ray radiation from a first photon energy to a second photon energy is performed by changing the radio frequency power. It is understood, that other technical possibilities may also be utilized for changing the energy.
The crosses 126 inside the bending loop 128 of the vacuum envelope 4 at the output of the linear accelerator waveguide 90 generally designate the magnetic field of the bending magnet of the vacuum envelope.
Figs. 4 and 5 illustrate an embodiment for the first compensating member 14. The first compensating member 14, which is fabricated, for example of tungsten, has diameters Dl to D4 approximately as follows: Dl = 2.7 inch, D2 = 2.2 inch, D3 = 1.3 inch and D4 = 0.15 inch. The approximate heights of Hl to H3 are as follows: HI = 0.05 inch, H2 = 0.3 inch and H3 = 1.7 inch. The mounting bores are 130.
Figs. 6 and 7 illustrate an embodiment for the second compensating member 80. The second compensating member 80, which is also preferably fabricated of tungsten alone or in combination with other materials, has diameters dl to d3 approximately as follows: dl = 7.5 inch, d2 = 6.5 inch and d3 = 0.3 inch. The approximate heights of hl to _h3 are are follows: hl = 0.13 inch, h2 = 0.3 inch and h3 = 0.35 inch. Again, the mounting bores are 132.
Under these circumstances, the ratio between the basic diameters D2, d2 of both compensating member lies in the range of D2:d2 = 1:3. The ratio between the total heights H3, h3 lies in the range of H3:h3 = 5:1.
Figs. 8 and 9 afford a view of the relative positions of the exit window 202 of an electron accelerator vacuum envelope 204, of a target 206, for generating x-rays when hit by high energy electrons, a collimator 208, an x-ray shielding jaws system 210, an absorption member 212 for electron absorption, and a first conical compensating member 214 (flattening filter), in an x-ray beam defining system of an electron accelerator. The target 206 is arranged on a target slide 216 in the radiation direction directly behind the exit window 202 of the vacuum envelope 204. The collimator 208 comprises three collimator portions 218, 220 and 222.
The first collimator portion 218 which includes a first conical passageway 224 (which may be stepped according to U.S. Patent 4,343,997) for the x-ray radiation of the target 206, is mounted by means of a first slide 226 on a support 228 of a steel housing 230. The first collimator portion 218 bears the first conical compensating member 214 inside the first x-ray passageway 224. It also contains a recess 232.
The second collimator portion 220 which includes a second passageway 234 for the x-ray radiation of the target 206, is for example mounted by means of a second slide 236 on a top surface 238 of the first slide 226. The second collimator portion 220 bears a second conical compensating member 240 inside the second x-ray passageway 234. The second slide 236 can be moved between a first position, wherein the second collimator portion 220 does not fill the recess 232 of the first collimator portion 218, and a second position, wherein the second collimator portion 220 fills the recess 232 of the first collimator portion 218. In the first position, as indicated in Fig. 8, the second collimator passageway 234 and thus also the second compensating member 240 lies outside the x-ray radiation (generally designated by 242). In the second position, as indicated in Fig. 9, the second collimator passageway 234 supplements the first collimator passageway 224 to one passing through passageway 224 plus 234 for the x-ray radiation 242. Also the second compensating member 240 is now arranged in the x-ray radiation 242 in addition to the first compensating member 214.
In Figs. 8 and 9 the first conical compensating member 214 comprises a cone base 244 and a cone tip 246. The second conical compensating member 240 also comprises a cone base 248 and a cone tip 250. As can be seen from Figs. 8 and 9 the first compensating member 214 is arranged in the first collimator passageway 224 with its tip aligned toward the target 206. The second comoensating member 240, however, is arranged in the second collimator passageway 234 with its tip aligned in the opposite direction. Due to this the cone tip 250 of the second compensating member 240 will be aligned away from the target 206, when the second collimator portion 220 is in the second position and 'thus the second compensating member is arranged in the x-ray radiation 242 in addition to the first compensating member 214. As mentioned before, due to this the influence of errors in positioning is further decreased. When arranged in the x-ray radiation the cone bases 244 and 248 of the first and second compensating members face each other in close proximity.
The third collimator portion 222 is mounted on a support 252 of the steel housing 230.
The first slide 226 comprises a first window 254 and a second window 256. In the first window 254 of slide 226 an x-ray dose chamber 258 is arranged for the purpose of monitoring the issued x-ray radiation. In the second window 256 of slide 226 an electron dose member (not shown) may be positioned. By moving the slide 226 in the direction of arrow 260 the first collimator portion 218 with x-ray passageway 224 together with slide window 254 and inserted x-ray dose chamber 258 are removed from electron exit window 202 of the vacuum envelope 204 into the same position as shown for the second collimator portion 220 in Fig. 8. Instead, window 256 is moved in a position beneath exit window 202 of the vacuum envelope 204. Thus, the system may be switched from x-ray mode to electron mode.
In Figs. 8 and 9 the x-ray shielding jaws system 210 comprises a pair of inner x-ray shielding jaw 262 and a pair of outer x-ray shielding jaws 264. The element 266 is a light field mirror for the x-ray field. The associated light source is generally designated by 268. The piece 270 is a blade disc in front of the light source 268. The x-ray shielding jaws system 210 is arranged in a steel housing 272. The steel housing 272 is attached to the steel housing 232 by means of a bearing 274 for rotation.
As illustrated in Figs. 8 and 9 electrons e^- of high energy are generated at exit window 202 of vacuum envelope 204 after acceleration within a waveguide and beam bending within the vacuum envelope 204. The x-ray radiation 242 is produced by collision of accelerated electrons with the target 206. The x-ray radiation 242 is in the shape of a cone. The maximum cone surface in the limits of the passageway 224 and 234 of the first and second collimator portions is generally designated with 276. A cone surface as limited by the x-ray shielding jaws system 210 is indicated with 278. The intensity maximum of the x-ray radiation 242 coincides with the direction of the impinging electron beam, that means in the direction of center ray 280 of the x-ray radiation 242.
The first conical compensating member 214 installed in the first x-ray passageway 224 of the first collimator portion 218 is precisely adapted with regard to its absorption value and its shape to the intensity characteristic of the x-ray radiation issuing from the target 206, when the x-ray radiation is switched to a first photon energy, for example 10 MV. This situation is for example illustrated in Fig. 8. In this case, the second compensating member 240 is not arranged in the x-ray radiation 242. Thus, when the x-ray radiation is switched to the lower first photon energy, merely the first compensating member 214 flattens the radiation intensity distribution.
Before switching to the higher second photon energy, e.g. 20 MV, the second compensating member 240 has to be arranged centrally in the x-ray radiation 242 in addition to the first compensating member 214. This situation is illustrated in Fig. 9. In this case the second collimator portion 220 has been moved by means of slide 236 into the recess 232 of the first collimator portion 218 such that it fills the recess. The slide 236 may be moved by hand. It may also be moved by means of a motor drive, as will be described later in more detail with respect to Fiq. 10.
After having'arranged the second compensating member 240 as shown in Fig. 9, the x-ray deceleration radiation can now be switched from the lower first photon energy to the higher second photon energy. After having switched the x-ray deceleration radiation to the higher second photon energy the first compensating member 214 in combination with the second compensating member 240 now flattens the radiation intensity distribution.
Fig. 10 shows in a schematic block diagram a linear accelerator waveguide 290 comprising an electron gun 292, a suitable radio frequency (RF) source 294, a radio frequency coupling element 296, a radio frequency input window 298 and an electron exit window 300. The power supply for the electron gun 292 is generally designated by 302. The power supply 302 is connected with a power supply adjusting means 304. The radio frequency source 294 comprises a radio frequency power adjusting means 306, which is connected with the output of a radio frequency power control circuit 308. The radio frequency power control circuit 308 comprises a first input switch 310 and a second input switch 312. Both input switches 310, 312 are controlled such that when one switch is closed the other one is open. When closing the first input switch 310 the electron beam of the accelerator will be switched to a first electron energy by means of the radio frequency power adjusting means 306 via radio frequency Dower control circuit 308. Due to this the x-ray deceleration radiation will be switched to a lower first photon energy, e.g. 10 MV. When closing the second input switch 312 the electron beam will be switched to a second electron energy and due to this the x-ray deceleration radiation will be switched_;to a higher second photon energy, e.g., 20 MV.
As mentioned above, the second compensating member 240 may be arranged centrally in the x-ray radiation in addition to the first compensating member 214 by moving slide 236 by hand. However, slide 236 may also be moved by means of a motor drive 314 as illustrated for example in Fig. 10. The motor drive 314 in Fig. 10 comprises a motor 316, a rack wheel 318 and a rack 320. The rack 320 is mounted on the bottom of slide 236 for the second compensating member 240. The motor 316 may be connected by control line 322 with the second input switcn 312 of the radio frequency power control circuit 308.
The motor 316 is rotatable between a first position and a second position according to double arrow 323. In the first motor position the second collimator portion 220 and the second compensating member 240 are in a position I (as indicated in Fig. 10 by solid lines). In the second motor position the second collimator portion 220 and the second compensating member 240 are in a position II (as depicted in Fig. 10 by dotted lines). In position I the second compensating member 240 is arranged outside the x-ray radiation 242. However, in position II the second compensating member 240 is positioned centrally in the x-ray radiation 242.
As long as input switch 310 of the radio frequency power control circuit 308 is closed (as indicated in Fig. 10), the control signal on line 322 keeps motor 316 in the first motor position. Thus, also the second compensating member 240 is in position I. As desired, the second compensating member 240 is arranged outside of the cone 244 of the x-ray radiation, when the x-ray radiation is switched to the lower first photon energy, i.e., 10 MV. Therefore, the first compensating member 214 alone flattens the radiation intensity distribution.
In the case input switch 312 is closed a control signal is generated on control line 322 which rotates the motor counter-clockwise into its second motor position. Due to this the second collimator portion 220 and the second compensating member 240 are moved into the position II. Thus, when switching from the lower first photon energy, i.e. 10 MV, to the higher second photon energy, i.e. 20 MV, the second compensating member 240 is driven into a position, where it is arranged centrally in the x-ray radiation 242. Now, at the higher photon energy the second compensating member 240 in combination with the first compensating member 214 flattens the radiation intensity distribution.
As indicated in Fig. 10 by dotted lines a delay member 324 may be inserted between the radio frequency power adjusting circuit 308 and the second input switch 312. The delay member 324 delays switching of the x-ray radiation to the higher photon energy until the motor. 316 has completed its rotation into the second position. This preventive measure guarantees that the second compensating member 240 is already in its position II before the x-ray radiation is switched to the higher second photon energy.
In the embodiment of Fig. 10 switching the x-ray radiation from a first photon energy to a second photon energy is performed by changing the radio frequency power. It is understood, that other technical possibilities may also be utilized for changing the energy.
The crosses 326 inside the bending loop 328 of the vacuum envelope 4 at the output of the linear accelerator waveguide 290 generally designate the magnetic field of the bending magnet of the vacuum envelope.
Having thus described the invention with particular reference to the preferred forms thereof, it will be obvious to those skilled in the art to which the invention pertains, after understanding the invention, that various changes and modifications may be made therein without departing from the soirit and scope of the invention as defined by the claims appended hereto.

Claims

1. An electron accelerator comprising:

a) an electron beam (e^-);

b) a target means (6) exoosed to the electron beam for producing x-ray deceleration radiation;

c) means (106, 108, 110, 112) for switching the x-ray deceleration radiation between a lower first photon energy and a higher second photon energy;

d) a first compensating member (14) for flattening the radiation intensity distribution when the x-ray radiation is switched to the first photon energy;

e) a second compensating member (80);

f) means (18) for arranging the first compensating member (14) centrally in the x-ray radiation in a first distance from the target means; and

g) means (52, 114) for arranging the second compensating member (80) centrally in the x-ray radiation in addition to the first compensating member in a second, with respect to the first distance longer distance from the target means, when the x-ray radiation is switched to the second photon energy such that the first compensating member in combination with the second compensating member flattens the radiation intensity distribution at the second photon energy.

2. The electron accelerator according to claim 1, wherein the second compensating member arranging means (52, 114) comprises a mounting device (52) which contains two slots (58, 60) which form a guide rail means for inserting the second compensating member.

3. The electron accelerator according to claim 1 or 2, further comprising a mounting tray for the second compensating member (80), wherein the mounting tray (86) is provided for insertion into the slots of the mounting device.

4. The electron accelerator according to one of the claims 1 to 3, further comprising an accessory holder (54), wherein the second compensating member arranging means are mounted in the accessory holder and wherein the accessory holder comprises a window (62) for inserting the second compensating member into the second compensating member arranging means.

5. The electron accelerator according to one of the claims 1 to 4, wherein the second compensating member arranging means comprises drive means (114) for driving the second compensating member from a first position (I) outside the x-ray radiation into a second position (II) in the center of the x-ray radiation.

6. The electron accelerator according to one of the claims 1 to 5, wherein the ratio between the first distance of the first compensating member (14) and the second distance of the second compensating member (80) is approximately 1:4.

7. The electron accelerator according to one of the claims 1 to 6, wherein the first distance of the first compensating member (14) from the target means (6) is smaller than 15 cm, e.g. approximately 9 cm and wherein the second distance of the second compensating member (80) is larger than 35 cm, e.g. approximately 40 cm.

8. The electron accelerator according to one of the claims 1 to 7, wherein the second compensating member (80) is relatively flat with respect to the first compensating member (14).

9. The electron accelerator according to claim 8, wherein the first compensating member (14) has a first base to tip height and the second compensating member has a second base to tip height and wherein the ratio between the first base to tip height and the second base to tip height is aDproximately 5:1.

10. The electron accelerator according to one of the claims 1 to 9, wherein the second compensating member (80) has a larger basic diameter than the first compensating member, e.g. wherein the ratio between the basic diameter of the second compensating member and the basic diameter of the first compensating member is approximately 3:1.

11. An electron accelerator comprising:

a) an electron beam (e^-);

b) a target means (206) exposed to the electron beam for producing x-ray deceleration radiation;

c) means (306, 308, 310, 312) for switching the x-ray deceleration radiation between a lower first photon energy and a higher second photon energy;

d) a first compensating member (214) for flattening the radiation intensity distribution when the x-ray radiation is switched to the first photon energy;

e) a second compensating member (240);

f) an x-ray dose measuring means (258);

g) means (218, 224) for arranging the first compensating member (214) centrally in the x-ray radiation between the target means and the x-ray dose measuring means in a distance from the latter one, such that a free space is formed between the first compensating member and the x-ray dose measuring means; and

h) means (220, 236) for arranging the second compensating member (240) centrally in the x-ray radiation in addition to the first compensating member in the free space (232) between the first compensating member and the x-ray dose measuring means, when the x-ray radiation is switched to the second photon energy, such that the first compensating member is combination with the second compensating member flattens the radiation intensity distribution at the second photon energy.

12. The electron accelerator according to claim 11, wherein the second compensating member (240) is formed as a cone having a cone base (248) and a cone tip (250) and wherein the second compensating member arranging means (220, 236) are designated for arranging the cone in the free space (232) between the first compensating member (214) and the x-ray dose measuring means (258) with its cone tip aligned away from the target means.

13. The electron accelerator according to claim 11 or 12, wherein the first compensating member (214) is also formed as a cone having a cone base (244) and a cone tip (246) and wherein the cone of the first compensating member is arranged in the x-ray radiation with its cone tip aligned towards the target means (206) and wherein the second compensating member arranging means are designated for arranging the cone of the second compensating member with respect to the cone of the first compensating member such that both cone bases face each other in close proximity.

14. The electron accelerator according to one of the claims 11 to 13, wherein

a) the first compensating member arranging means comprises a first collimator portion (218) located between the target means (206) and the x-ray dose measuring means (258), such that a free collimator space (232) is formed between the first compensating member (214) and the x-ray dose measuring means, said first collimator portion having a first passageway (224) for the x-ray radiation and the first compensating member arranged therein; and

b) the second compensating member arranging means (220, 236) comprises a second collimator portion (220) having a second passageway (234) for the x-ray radiation and the second compensating member (240) arranged therein and being designated for being inserted in the free collimator space (232), when the x-ray radiation is switched to the higher second photon energy such that the first and the second collimator portions add to one compact collimator and the first and second passageways supplement to one passing through collimator passageway for the x-ray radiation and both the first and the second compensating member are arranged in the passing through collimator passageway.

15. The electron accelerator according to claim 14, further comprisinq means (236) for driving the first collimator portion and the second collimator portion relatively to each other between a first position (I), wherein the second collimator portion is located outside the free collimator space of the first collimator portion and a second position (II), wherein the second collimator is inserted in the free collimator space of the first collimator portion.

16. The electron accelerator according to claim 14, wherein the free collimator space (232) is a recess in the first collimator portion.

17. The electron accelerator according to claim 15, wherein the free collimator space is a recess in the first collimator portion and wherein the second collimator portion fills the recess in the second position.

18. The electron accelerator according to claim 15, further comprising a support wherein the first collimator portion is arranged at the support in a higher first plane and the second collimator portion is arranged at the support in a lower second plane and wherein the driving means is mounted at the support for driving the collimator portions in the first and second planes relatively to each other.

19. The electron accelerator according to claim 5 or 15, wherein the drive means (114; 314) comprises a slide support for a second compensating member and/or a second collimator portion.

20. The electron accelerator according to claim 5 or 19, further comprising a drive motor (116; 316) connected with the slide support.

21. The electron accelerator according to claim 5 or 15, wherein the drive means is connected with the switching means (110, 112; 310, 312) for driving a second compensating member and/or a first and second collimator portions relatively to each other from a first position to a second position when the x-ray deceleration radiation is switched from the first photon energy to the second photon energy.

22. The electron accelerator according to one of the claims 1 to 21, further comprising means (324) connected with the switching means for delaying switching to the second photon energy until a second compensating member and/or a first and second collimator portions are in the second position, such that the second compensating member has been arranged in the x-ray radiation in addition to the first compensating member.

23. The electron accelerator according to one of the claims 1 to 22, wherein the x-ray deceleration radiation is in the shape of a cone and wherein the first compensating member arranging means and the second compensating member arranging means are provided for arranging the first compensating member and the second compensating member centrally in the cone of the x-ray radiation.